Reviews|109 Article(s)
Design of Imaging and Display Systems Combining Freeform Optics and Holographic Optical Elements
Tong Yang, Yongdong Wang, Lü Xin, Dewen Cheng, and Yongtian Wang
SignificanceProgress in imaging and display optical systems exerts significant influences on the development of science and technology. Imaging and display systems intrinsically utilize optical elements (geometric or phase elements) to modulate optical wavefronts and achieve expectational imaging relationships, system specifications, and structure requirements. As the representative elements of geometric and phase elements respectively, freeform optical elements (FOEs) and holographic optical elements (HOEs) have significant advantages in optical system design. FOEs possess high degrees of design freedom, which can greatly enhance the ability to modulate wavefronts and improve imaging performance. Additionally, freeform surfaces can correct the aberrations of optical systems with off-axis nonsymmetric structures. Meanwhile, HOEs can unconventionally deflect rays at large angles due to their unique ability to modulate optical wavefronts. They can dramatically reduce the weight and volume of optical systems due to the lightweight form factor, and realize better optical see-through experiences and full-color display due to unique selectivity and multiplex ability, achieving mass productions owing to relatively simple fabrication methods and low costs. Meanwhile, it is easy to fabricate HOEs with large sizes due to the unique fabrication methods. Considering the above-mentioned advantages, designers may design imaging and display optical systems that combine FOEs and HOEs, significantly improving the degrees of design freedom and the ability to correct aberrations. Additionally, we can achieve advanced system specifications, excellent system performance, compact and lightweight system forms, and unconventional system structures with off-axis nonsymmetry, with further development of optical systems promoted. It is important to summarize the existing design methods of imaging and display systems combining FOEs and HOEs, analyze the problems restricting their further development, and predict the development trends. Meanwhile, it is essential to summarize the existing designs and applications of these systems to better guide and promote the development.ProgressWe describe the basic principles, ray-tracing models, advantages, and applications of FOEs and HOEs respectively, summarize the system design methods, review the designs and applications of these systems, and analyze current restrictions and future development trends. The design of these systems can be divided into three types. 1) FOEs and HOEs are simultaneously utilized to correct the aberrations of optical systems. 2) The freeform surface is adopted as the substrate shape of HOEs. 3) During HOE fabrication, FOEs are introduced to modulate the recording waves of HOEs. In practical optical system designs, the design can be a combination of the above three ways. The first way directly builds ray-tracing models of freeform optics and HOEs in the optical system design and then adopts the optimization strategy to achieve expectational requirements. The second way coats the holographic recording medium on the freeform substrate to yield HOEs with freeform substrates. The third way bridges the numerical relationship between freeform optics and recording waves of HOEs to fabricate HOEs with unconventional profiles of holographic phase function or grating vector. The methods for defining HOEs based on ray tracing are described in detail, including the phase functions (direction cosines) of the recording waves, holographic phase function, and holographic grating vector, which guides the basic combined design schemes. We review the ways of fabricating HOEs including the whole-area exposing and sub-area exposing (holographic printing) to provide references for combined design fabrication. The calculation methods of starting points of optical systems based on HOEs are summarized in detail, including point-by-point construction and iteration methods, confocal methods, and simultaneous multiple surface (SMS) methods, which guide the design of the optical system combining FOEs and HOEs. The designs and applications of these systems are summarized based on the classifications of HOEs, including augmented reality (AR) near-eye display systems, head-up display (HUD) systems, and HOE-lens imaging systems. Additionally, combined designs of freeform optics and other types of phase elements are also presented, such as liquid crystal polarization hologram (LCPH) based on freeform exposure, and metasurfaces with freeform substrate, which has certain guidance for the combined design of FOEs and HOEs.Conclusions and ProspectsStudies on the system design combining FOEs and HOEs make significant progress in the basic principles, design frameworks, and fabrication methods, which has been employed for developing imaging and display systems with high performance, novel structure, and lightweight form factor. There are also some problems and challenges for the research on the system design combining FOEs and HOEs. They include how to fabricate HOEs with freeform substrates by innovative coating technologies of the holographic recording medium, how to correct chromatic aberrations in the imaging and display system using HOEs, how to reduce the nonuniformity of diffraction efficiency and stray light of systems combining FOEs and HOEs, and how to conduct tolerance analysis of such systems. In summary, the research on the design of imaging and display systems combining FOEs and HOEs will promote the development of next-generation high-performance and compact optical systems. SignificanceProgress in imaging and display optical systems exerts significant influences on the development of science and technology. Imaging and display systems intrinsically utilize optical elements (geometric or phase elements) to modulate optical wavefronts and achieve expectational imaging relationships, system specifications, and structure requirements. As the representative elements of geometric and phase elements respectively, freeform optical elements (FOEs) and holographic optical elements (HOEs) have significant advantages in optical system design. FOEs possess high degrees of design freedom, which can greatly enhance the ability to modulate wavefronts and improve imaging performance. Additionally, freeform surfaces can correct the aberrations of optical systems with off-axis nonsymmetric structures. Meanwhile, HOEs can unconventionally deflect rays at large angles due to their unique ability to modulate optical wavefronts. They can dramatically reduce the weight and volume of optical systems due to the lightweight form factor, and realize better optical see-through experiences and full-color display due to unique selectivity and multiplex ability, achieving mass productions owing to relatively simple fabrication methods and low costs. Meanwhile, it is easy to fabricate HOEs with large sizes due to the unique fabrication methods. Considering the above-mentioned advantages, designers may design imaging and display optical systems that combine FOEs and HOEs, significantly improving the degrees of design freedom and the ability to correct aberrations. Additionally, we can achieve advanced system specifications, excellent system performance, compact and lightweight system forms, and unconventional system structures with off-axis nonsymmetry, with further development of optical systems promoted. It is important to summarize the existing design methods of imaging and display systems combining FOEs and HOEs, analyze the problems restricting their further development, and predict the development trends. Meanwhile, it is essential to summarize the existing designs and applications of these systems to better guide and promote the development.ProgressWe describe the basic principles, ray-tracing models, advantages, and applications of FOEs and HOEs respectively, summarize the system design methods, review the designs and applications of these systems, and analyze current restrictions and future development trends. The design of these systems can be divided into three types. 1) FOEs and HOEs are simultaneously utilized to correct the aberrations of optical systems. 2) The freeform surface is adopted as the substrate shape of HOEs. 3) During HOE fabrication, FOEs are introduced to modulate the recording waves of HOEs. In practical optical system designs, the design can be a combination of the above three ways. The first way directly builds ray-tracing models of freeform optics and HOEs in the optical system design and then adopts the optimization strategy to achieve expectational requirements. The second way coats the holographic recording medium on the freeform substrate to yield HOEs with freeform substrates. The third way bridges the numerical relationship between freeform optics and recording waves of HOEs to fabricate HOEs with unconventional profiles of holographic phase function or grating vector. The methods for defining HOEs based on ray tracing are described in detail, including the phase functions (direction cosines) of the recording waves, holographic phase function, and holographic grating vector, which guides the basic combined design schemes. We review the ways of fabricating HOEs including the whole-area exposing and sub-area exposing (holographic printing) to provide references for combined design fabrication. The calculation methods of starting points of optical systems based on HOEs are summarized in detail, including point-by-point construction and iteration methods, confocal methods, and simultaneous multiple surface (SMS) methods, which guide the design of the optical system combining FOEs and HOEs. The designs and applications of these systems are summarized based on the classifications of HOEs, including augmented reality (AR) near-eye display systems, head-up display (HUD) systems, and HOE-lens imaging systems. Additionally, combined designs of freeform optics and other types of phase elements are also presented, such as liquid crystal polarization hologram (LCPH) based on freeform exposure, and metasurfaces with freeform substrate, which has certain guidance for the combined design of FOEs and HOEs.Conclusions and ProspectsStudies on the system design combining FOEs and HOEs make significant progress in the basic principles, design frameworks, and fabrication methods, which has been employed for developing imaging and display systems with high performance, novel structure, and lightweight form factor. There are also some problems and challenges for the research on the system design combining FOEs and HOEs. They include how to fabricate HOEs with freeform substrates by innovative coating technologies of the holographic recording medium, how to correct chromatic aberrations in the imaging and display system using HOEs, how to reduce the nonuniformity of diffraction efficiency and stray light of systems combining FOEs and HOEs, and how to conduct tolerance analysis of such systems. In summary, the research on the design of imaging and display systems combining FOEs and HOEs will promote the development of next-generation high-performance and compact optical systems.
Acta Optica Sinica
- Publication Date: May. 10, 2024
- Vol. 44, Issue 9, 0900001 (2024)
Recent Progress in Optical Lateral Forces (Invited)
Yuzhi Shi, Chengxing Lai, Weicheng Yi, Haiyang Huang, Chao Feng, Tao He, Aiqun Liu, Weicheng Qiu, Zhanshan Wang, and Xinbin Cheng
SignificanceMomentum is an important backbone of wave fields such as electromagnetic waves, matter waves, sound waves, and fluid waves. As the carriers of electromagnetic waves, photons possess both linear and angular momenta, which can interact with matter and generate optical forces. The technique, or“optical tweezers”which utilize optical forces to manipulate micro/nano-objects, was established by Arthur Ashkin between the 1970s and 1980s. Optical tweezers have unparalleled advantages in capturing and manipulating microscopic particles and provide new research tools for research fields such as biomedicine, physics, and chemistry. In 1997, Steven Chu, Claude Cohen-Tannoudji, and William D. Phillips won the Nobel Prize in Physics for employing optical forces to achieve atomic cooling. It was later in 2018 that Ashkin won half of the Nobel Prize in Physics for his pioneering contributions to optical tweezers and implementing them for biomedical applications.Optical forces adopted in optical manipulation mainly include two popular types of radiation pressure and optical gradient force. The radiation pressure is the force along the direction of the Poynting vector due to the light scattering and absorption and has important applications in atomic cooling, optical sorting, and particle propulsion. The optical gradient force is the force generated by the inhomogeneous intensity or phase distribution of the light field, with great potential in numerous physical and biomedical applications.The optical lateral force (OLF) is an extraordinary force that is perpendicular to the light propagation direction and independent of the intensity or phase gradient of the light field. It is related to the intrinsic and structural properties of light and matter. The strategy to realize an OLF is to break the system symmetry to make photons exert transverse momenta and consequently generate optical forces on particles. Since OLF was first proposed by Nori's and Chan's groups on the same day in 2014, various methods and mechanisms have been reported to configure and explore OLF, such as utilizing the spin and orbital momenta of light, coupling of light and particle chirality, spin-orbit interaction (SOI), and surface plasmon polariton (SPP).In the past ten years, the understanding of transverse momenta and relevant light-matter interaction has reached a new stage. Transverse momentum, whether linear or angular, is closely related to OLF since the optical force is a consequence of momentum exchange or translation between light and matter. For example, transverse spin momentum, also known as the Belinfante spin momentum (BSM), or transverse spin angular momentum (SAM), can generate spin-correlated OLF. Additionally, the imaginary Poynting momentum (IPM) can also induce OLF. The chirality of particles can couple with light and generate transverse energy flux and force near the surface. The conversion of spin angular momentum to orbital angular momentum, or the so-called SOI, endows a new way to generate the OLF.Investigations of such extraordinary transverse light momenta and OLF deepen the understanding of light-matter interactions and have tremendous applications in bidirectional enantioselective separation, meta-robots, spintronics, and quantum physics.ProgressWe review the current theoretical and experimental research progress on OLF, including different mechanisms, experimental methods, and potential applications. We first introduce some fundamentals of transverse momenta, and representative mechanisms for generating OLF from both theoretical and experimental perspectives, including the BSM, chirality, SOI, IPM, and some other effects such as heat, electricity, bubbles, and topology. Meanwhile, we review some representative applications based on OLF, such as meta-robots, particle sorting, and some other biomedical and chemical applications. Finally, we summarize this research direction and provide our vision of new physical mechanisms and more applications that may emerge in the future.Conclusions and ProspectsMomentum and force are two fundamental quantities in electromagnetics. With the innovation and burgeoning development of optical theories of transverse light momenta, mechanisms of OLF are also advancing. The optical force has also become an essential platform and effective tool for testing and validating numerous optical phenomena including transverse momenta.Traditional optical gradient force and radiation pressure have been widely studied in the past four decades, and their technical limitations in some applications have been well comprehended. Some peculiar optical forces discovered in recent years such as the optical pulling force and OLF are playing increasingly important roles in high-precision optical manipulation. OLF provide new possibilities for nanometer-precision sorting, enantioselective separation, and minuscule momentum probing. Additionally, unprecedented advantages of metasurfaces in electromagnetic wave guidance and steering also present more possibilities for manipulating particles. Especially in recent years, with the rapid development of nanofabrication technology, a type of“meta-robot”driven by the OLF has emerged. Although it has not been implemented in practice, its interesting properties and the new degree of freedom in optical manipulation are expected to find many biomedical applications in the future, such as cargo transporting, biotherapy, and local probing. We can also envision various biological applications of OLF, such as bilaterally sorting and binding tiny bioparticles, cargo transporting using metavehicles, stretching and folding DNA and protein molecules in line-shaped beams, enantioselective separation, and high-sensitive sensing by the helical dichroism. Therefore, we can conclude that with the development of modern optics and photonics, the two interrelated quantities of momentum and force will be explored more deeply and have wide applications in material science, biophysics, quantum science, spintronics, optical manipulation, and sensing. SignificanceMomentum is an important backbone of wave fields such as electromagnetic waves, matter waves, sound waves, and fluid waves. As the carriers of electromagnetic waves, photons possess both linear and angular momenta, which can interact with matter and generate optical forces. The technique, or“optical tweezers”which utilize optical forces to manipulate micro/nano-objects, was established by Arthur Ashkin between the 1970s and 1980s. Optical tweezers have unparalleled advantages in capturing and manipulating microscopic particles and provide new research tools for research fields such as biomedicine, physics, and chemistry. In 1997, Steven Chu, Claude Cohen-Tannoudji, and William D. Phillips won the Nobel Prize in Physics for employing optical forces to achieve atomic cooling. It was later in 2018 that Ashkin won half of the Nobel Prize in Physics for his pioneering contributions to optical tweezers and implementing them for biomedical applications.Optical forces adopted in optical manipulation mainly include two popular types of radiation pressure and optical gradient force. The radiation pressure is the force along the direction of the Poynting vector due to the light scattering and absorption and has important applications in atomic cooling, optical sorting, and particle propulsion. The optical gradient force is the force generated by the inhomogeneous intensity or phase distribution of the light field, with great potential in numerous physical and biomedical applications.The optical lateral force (OLF) is an extraordinary force that is perpendicular to the light propagation direction and independent of the intensity or phase gradient of the light field. It is related to the intrinsic and structural properties of light and matter. The strategy to realize an OLF is to break the system symmetry to make photons exert transverse momenta and consequently generate optical forces on particles. Since OLF was first proposed by Nori's and Chan's groups on the same day in 2014, various methods and mechanisms have been reported to configure and explore OLF, such as utilizing the spin and orbital momenta of light, coupling of light and particle chirality, spin-orbit interaction (SOI), and surface plasmon polariton (SPP).In the past ten years, the understanding of transverse momenta and relevant light-matter interaction has reached a new stage. Transverse momentum, whether linear or angular, is closely related to OLF since the optical force is a consequence of momentum exchange or translation between light and matter. For example, transverse spin momentum, also known as the Belinfante spin momentum (BSM), or transverse spin angular momentum (SAM), can generate spin-correlated OLF. Additionally, the imaginary Poynting momentum (IPM) can also induce OLF. The chirality of particles can couple with light and generate transverse energy flux and force near the surface. The conversion of spin angular momentum to orbital angular momentum, or the so-called SOI, endows a new way to generate the OLF.Investigations of such extraordinary transverse light momenta and OLF deepen the understanding of light-matter interactions and have tremendous applications in bidirectional enantioselective separation, meta-robots, spintronics, and quantum physics.ProgressWe review the current theoretical and experimental research progress on OLF, including different mechanisms, experimental methods, and potential applications. We first introduce some fundamentals of transverse momenta, and representative mechanisms for generating OLF from both theoretical and experimental perspectives, including the BSM, chirality, SOI, IPM, and some other effects such as heat, electricity, bubbles, and topology. Meanwhile, we review some representative applications based on OLF, such as meta-robots, particle sorting, and some other biomedical and chemical applications. Finally, we summarize this research direction and provide our vision of new physical mechanisms and more applications that may emerge in the future.Conclusions and ProspectsMomentum and force are two fundamental quantities in electromagnetics. With the innovation and burgeoning development of optical theories of transverse light momenta, mechanisms of OLF are also advancing. The optical force has also become an essential platform and effective tool for testing and validating numerous optical phenomena including transverse momenta.Traditional optical gradient force and radiation pressure have been widely studied in the past four decades, and their technical limitations in some applications have been well comprehended. Some peculiar optical forces discovered in recent years such as the optical pulling force and OLF are playing increasingly important roles in high-precision optical manipulation. OLF provide new possibilities for nanometer-precision sorting, enantioselective separation, and minuscule momentum probing. Additionally, unprecedented advantages of metasurfaces in electromagnetic wave guidance and steering also present more possibilities for manipulating particles. Especially in recent years, with the rapid development of nanofabrication technology, a type of“meta-robot”driven by the OLF has emerged. Although it has not been implemented in practice, its interesting properties and the new degree of freedom in optical manipulation are expected to find many biomedical applications in the future, such as cargo transporting, biotherapy, and local probing. We can also envision various biological applications of OLF, such as bilaterally sorting and binding tiny bioparticles, cargo transporting using metavehicles, stretching and folding DNA and protein molecules in line-shaped beams, enantioselective separation, and high-sensitive sensing by the helical dichroism. Therefore, we can conclude that with the development of modern optics and photonics, the two interrelated quantities of momentum and force will be explored more deeply and have wide applications in material science, biophysics, quantum science, spintronics, optical manipulation, and sensing.
Acta Optica Sinica
- Publication Date: Apr. 25, 2024
- Vol. 44, Issue 7, 0700001 (2024)
Hyperspectral Remote Sensing Technology of Far-Infrared Radiation and Its Application in Ice Cloud Retrievals (Invited)
Lei Liu, Shulei Li, Shuai Hu, and Qingwei Zeng
SignificanceThe observation of atmospheric far-infrared radiation is of significance for a deeper understanding of radiation exchange and balance in the earth-atmosphere system, especially in polar regions. More importantly, compared with other bands, far-infrared bands have inimitable advantages in remote sensing of ice clouds, upper layer water vapor in the troposphere, and atmosphere ingredients.On the one hand, far-infrared radiation plays a crucial role in regulating climate and energy balance. Far-infrared radiation accounts for about 40% to 65% of the Earth's energy emitted to space and thus makes great contributions to the Earth's OLR (outgoing longwave radiation) and atmospheric cooling. However, there is still significant uncertainty in addressing the key issues related to heat flux regulation factors in cold and dry polar conditions due to the limited observations of far-infrared radiation, which has a negative influence on the accuracy of climate models. On the other hand, in atmospheric remote sensing, far-infrared spectra are highly sensitive to low-concentration water vapor in low temperature conditions, making it important for remote sensing of water vapor in polar regions, and in the upper troposphere and lower stratosphere. Additionally, the complex refractive indices of water and ice exhibit different spectral characteristics in mid-infrared and far-infrared bands, further enhancing the ability for cloud detection and phase recognition. Meanwhile, far-infrared hyperspectral radiation is considered to have the potential to improve the retrieval accuracy of microphysical and optical properties of thin ice clouds.However, currently direct measurements of far-infrared radiation at hyperspectral resolution are still relatively limited due to technical issues related to precise spectroscopic and highly sensitive measurements. The most recent measurement of spaceborne far-infrared hyperspectral spectra can be traced back to the 1970s when the National Aeronautics and Space Administration of the United States (NASA) launched the Nimbus-III and Nimbus-IV using the IRIS (infrared interferometer sound) infrared interferometer, which measured far-infrared to mid-infrared radiation with a relatively rough spectral resolution (2.8 cm-1) and a spatial resolution ranging from 400 cm-1 to 1600 cm-1. However, this is still the only satellite borne far-infrared radiation spectral observation data that can be obtained on a global scale. The main technical difficulties for spaceborne far-infrared radiation measurements lie in high-sensitivity detectors and hyperspectral optical systems (such as beam splitters). Due to the low photon energy in the infrared band, traditional infrared hyperspectral interferometers often require cooling by liquid helium (or liquid nitrogen) to improve measurement accuracy and signal-to-noise ratio, and this cannot be extended to satellite applications. Additionally, the moving mirror system of the Fourier spectrometer must also consider tilt and other errors when carried in space. These factors have become the main constraints on the development of high-precision and hyperspectral measurements of atmospheric far-infrared radiation for spaceborne payloads.In recent years, with the development of high-sensitive uncooled detectors and beam splitters, a few comprehensive observation experiments of atmospheric far-infrared radiation at hyperspectral resolution have been conducted based on ground-based and airborne prototypes. Institutions such as the European Space Agency (ESA) and the NASA have also proposed a series of missions to observe far-infrared radiation by satellite instruments. Retrievals of ice cloud characteristics using hyperspectral far-infrared radiation have become an important frontier field and research hotspot. Thus, it is important and necessary to summarize the existing research to guide the future development of this field more rationally.ProgressThe main theoretical basis of far-infrared hyperspectral remote sensing is reviewed and summarized. We also introduce the advantages of far-infrared hyperspectral remote sensing of ice clouds from atmospheric absorption and ice crystal particle scattering sensitivities. Afterward, the development of far-infrared hyperspectral instruments for atmospheric remote sensing is sorted and summarized, with a focus on the technical parameters and key technical issues of the relevant instruments. From the perspective of technological breakthroughs in far-infrared radiation measurements, the key technologies associated with detectors, spectrometers, and beam splitters currently adopted have been classified and introduced (Tables 1-3). From the perspective of the platforms, the corresponding instruments and observation experiments of ground-based and airborne, and the main experimental results are introduced. Then, the main spaceborne missions to measure atmospheric far-infrared at hyperspectral resolution are summarized, including FORUM (ESA) and PREFIRE (NASA). Subsequently, the advantages and research progress of far-infrared hyperspectral technology for remote sensing of ice clouds are discussed. Since far-infrared spectra can provide complementary information on remote sensing of ice clouds, we compare studies about synergistic retrievals of ice cloud parameters and phase recognition by far-infrared and mid-infrared spectrum. In the end, the problems and the ongoing research trends in this field are discussed, including possible technological breakthroughs in the future and possible innovations in the future. The potential applications of far-infrared hyperspectral technology in ice cloud remote sensing in the future are also pointed out.Conclusions and ProspectsFar-infrared radiation measurements with hyperspectral resolution and highly sensitive measurements are gradually becoming a popular tool for atmospheric remote sensing. In summary, conducting global ice cloud remote sensing by hyperspectral far-infrared in the future still calls for in-depth and detailed explorations to promote the development of instrument technology, and also calls for a large number of observational experiments to develop accurate forward and retrieval algorithms. SignificanceThe observation of atmospheric far-infrared radiation is of significance for a deeper understanding of radiation exchange and balance in the earth-atmosphere system, especially in polar regions. More importantly, compared with other bands, far-infrared bands have inimitable advantages in remote sensing of ice clouds, upper layer water vapor in the troposphere, and atmosphere ingredients.On the one hand, far-infrared radiation plays a crucial role in regulating climate and energy balance. Far-infrared radiation accounts for about 40% to 65% of the Earth's energy emitted to space and thus makes great contributions to the Earth's OLR (outgoing longwave radiation) and atmospheric cooling. However, there is still significant uncertainty in addressing the key issues related to heat flux regulation factors in cold and dry polar conditions due to the limited observations of far-infrared radiation, which has a negative influence on the accuracy of climate models. On the other hand, in atmospheric remote sensing, far-infrared spectra are highly sensitive to low-concentration water vapor in low temperature conditions, making it important for remote sensing of water vapor in polar regions, and in the upper troposphere and lower stratosphere. Additionally, the complex refractive indices of water and ice exhibit different spectral characteristics in mid-infrared and far-infrared bands, further enhancing the ability for cloud detection and phase recognition. Meanwhile, far-infrared hyperspectral radiation is considered to have the potential to improve the retrieval accuracy of microphysical and optical properties of thin ice clouds.However, currently direct measurements of far-infrared radiation at hyperspectral resolution are still relatively limited due to technical issues related to precise spectroscopic and highly sensitive measurements. The most recent measurement of spaceborne far-infrared hyperspectral spectra can be traced back to the 1970s when the National Aeronautics and Space Administration of the United States (NASA) launched the Nimbus-III and Nimbus-IV using the IRIS (infrared interferometer sound) infrared interferometer, which measured far-infrared to mid-infrared radiation with a relatively rough spectral resolution (2.8 cm-1) and a spatial resolution ranging from 400 cm-1 to 1600 cm-1. However, this is still the only satellite borne far-infrared radiation spectral observation data that can be obtained on a global scale. The main technical difficulties for spaceborne far-infrared radiation measurements lie in high-sensitivity detectors and hyperspectral optical systems (such as beam splitters). Due to the low photon energy in the infrared band, traditional infrared hyperspectral interferometers often require cooling by liquid helium (or liquid nitrogen) to improve measurement accuracy and signal-to-noise ratio, and this cannot be extended to satellite applications. Additionally, the moving mirror system of the Fourier spectrometer must also consider tilt and other errors when carried in space. These factors have become the main constraints on the development of high-precision and hyperspectral measurements of atmospheric far-infrared radiation for spaceborne payloads.In recent years, with the development of high-sensitive uncooled detectors and beam splitters, a few comprehensive observation experiments of atmospheric far-infrared radiation at hyperspectral resolution have been conducted based on ground-based and airborne prototypes. Institutions such as the European Space Agency (ESA) and the NASA have also proposed a series of missions to observe far-infrared radiation by satellite instruments. Retrievals of ice cloud characteristics using hyperspectral far-infrared radiation have become an important frontier field and research hotspot. Thus, it is important and necessary to summarize the existing research to guide the future development of this field more rationally.ProgressThe main theoretical basis of far-infrared hyperspectral remote sensing is reviewed and summarized. We also introduce the advantages of far-infrared hyperspectral remote sensing of ice clouds from atmospheric absorption and ice crystal particle scattering sensitivities. Afterward, the development of far-infrared hyperspectral instruments for atmospheric remote sensing is sorted and summarized, with a focus on the technical parameters and key technical issues of the relevant instruments. From the perspective of technological breakthroughs in far-infrared radiation measurements, the key technologies associated with detectors, spectrometers, and beam splitters currently adopted have been classified and introduced (Tables 1-3). From the perspective of the platforms, the corresponding instruments and observation experiments of ground-based and airborne, and the main experimental results are introduced. Then, the main spaceborne missions to measure atmospheric far-infrared at hyperspectral resolution are summarized, including FORUM (ESA) and PREFIRE (NASA). Subsequently, the advantages and research progress of far-infrared hyperspectral technology for remote sensing of ice clouds are discussed. Since far-infrared spectra can provide complementary information on remote sensing of ice clouds, we compare studies about synergistic retrievals of ice cloud parameters and phase recognition by far-infrared and mid-infrared spectrum. In the end, the problems and the ongoing research trends in this field are discussed, including possible technological breakthroughs in the future and possible innovations in the future. The potential applications of far-infrared hyperspectral technology in ice cloud remote sensing in the future are also pointed out.Conclusions and ProspectsFar-infrared radiation measurements with hyperspectral resolution and highly sensitive measurements are gradually becoming a popular tool for atmospheric remote sensing. In summary, conducting global ice cloud remote sensing by hyperspectral far-infrared in the future still calls for in-depth and detailed explorations to promote the development of instrument technology, and also calls for a large number of observational experiments to develop accurate forward and retrieval algorithms.
Acta Optica Sinica
- Publication Date: Mar. 25, 2024
- Vol. 44, Issue 6, 0600002 (2024)
Research Progress of Laser Remote Sensing in Slant Visibility Measurements(Invited)
Yufeng Wang, and Dengxin Hua
SignificanceVisibility is a basic meteorological parameter and is regarded as a weather index to understand atmospheric stability and vertical structure. Unlike horizontal visibility, slant visibility is a crucial parameter that pilots are actually concerned about, and it directly determines the safety of aircraft take-off and landing in the aviation field. In addition, slant visibility is an important parameter for space target recognition and plays an important role in the field of weather analysis, sea, land, and air traffic, astronomical observation, sea fog warning, and so on. Therefore, slant visibility, as a highly concerned atmospheric optical and meteorological parameter in recent years, has shown important scientific research significance and application value in atmospheric research, civil aviation, space exploration aerospace, military, and other fields.With the development of lidar technology, few lidar visibility meters have appeared in recent years, with single-wavelength Mie-scattering lidar as the core, and two types of inversion algorithms have been developed. One is the slant visibility inversion method based on Koschmieder's visibility law, in which the measurement of slant visibility only depends on the inversion of atmospheric aerosol extinction coefficients; the other is the inversion method based on optical thickness, in which the atmospheric optical thickness obtained by multi-elevation angle lidar detection is used to estimate the slant visibility. However, the main limitation lies in the neglect of the influence of scattered radiance and the uniform path assumption. As a result, these inversion methods have certain inversion defects. In addition, previous research has begun to pay attention to the atmospheric scattered radiance. However, most of them focus on theoretical modeling and simulation analysis, and thus in-depth study and further exploration are greatly required.To solve the difficulty of atmospheric scattered radiance, the research team of Xi'an University of Technology recently developed a new slant visibility measurement method by lidar and the radiative transfer model (Fig. 6). By taking full advantage of laser remote sensing, aerosol lidar detection was carried out with the high spatial-temporal resolution and high-precision, and the real-time aerosol information including optical, micro-physical, and scattering parameters was provided. The radiative transfer model realized the path distribution of actual atmospheric scattered radiance (Fig. 9), which fundamentally solved the current technical bottleneck of slant visibility measurements. Moreover, a reflectance measurement system was supplemented to provide the intrinsic contrast of the object and the background, and the slant visibility measurement considering the correction of atmospheric scattered radiance was ultimately achieved (Fig. 10).ProgressAtmospheric visibility can be measured by visual methods or visibility instruments. Both the forward-scattering-type and the transmission-type visibility meters can measure horizontal visibility. However, they are unable to provide information on slant visibility.Conclusions and ProspectsWe comprehensively review the main techniques and research progress of laser remote sensing in slant visibility measurements, and several slant visibility inversion methods are sorted out. The shortcomings and limitations of the existing techniques are investigated as well. In view of the difficulty of actual atmospheric scattered radiance, a new remote sensing technique combining lidar and the radiative transfer model is mainly introduced, which has effectively broken through the bottleneck of slant visibility measurements. In the future, with the development of satellite remote sensing technology, it is envisioned to achieve the global map of slant visibility by laser remote sensing technology. SignificanceVisibility is a basic meteorological parameter and is regarded as a weather index to understand atmospheric stability and vertical structure. Unlike horizontal visibility, slant visibility is a crucial parameter that pilots are actually concerned about, and it directly determines the safety of aircraft take-off and landing in the aviation field. In addition, slant visibility is an important parameter for space target recognition and plays an important role in the field of weather analysis, sea, land, and air traffic, astronomical observation, sea fog warning, and so on. Therefore, slant visibility, as a highly concerned atmospheric optical and meteorological parameter in recent years, has shown important scientific research significance and application value in atmospheric research, civil aviation, space exploration aerospace, military, and other fields.With the development of lidar technology, few lidar visibility meters have appeared in recent years, with single-wavelength Mie-scattering lidar as the core, and two types of inversion algorithms have been developed. One is the slant visibility inversion method based on Koschmieder's visibility law, in which the measurement of slant visibility only depends on the inversion of atmospheric aerosol extinction coefficients; the other is the inversion method based on optical thickness, in which the atmospheric optical thickness obtained by multi-elevation angle lidar detection is used to estimate the slant visibility. However, the main limitation lies in the neglect of the influence of scattered radiance and the uniform path assumption. As a result, these inversion methods have certain inversion defects. In addition, previous research has begun to pay attention to the atmospheric scattered radiance. However, most of them focus on theoretical modeling and simulation analysis, and thus in-depth study and further exploration are greatly required.To solve the difficulty of atmospheric scattered radiance, the research team of Xi'an University of Technology recently developed a new slant visibility measurement method by lidar and the radiative transfer model (Fig. 6). By taking full advantage of laser remote sensing, aerosol lidar detection was carried out with the high spatial-temporal resolution and high-precision, and the real-time aerosol information including optical, micro-physical, and scattering parameters was provided. The radiative transfer model realized the path distribution of actual atmospheric scattered radiance (Fig. 9), which fundamentally solved the current technical bottleneck of slant visibility measurements. Moreover, a reflectance measurement system was supplemented to provide the intrinsic contrast of the object and the background, and the slant visibility measurement considering the correction of atmospheric scattered radiance was ultimately achieved (Fig. 10).ProgressAtmospheric visibility can be measured by visual methods or visibility instruments. Both the forward-scattering-type and the transmission-type visibility meters can measure horizontal visibility. However, they are unable to provide information on slant visibility.Conclusions and ProspectsWe comprehensively review the main techniques and research progress of laser remote sensing in slant visibility measurements, and several slant visibility inversion methods are sorted out. The shortcomings and limitations of the existing techniques are investigated as well. In view of the difficulty of actual atmospheric scattered radiance, a new remote sensing technique combining lidar and the radiative transfer model is mainly introduced, which has effectively broken through the bottleneck of slant visibility measurements. In the future, with the development of satellite remote sensing technology, it is envisioned to achieve the global map of slant visibility by laser remote sensing technology.
Acta Optica Sinica
- Publication Date: Mar. 25, 2024
- Vol. 44, Issue 6, 0600001 (2024)
Metasurfaces-Empowered Optical Micromanipulation (Invited)
Xiaohao Xu, Wenyu Gao, Tianyue Li, Tianhua Shao, Xingyi Li, Yuan Zhou, Geze Gao, Guoxi Wang, Shaohui Yan, Shuming Wang, and Baoli Yao
SignificanceOptical micromanipulation utilizes optical force to dynamically control particles, which has the characteristics of non-contact and can be operated in a vacuum environment. Since the invention of optical tweezers in the 1980s, the field has experienced rapid development and has given rise to many emerging research directions, such as holographic optical tweezers, near-field evanescent wave optical tweezers, fiber optic tweezers, optoelectronic tweezers, and photo-induced temperature field optical tweezers, providing rich and powerful tools for fields such as biology, chemistry, nanoscience, and quantum technology. These methods can not only capture, separate, and transport small objects but also allow more precise manipulation, such as the rotation of small objects. However, traditional manipulation methods rely on tightly focused local light, greatly limiting the action range of optical force. In addition, in order to generate a structured light field, larger optical components such as spatial light modulators are usually required, making it difficult to miniaturize and integrate the optical manipulation system.In recent years, metasurfaces have emerged as integrated devices composed of subwavelength nanoantennas, promising new opportunities for optical micromanipulation. This ultra-thin artificial microstructure device can flexiblely control multiple degrees of freedom such as amplitude, phase, and polarization of light, by specially designing the geometric shape, size, and material of its own micro/nanostructure. Compared with traditional optical components such as liquid crystal spatial light modulators, gratings, and lenses, metasurfaces exhibit higher operating bandwidth, structural compactness, and integration. With the merits of miniaturization, integration, and excellent performance in light tailoring, optical metasurfaces have been extensively incorporated into the realm of optical micromanipulation. Especially, owing to their peculiar photomechanical properties, the metasurfaces hold the ability to be actuated by light fields, paving the way to the next generation of light-driven artificial micro-robots. The fast development of this subject indicates that the time is now ripe to overview recent progress in this cross-field.ProgressWe summarized principles of optical micromanipulation and metasurfaces (Fig. 1) and overviewed meta-manipulation devices, including metasurface-based optical tweezers (Fig. 2), tractor beams (Fig. 5), multifunctional micro-manipulation systems (Fig. 3), and metamachines (Figs. 7 and 8). Furthermore, we provided a detailed discussion of novel mechanical effects, such as topological light manipulation, which stems from the topological characteristics of nanostructures (Fig. 6).Conclusions and ProspectsWe review the cutting-edge developments in the field of optical micromanipulation based on metasurfaces. The metasurface-based micromanipulation technology is expected to evolve toward higher temporal resolution, higher spatial accuracy, and lower manipulation power. To this end, more urgent requirements have been imposed on the underlying design scheme and experimental preparation standards of the metasurface. Although the introduction of metasurfaces has benefited micromanipulation systems and significantly reduced their sizes, there is still much room for further development and improvement in wide bands, multi-dimensional responses, and device thresholds.In terms of micromanipulation systems, the subwavelength-scale structure of metasurfaces will continue to be a key focus of research. Especially in the field of topological light manipulation, it is expected to further expand its research scope, combining non-Abelitan, non-Hermitian, and nonlinear effects to discover new physical phenomena. In the fields of biology and chemistry, metasurface technology is expected to be flexibly applied on smaller scales, even achieving manipulation of single molecule-level objects. This technology is expected to be further applied to the fields such as battery quality inspection and targeted therapy, bringing changes to the basic research and practical applications of energy and life sciences. Specifically, in the development of ultrafast optics, metasurfaces are gradually exhibiting unique advantages. Nanoscale superlattice enables high-resolution spectral measurements, and the design of nonlinear superlattice surfaces can be used to enhance nonlinear effects or generate high-order harmonics, making high time resolution transient micromanipulation technology possible.Overall, the technological evolution from traditional optical micromanipulation to meta-manipulation will continue to drive the vigorous development of nanophotonics. This technological paradigm not only meets the needs of various basic research but also arouses more innovative applications, opening up new prospects for branched sciences and technologies. SignificanceOptical micromanipulation utilizes optical force to dynamically control particles, which has the characteristics of non-contact and can be operated in a vacuum environment. Since the invention of optical tweezers in the 1980s, the field has experienced rapid development and has given rise to many emerging research directions, such as holographic optical tweezers, near-field evanescent wave optical tweezers, fiber optic tweezers, optoelectronic tweezers, and photo-induced temperature field optical tweezers, providing rich and powerful tools for fields such as biology, chemistry, nanoscience, and quantum technology. These methods can not only capture, separate, and transport small objects but also allow more precise manipulation, such as the rotation of small objects. However, traditional manipulation methods rely on tightly focused local light, greatly limiting the action range of optical force. In addition, in order to generate a structured light field, larger optical components such as spatial light modulators are usually required, making it difficult to miniaturize and integrate the optical manipulation system.In recent years, metasurfaces have emerged as integrated devices composed of subwavelength nanoantennas, promising new opportunities for optical micromanipulation. This ultra-thin artificial microstructure device can flexiblely control multiple degrees of freedom such as amplitude, phase, and polarization of light, by specially designing the geometric shape, size, and material of its own micro/nanostructure. Compared with traditional optical components such as liquid crystal spatial light modulators, gratings, and lenses, metasurfaces exhibit higher operating bandwidth, structural compactness, and integration. With the merits of miniaturization, integration, and excellent performance in light tailoring, optical metasurfaces have been extensively incorporated into the realm of optical micromanipulation. Especially, owing to their peculiar photomechanical properties, the metasurfaces hold the ability to be actuated by light fields, paving the way to the next generation of light-driven artificial micro-robots. The fast development of this subject indicates that the time is now ripe to overview recent progress in this cross-field.ProgressWe summarized principles of optical micromanipulation and metasurfaces (Fig. 1) and overviewed meta-manipulation devices, including metasurface-based optical tweezers (Fig. 2), tractor beams (Fig. 5), multifunctional micro-manipulation systems (Fig. 3), and metamachines (Figs. 7 and 8). Furthermore, we provided a detailed discussion of novel mechanical effects, such as topological light manipulation, which stems from the topological characteristics of nanostructures (Fig. 6).Conclusions and ProspectsWe review the cutting-edge developments in the field of optical micromanipulation based on metasurfaces. The metasurface-based micromanipulation technology is expected to evolve toward higher temporal resolution, higher spatial accuracy, and lower manipulation power. To this end, more urgent requirements have been imposed on the underlying design scheme and experimental preparation standards of the metasurface. Although the introduction of metasurfaces has benefited micromanipulation systems and significantly reduced their sizes, there is still much room for further development and improvement in wide bands, multi-dimensional responses, and device thresholds.In terms of micromanipulation systems, the subwavelength-scale structure of metasurfaces will continue to be a key focus of research. Especially in the field of topological light manipulation, it is expected to further expand its research scope, combining non-Abelitan, non-Hermitian, and nonlinear effects to discover new physical phenomena. In the fields of biology and chemistry, metasurface technology is expected to be flexibly applied on smaller scales, even achieving manipulation of single molecule-level objects. This technology is expected to be further applied to the fields such as battery quality inspection and targeted therapy, bringing changes to the basic research and practical applications of energy and life sciences. Specifically, in the development of ultrafast optics, metasurfaces are gradually exhibiting unique advantages. Nanoscale superlattice enables high-resolution spectral measurements, and the design of nonlinear superlattice surfaces can be used to enhance nonlinear effects or generate high-order harmonics, making high time resolution transient micromanipulation technology possible.Overall, the technological evolution from traditional optical micromanipulation to meta-manipulation will continue to drive the vigorous development of nanophotonics. This technological paradigm not only meets the needs of various basic research but also arouses more innovative applications, opening up new prospects for branched sciences and technologies.
Acta Optica Sinica
- Publication Date: Mar. 10, 2024
- Vol. 44, Issue 5, 0500001 (2024)
All-Optical Inverse Compton Scattering
Jianmeng Wei, Changquan Xia, Ke Feng, Hong Zhang, Hai Jiang, Yanjie Ge, Wentao Wang, Yuxin Leng, and Ruxin Li
SignificanceInverse Compton scattering (ICS) sources can generate high-energy radiation and have significant applications in various fields. In traditional ICS light sources, the electron beams are primarily sourced from storage rings. Storage rings provide high repetition rate electron beams, operate stably, and allow for multiple collisions with lasers, making it easier to achieve higher photon flux and enhance the average γ-ray flux. However, storage ring-based ICS devices cannot produce radiation with short duration, limiting their applications in ultrafast processes. In addition to storage ring electron accelerators, there are linear electron accelerators capable of providing high-brightness electron beams at high average currents. In recent years, with the continuous advancement of ultra-intense and ultra-short laser technology, ICS devices combining linear electron accelerators with ultra-intense and ultra-short lasers have begun to emerge. For example, the under-construction ELI-NP facility is based on this design and can generate X/γ-rays with shorter pulse widths, making it a highly promising source for ultra-short gamma radiation.However, both storage ring-based ICS devices and linear accelerator-based ones are costly. Furthermore, their bulky size limits their applications, particularly in desktop radiation sources. The progress in ultra-intense and ultra-short laser technology has propelled the development of laser plasma accelerators, especially laser Wakefield accelerators. Laser plasma accelerators offer a three-order-of-magnitude increase in acceleration gradient compared to traditional accelerators, significantly reducing the size of accelerators. Laser plasma accelerators open up a new technological pathway for high-energy radiation sources. Using electron beams generated by laser plasma accelerators for ICS enables all-optical inverse Compton scattering sources (AOCSs).The AOCS promotes the desktop applications of radiation sources and reduces their cost. Another prominent advantage of AOCSs compared to traditional accelerator-based ICS devices is their ability to generate higher brightness and ultra-short pulse γ-rays. The novel AOCSs, with their unique advantages such as high energy, high peak brightness, small source size, and quasi-monochromatic characteristics, have now become a crucial tool in many cutting-edge scientific fields. While significant progress has been made in AOCSs, there are still some challenges. We provide insights for future designs by summarizing past developments.ProgressThe ICS sources have made significant progress in generating high brightness, high-energy, quasi-monochromatic radiation, etc. The current all-optical ICS experimental schemes can be classified into two categories based on the source of scattering beams. One is the single beam combined with a plasma mirror approach, and the other is the dual-beam approach (Fig. 2). In the former, the scattering laser is derived from the driving laser reflected by a plasma mirror, while in the latter, the scattering laser comes from a separate laser source.AOCS is particularly suitable for generating high-brightness radiation. In 2012, the research team at the Laboratoire d'Optique Appliquée in France first employed the single-beam approach combined with a plasma mirror to achieve self-synchronized ICS, resulting in X-rays with energies of approximately 100 keV, a total photon count of 1×108, and a brightness of 1×1021 photon·s-1·mm-2·mrad-2 per 0.1% BW (bandwidth). In 2014, Sarri et al. reported experimental evidence of nonlinear relativistic Thomson scattering (TS) in dual-beam and head-on propagation conditions, resulting in peak brightness of γ-ray exceeding 1.8×1020 photon·s-1·mm-2·mrad-2 per 0.1% BW at 15 MeV. In 2016, the research team at the Shanghai Institute of Optics and Fine Mechanics, the Chinese Academy of Sciences, used a self-synchronized all-optical Compton scattering scheme to produce quasi-monochromatic and ultra-bright MeV γ-rays, with a brightness of 3×1022 photon·s-1·mm-2·mrad-2 per 0.1% BW. In 2022, a research team from Peking University obtained radiation with an estimated brightness of up to 1022 photon·s-1·mm-2·mrad-2 per 0.1% BW at 10 MeV.AOCS is also well-suited for producing high-energy radiation. In 2014, Liu et al. produced gamma photons with energies exceeding 9 MeV. In 2017, Yan et al. employed the dual-beam approach, utilizing ultra-intense lasers [a0(the magnitude of the normalized vector potential of the incident laser field)~12] and high-order (n>500) multiphoton ICS with electron beams to achieve γ-rays with a critical energy of approximately 27.9 MeV. In 2018, Cole et al. also used the dual-beam all-optical ICS approach with high-intensity lasers (a0~24.7) to collide with electron beams, resulting in γ-rays with critical energies exceeding 30 MeV. Due to the requirement for narrow-bandwidth X/γ rays in multiple application fields, researchers have focused on optimizing the monochromaticity of radiation. In 2014, Powers et al. reported tunable quasi-monochromatic X-rays with energies ranging from 70 to 1000 keV by varying the electron energies. In 2015, Khrennikov et al. achieved tunable quasi-monochromatic X-rays with energies ranging from 5 to 42 keV by controlling electron energies. Additionally, generating high photon yields in radiation is crucial. In 2019, Lemos et al. employed a scheme involving direct laser acceleration of electrons, followed by collision with a plasma mirror-reflecting high-energy electron beam, to obtain X-rays with energies ranging from 80 to 250 keV and photon counts of up to 1011.From the radiation parameters obtained in recent years of all-optical ICS experiments, it is evident that source sizes can reach the micrometer scale, and photon energies cover the range from tens of keV to tens of MeV. Photon yields range from 107 to 1011, and brightness can reach 1022 photon·s-1·mm-2·mrad-2 per 0.1% BW. Consequently, AOCS stand out in terms of brightness, spatial distribution, and photon flux, possessing unique advantages in various application domains. We summarize the design approach and outline relevant applications (Figs. 5 and 6) to serve as future application goals for the design of ICSs.Conclusions and ProspectsCompared to traditional ICS devices, AOCSs offer several key advantages: smaller size, lower cost, excellent spatial and temporal characteristics, and higher brightness. Therefore, AOCSs hold significant value for various applications. While AOCSs show great promise, they are currently in the experimental exploration and development phase and have not yet been widely deployed in large-scale projects. Enhancing the photon quality of AOCSs to meet application requirements remains a pressing challenge for research teams. Furthermore, some unique features of AOCSs are still waiting to be fully explored and exploited. If these issues can be addressed, AOCSs will bring new opportunities to the development of multiple fields. SignificanceInverse Compton scattering (ICS) sources can generate high-energy radiation and have significant applications in various fields. In traditional ICS light sources, the electron beams are primarily sourced from storage rings. Storage rings provide high repetition rate electron beams, operate stably, and allow for multiple collisions with lasers, making it easier to achieve higher photon flux and enhance the average γ-ray flux. However, storage ring-based ICS devices cannot produce radiation with short duration, limiting their applications in ultrafast processes. In addition to storage ring electron accelerators, there are linear electron accelerators capable of providing high-brightness electron beams at high average currents. In recent years, with the continuous advancement of ultra-intense and ultra-short laser technology, ICS devices combining linear electron accelerators with ultra-intense and ultra-short lasers have begun to emerge. For example, the under-construction ELI-NP facility is based on this design and can generate X/γ-rays with shorter pulse widths, making it a highly promising source for ultra-short gamma radiation.However, both storage ring-based ICS devices and linear accelerator-based ones are costly. Furthermore, their bulky size limits their applications, particularly in desktop radiation sources. The progress in ultra-intense and ultra-short laser technology has propelled the development of laser plasma accelerators, especially laser Wakefield accelerators. Laser plasma accelerators offer a three-order-of-magnitude increase in acceleration gradient compared to traditional accelerators, significantly reducing the size of accelerators. Laser plasma accelerators open up a new technological pathway for high-energy radiation sources. Using electron beams generated by laser plasma accelerators for ICS enables all-optical inverse Compton scattering sources (AOCSs).The AOCS promotes the desktop applications of radiation sources and reduces their cost. Another prominent advantage of AOCSs compared to traditional accelerator-based ICS devices is their ability to generate higher brightness and ultra-short pulse γ-rays. The novel AOCSs, with their unique advantages such as high energy, high peak brightness, small source size, and quasi-monochromatic characteristics, have now become a crucial tool in many cutting-edge scientific fields. While significant progress has been made in AOCSs, there are still some challenges. We provide insights for future designs by summarizing past developments.ProgressThe ICS sources have made significant progress in generating high brightness, high-energy, quasi-monochromatic radiation, etc. The current all-optical ICS experimental schemes can be classified into two categories based on the source of scattering beams. One is the single beam combined with a plasma mirror approach, and the other is the dual-beam approach (Fig. 2). In the former, the scattering laser is derived from the driving laser reflected by a plasma mirror, while in the latter, the scattering laser comes from a separate laser source.AOCS is particularly suitable for generating high-brightness radiation. In 2012, the research team at the Laboratoire d'Optique Appliquée in France first employed the single-beam approach combined with a plasma mirror to achieve self-synchronized ICS, resulting in X-rays with energies of approximately 100 keV, a total photon count of 1×108, and a brightness of 1×1021 photon·s-1·mm-2·mrad-2 per 0.1% BW (bandwidth). In 2014, Sarri et al. reported experimental evidence of nonlinear relativistic Thomson scattering (TS) in dual-beam and head-on propagation conditions, resulting in peak brightness of γ-ray exceeding 1.8×1020 photon·s-1·mm-2·mrad-2 per 0.1% BW at 15 MeV. In 2016, the research team at the Shanghai Institute of Optics and Fine Mechanics, the Chinese Academy of Sciences, used a self-synchronized all-optical Compton scattering scheme to produce quasi-monochromatic and ultra-bright MeV γ-rays, with a brightness of 3×1022 photon·s-1·mm-2·mrad-2 per 0.1% BW. In 2022, a research team from Peking University obtained radiation with an estimated brightness of up to 1022 photon·s-1·mm-2·mrad-2 per 0.1% BW at 10 MeV.AOCS is also well-suited for producing high-energy radiation. In 2014, Liu et al. produced gamma photons with energies exceeding 9 MeV. In 2017, Yan et al. employed the dual-beam approach, utilizing ultra-intense lasers [a0(the magnitude of the normalized vector potential of the incident laser field)~12] and high-order (n>500) multiphoton ICS with electron beams to achieve γ-rays with a critical energy of approximately 27.9 MeV. In 2018, Cole et al. also used the dual-beam all-optical ICS approach with high-intensity lasers (a0~24.7) to collide with electron beams, resulting in γ-rays with critical energies exceeding 30 MeV. Due to the requirement for narrow-bandwidth X/γ rays in multiple application fields, researchers have focused on optimizing the monochromaticity of radiation. In 2014, Powers et al. reported tunable quasi-monochromatic X-rays with energies ranging from 70 to 1000 keV by varying the electron energies. In 2015, Khrennikov et al. achieved tunable quasi-monochromatic X-rays with energies ranging from 5 to 42 keV by controlling electron energies. Additionally, generating high photon yields in radiation is crucial. In 2019, Lemos et al. employed a scheme involving direct laser acceleration of electrons, followed by collision with a plasma mirror-reflecting high-energy electron beam, to obtain X-rays with energies ranging from 80 to 250 keV and photon counts of up to 1011.From the radiation parameters obtained in recent years of all-optical ICS experiments, it is evident that source sizes can reach the micrometer scale, and photon energies cover the range from tens of keV to tens of MeV. Photon yields range from 107 to 1011, and brightness can reach 1022 photon·s-1·mm-2·mrad-2 per 0.1% BW. Consequently, AOCS stand out in terms of brightness, spatial distribution, and photon flux, possessing unique advantages in various application domains. We summarize the design approach and outline relevant applications (Figs. 5 and 6) to serve as future application goals for the design of ICSs.Conclusions and ProspectsCompared to traditional ICS devices, AOCSs offer several key advantages: smaller size, lower cost, excellent spatial and temporal characteristics, and higher brightness. Therefore, AOCSs hold significant value for various applications. While AOCSs show great promise, they are currently in the experimental exploration and development phase and have not yet been widely deployed in large-scale projects. Enhancing the photon quality of AOCSs to meet application requirements remains a pressing challenge for research teams. Furthermore, some unique features of AOCSs are still waiting to be fully explored and exploited. If these issues can be addressed, AOCSs will bring new opportunities to the development of multiple fields.
Acta Optica Sinica
- Publication Date: Feb. 25, 2024
- Vol. 44, Issue 4, 0400004 (2024)
Research and Development of SiC Ceramic Fabrication Technologies for Optics and Fine Structures
Ge Zhang, Congcong Cui, Wei Li, Binchao Dong, Qi Cao, Lixun Zhou, Conghui Guo, Wei Zhang, Chuanxiang Xu, Wanli Zhu, and Jianxun Bao
SignificanceDue to the outstanding thermal-mechanical properties and the high resistance to radiation, abrasion, and corrosion, SiC ceramics can be ranked as the optimal materials for the manufacture of the optics and the precision structures for space/ground-based advanced opto-mechanical systems. They fulfill the increasing demands of aperture enlargement, weight budget reduction, thermomechanical management simplification, and long-term stability. During the past three decades, ESA, NASA, JAXA, CASC, China Academy of Sciences, and so forth have been making great efforts to develop SiC components for remote sensors and telescopes for civilian and military applications at the cutting edge of the new generation optomechanical system development. The material preparation technologies and the relevant fabrication technologies, which determine the performance of the SiC components, the modules, and even the whole system, are the focus of the investigation and study.ProgressThe major concerns of the great efforts paid to the SiC preparation technologies are the accomplishment of optical surface density, the homogeneity, and the isotropy of the SiC blanks, which are essential for the optomechanical application, as well as the improvement of the thermomechanical properties such as specific modulus and thermal stability, and the manufacturability of the large-scale structural complexities.Among various SiC preparing technologies presented in Fig. 1, the densification methods of pressureless sintering, the reaction sintering/bonding and the chemical vapor composition/converting (CVC), combining the suited forming techniques for preforms, are proven to be effective for the SiC optics and precision structures. The pressureless sintered SiC possesses relatively better mechanical performance and homogeneity. It has presented isotropy, thermostability, and machinability during the development and in-orbit services of Herschel Space Observatory's (2009) Φ3.5 m primary mirror, GAIA (2014) and Euclid's (2023) all-SiC optomechanical structures. The maximum sizes of monolithic pressureless sintered SiC (S-SiC) optics reported reach 1.7 m×1.2 m (BOOSTEC) and Φ1.5 m (Shanghai Institute of Ceramics, China Academy of Sciences). However, further enlargement encounters the difficulties of the large-size high-temperature sintering equipment construction, the high sintering shrinkage, and the resulting ununiform deformation and stress that might cause cracking. CVD or PVD cladding on the S-SiC surface is necessary for optical polishing due to the residual micropores. Typical reaction sintered/bonded SiC (RB-SiC) comprises SiC, free Si, and residual C that is detrimental to the materials. The results show that reaction sintering/bonding densification methods are suitable for various ceramic forming techniques and the shrinkages of the whole process can be kept lower than 1%. The sintering temperatures are as low as the melting point of Si and the homogeneous bonding of parts is practicable, which are the essential processes to realize the reported largest Φ4.03 m SiC mirror (Changchun Institute of Optics, Fine Mechanics and Physics, China Academy of Sciences, 2016), except for lower Young's modulus than that of single-phase SiC ceramics. The results of the previous study also demonstrated the effectiveness of the microstructure refining and the phase composition regulation on the improvement of the RB-SiC performance and optical manufacturability. SiC via chemical vapor composition/converting (CVC SiC) is a single-phase ceramic with high purity. Trex Enterprise developed the co-deposition of micro SiC powder and precursor derived SiC onto the mold along with the densification process. POCO company adapted the pure porous graphite as preforms, and the vapor SiO and Si as infiltration matters, which would react with the graphite to convert into beta SiC meanwhile promoting densification. The introduction of the heterogeneous nucleation cores of the micro SiC powders or the graphite surface increases the rate of the crystal growth via the vapor phase by 10 times more than that of the CVD process and helps to overcome the heterogeneity of the materials due to the columnar crystal growth and to reduce the stress between the interface of the sequential solidified phases, which enables the fabrication of 1.5 m class CVC SiC mirror blank. The properties of Trex's CVC SiC are as excellent as pure full-dense SiC ceramic and facilitate the direction polishing without additional surface modification for optical surface finishing. However, the deposit efficiency and the capability of the complex component fabrication are yet the bottleneck of the promotion of the CVC techniques.As another determining factor for the performance of the SiC components, the improved structural configurations, such as topology-optimized structures (Fig. 12) and structures with the integrated cooling medium channels, exceed the capability of conventional technologies. Additive manufacturing (AM) or 3D printing techniques enable the free-form components manufacture. According to Goodman's investigation, based on the AM or 3D printing techniques, the weight reduction of the SiC optics comes up to 39% for 1-2.5 m class SiC mirrors for FIR application compared to JWST, and up to 40% of cost reduction. Investigation results show that additive manufacturing shaping combined with reaction sintering densifying is optimal for the preparation of the SiC materials for optics and precision structures. Binder jet printing, stereolithography/digital light processing, fused deposition modeling, and selective laser sintering are promising candidate methods for SiC or SiC-C preform forming. However, the problems of the lower performance compared to the materials via conventional methods, the heterogeneities of the materials, and the difficulties in non-uniform deformation control during the debonding and the reaction sintering are yet to be resolved.The joint of SiC parts favors the large-scale optomechanical system construction less costly and risky. As a typical case of all-SiC structure, the Euclid payload demonstrated the bolt joint, epoxy bonding, ceramic bonding, and brazing of the pressureless sintered SiC parts (Figs. 17-18). The rigidity of the brazing joined components or the structural frames is more promising than that of the first two, though the bolt joint and the epoxy bonding might be realized at room temperature in a normal atmosphere and applicable for SiC and other materials. However, brazing will inevitably introduce residual stress due to the thermal mismatching of the base materials and the fillers, and due to the volume changing of the fillers during solidification. The residual stress cannot be eliminated through the post process, hence increasing the uncertainty for the dimensional stability of the precision structures. Reaction bonding techniques facilitate the homogeneous joint through Si-C reaction, which can be carried out simultaneously with the reaction sintering process and avoid the residual stress. The microstructure of the joining area can be tailored to be identical to the parent RB-SiC parts.The advantages of the SiC materials are expected to extend to the manufacture and applications of space/ground-based large aperture photoelectric imaging systems, short wave optics for ultraviolet to soft X-ray, high power laser optics, and other precision structures such as key components in semiconductor equipment. The merits brought about include the system rigidity and the weight lessening, and the improvement of the system sensitivity and reliability, thanks to the high specific stiffness, excellent thermal stability, high resistance to abrasion, and corrosion of the SiC ceramics.Conclusions and ProspectsThe pressureless sintered SiC, reaction sintered SiC, and CVC SiC ceramics exhibit advantages in the optomechanical system manufacture due to their thermal mechanical comprehensive properties. To further promote the application of silicon carbide in precision engineering, it is necessary to develop new fabrication methods such as additive manufacture of SiC ceramics, and advanced SiC joint technologies for the innovative structural forms within an acceptable cost space. The improvements of the material microstructures and the properties from micro to macro scale via technical breakthrough are needed in advanced material forming, densification sintering, connection technologies, and applied technologies. SignificanceDue to the outstanding thermal-mechanical properties and the high resistance to radiation, abrasion, and corrosion, SiC ceramics can be ranked as the optimal materials for the manufacture of the optics and the precision structures for space/ground-based advanced opto-mechanical systems. They fulfill the increasing demands of aperture enlargement, weight budget reduction, thermomechanical management simplification, and long-term stability. During the past three decades, ESA, NASA, JAXA, CASC, China Academy of Sciences, and so forth have been making great efforts to develop SiC components for remote sensors and telescopes for civilian and military applications at the cutting edge of the new generation optomechanical system development. The material preparation technologies and the relevant fabrication technologies, which determine the performance of the SiC components, the modules, and even the whole system, are the focus of the investigation and study.ProgressThe major concerns of the great efforts paid to the SiC preparation technologies are the accomplishment of optical surface density, the homogeneity, and the isotropy of the SiC blanks, which are essential for the optomechanical application, as well as the improvement of the thermomechanical properties such as specific modulus and thermal stability, and the manufacturability of the large-scale structural complexities.Among various SiC preparing technologies presented in Fig. 1, the densification methods of pressureless sintering, the reaction sintering/bonding and the chemical vapor composition/converting (CVC), combining the suited forming techniques for preforms, are proven to be effective for the SiC optics and precision structures. The pressureless sintered SiC possesses relatively better mechanical performance and homogeneity. It has presented isotropy, thermostability, and machinability during the development and in-orbit services of Herschel Space Observatory's (2009) Φ3.5 m primary mirror, GAIA (2014) and Euclid's (2023) all-SiC optomechanical structures. The maximum sizes of monolithic pressureless sintered SiC (S-SiC) optics reported reach 1.7 m×1.2 m (BOOSTEC) and Φ1.5 m (Shanghai Institute of Ceramics, China Academy of Sciences). However, further enlargement encounters the difficulties of the large-size high-temperature sintering equipment construction, the high sintering shrinkage, and the resulting ununiform deformation and stress that might cause cracking. CVD or PVD cladding on the S-SiC surface is necessary for optical polishing due to the residual micropores. Typical reaction sintered/bonded SiC (RB-SiC) comprises SiC, free Si, and residual C that is detrimental to the materials. The results show that reaction sintering/bonding densification methods are suitable for various ceramic forming techniques and the shrinkages of the whole process can be kept lower than 1%. The sintering temperatures are as low as the melting point of Si and the homogeneous bonding of parts is practicable, which are the essential processes to realize the reported largest Φ4.03 m SiC mirror (Changchun Institute of Optics, Fine Mechanics and Physics, China Academy of Sciences, 2016), except for lower Young's modulus than that of single-phase SiC ceramics. The results of the previous study also demonstrated the effectiveness of the microstructure refining and the phase composition regulation on the improvement of the RB-SiC performance and optical manufacturability. SiC via chemical vapor composition/converting (CVC SiC) is a single-phase ceramic with high purity. Trex Enterprise developed the co-deposition of micro SiC powder and precursor derived SiC onto the mold along with the densification process. POCO company adapted the pure porous graphite as preforms, and the vapor SiO and Si as infiltration matters, which would react with the graphite to convert into beta SiC meanwhile promoting densification. The introduction of the heterogeneous nucleation cores of the micro SiC powders or the graphite surface increases the rate of the crystal growth via the vapor phase by 10 times more than that of the CVD process and helps to overcome the heterogeneity of the materials due to the columnar crystal growth and to reduce the stress between the interface of the sequential solidified phases, which enables the fabrication of 1.5 m class CVC SiC mirror blank. The properties of Trex's CVC SiC are as excellent as pure full-dense SiC ceramic and facilitate the direction polishing without additional surface modification for optical surface finishing. However, the deposit efficiency and the capability of the complex component fabrication are yet the bottleneck of the promotion of the CVC techniques.As another determining factor for the performance of the SiC components, the improved structural configurations, such as topology-optimized structures (Fig. 12) and structures with the integrated cooling medium channels, exceed the capability of conventional technologies. Additive manufacturing (AM) or 3D printing techniques enable the free-form components manufacture. According to Goodman's investigation, based on the AM or 3D printing techniques, the weight reduction of the SiC optics comes up to 39% for 1-2.5 m class SiC mirrors for FIR application compared to JWST, and up to 40% of cost reduction. Investigation results show that additive manufacturing shaping combined with reaction sintering densifying is optimal for the preparation of the SiC materials for optics and precision structures. Binder jet printing, stereolithography/digital light processing, fused deposition modeling, and selective laser sintering are promising candidate methods for SiC or SiC-C preform forming. However, the problems of the lower performance compared to the materials via conventional methods, the heterogeneities of the materials, and the difficulties in non-uniform deformation control during the debonding and the reaction sintering are yet to be resolved.The joint of SiC parts favors the large-scale optomechanical system construction less costly and risky. As a typical case of all-SiC structure, the Euclid payload demonstrated the bolt joint, epoxy bonding, ceramic bonding, and brazing of the pressureless sintered SiC parts (Figs. 17-18). The rigidity of the brazing joined components or the structural frames is more promising than that of the first two, though the bolt joint and the epoxy bonding might be realized at room temperature in a normal atmosphere and applicable for SiC and other materials. However, brazing will inevitably introduce residual stress due to the thermal mismatching of the base materials and the fillers, and due to the volume changing of the fillers during solidification. The residual stress cannot be eliminated through the post process, hence increasing the uncertainty for the dimensional stability of the precision structures. Reaction bonding techniques facilitate the homogeneous joint through Si-C reaction, which can be carried out simultaneously with the reaction sintering process and avoid the residual stress. The microstructure of the joining area can be tailored to be identical to the parent RB-SiC parts.The advantages of the SiC materials are expected to extend to the manufacture and applications of space/ground-based large aperture photoelectric imaging systems, short wave optics for ultraviolet to soft X-ray, high power laser optics, and other precision structures such as key components in semiconductor equipment. The merits brought about include the system rigidity and the weight lessening, and the improvement of the system sensitivity and reliability, thanks to the high specific stiffness, excellent thermal stability, high resistance to abrasion, and corrosion of the SiC ceramics.Conclusions and ProspectsThe pressureless sintered SiC, reaction sintered SiC, and CVC SiC ceramics exhibit advantages in the optomechanical system manufacture due to their thermal mechanical comprehensive properties. To further promote the application of silicon carbide in precision engineering, it is necessary to develop new fabrication methods such as additive manufacture of SiC ceramics, and advanced SiC joint technologies for the innovative structural forms within an acceptable cost space. The improvements of the material microstructures and the properties from micro to macro scale via technical breakthrough are needed in advanced material forming, densification sintering, connection technologies, and applied technologies.
Acta Optica Sinica
- Publication Date: Feb. 25, 2024
- Vol. 44, Issue 4, 0400003 (2024)
Review on Multimodal Nonlinear Optical Microscopy Imaging Technology
Yanping Li, Yongqiang Chen, Yuqing Liu, Rui Hu, Junle Qu, and Liwei Liu
SignificanceNonlinear optical microscopy (NLOM) is a technology that combines nonlinear optical effect with optical microscopy to generate contrast images by nonlinear light-matter interactions. Additionally, NLOM differs from conventional microscopy, which is typically based on linear interactions such as absorption, scattering, refraction, and fluorescence. In the past few decades, nonlinear optical imaging techniques have become important tools for detecting biomolecules, cells, and tissues at the micrometer and nanometer levels. The NLOM advancements promote and enhance the basic research on biology, pharmacy, and medicine. The nonlinear imaging techniques mainly include second harmonic generation (SHG), third harmonic generation (THG), two-photon excited fluorescence (TPEF), three-photon excited fluorescence (3PEF), coherent anti-Stokes Raman scattering (CARS) microscopy, and stimulated Raman scattering (SRS) microscopy. These techniques rely on tight focusing of ultrashort pulses with high photon density to excite nonlinear processes, which feature diffraction-limited spatial resolution and optical sectioning. Additionally, nonlinear optical microscopes employ near-infrared light sources that provide strong penetration power and cause minimal photodamage to tissues, allowing label-free imaging at the subcellular level. The nonlinear optical properties of different molecules in biological tissues enable molecular specificity and selectivity, making nonlinear optical imaging techniques widely applicable in biomedical imaging.With the advances in biology, the applications of nonlinear optical imaging technology are expanding, and the complex structures and functions of living organisms pose new challenges to optical imaging. Biomedical research requires super-composite optical imaging technology to achieve multidimensional optical characterization of biological tissues and obtain comprehensive information about their microstructure and molecular metabolism. Multiple nonlinear contrastive imaging technologies eliminate the need for tedious tissue preparation and enable analysis of unlabeled tissue samples, which provides rich structural and functional information about complex organisms. Finally, the multimodal nonlinear optical imaging technology which integrates multiple optical characterization methods has emerged as a new direction in optical microscopy in recent years.It is necessary to summarize and explore the existing research progress and future development trends to further promote the development of multimodal nonlinear optical imaging technology and contribute to relevant biomedical research. This will provide references for researchers in related fields.ProgressThe generation of nonlinear optical effects relies on focusing ultrashort pulse lasers to achieve extremely high peak intensity. When multiple photons simultaneously interact with excited fluorophores or specific structures, nonlinear optical signals are generated by light-matter interactions. A deep understanding about the generation process of various nonlinear effects is necessary to obtain optical images with high signal contrast and signal-to-noise ratio (SNR). Furthermore, selecting appropriate excitation conditions and detection methods is crucial for effective nonlinear optical imaging. We introduce the generation process of different nonlinear optical signals and their imaging mechanisms, mainly including multiphoton excitation fluorescence (MPEF), SHG/THG, coherent Raman scattering (CRS), and two-photon fluorescence lifetime microscope (TP-FLIM).Multimodal nonlinear optical imaging technology allows for accurate and comprehensive multi-parameter optical physical information. It serves as an important tool in studying complex organisms and multi-threaded dynamic processes from a multi-dimensional perspective. This technology has extensive applications in biological research fields such as physiology, neurobiology, embryology, and tissue engineering. However, different nonlinear optical imaging systems have distinct requirements for optics and hardware in excitation conditions and detection methods. Therefore, the key to integrating multiple nonlinear optical imaging technologies lies in coordinating the synchronous excitation of multiple nonlinear effects and the simultaneous detection of multi-dimensional signals. Meanwhile, we elaborate on the technical challenges and solutions related to multimodal coupling in nonlinear optical imaging and introduce the research progress and biological applications of multimodal imaging with multiple coupling mechanisms.Additionally, we review the optimization schemes for multimodal nonlinear optical imaging from three aspects of imaging speed, spatial resolution, and SNR to further improve the performance of multimodal optical imaging system. System miniaturization is discussed, and multimodal nonlinear optical endoscopy is extended to enable dynamic monitoring of the epidermis and internal organs of living organisms. Furthermore, nonlinear optical imaging microscopes can visualize the tissue structure and molecules in organism specificity. The imaging results require combined image processing methods for the quantitative detection of biological molecules and tissue structures. Therefore, we further introduce quantitative analysis methods for different nonlinear optical images.Conclusions and ProspectsMultimodal nonlinear optical microscopy, along with corresponding quantitative analysis methods, can conduct imaging and characterize the structure and physiological dynamic processes of biological tissues from multiple information dimensions. It represents an important branch of nonlinear optical microscopy development, with extensive applications in biomedical fields such as cell detection, cancer diagnosis, and brain imaging. Additionally, it holds significant potential, particularly in clinical pathological diagnosis. However, there are still several aspects of this technology to be further developed and improved. Firstly, in multimodal imaging, TP-FLIM imaging based on time-correlated single photon counting (TCSPC) requires a longer accumulation time for photons to obtain the lifetime decay curve. Simultaneously, spectral scanning in stimulated Raman scattering (SRS) imaging necessitates changing the position of time delay displacement tables. The two imaging methods still limit the imaging speed of the system and hinder the multi-parameter optical characterization for certain dynamic physiological processes. Therefore, there is still a room for further research on fast multimodal nonlinear optical imaging schemes.Meanwhile, in practical applications, the images obtained from multi-parameter nonlinear optical imaging systems should be combined with corresponding analysis methods to extract relevant biochemical information. This requires extensive data processing and statistical analysis, particularly in the context of clinical pathological analysis. Exploring new analytical methods that enable rapid conversion from optical images to biological information will significantly enhance the clinical applicability of multimodal nonlinear optical imaging. In summary, despite the potential and utility in biomedical research presented by multimodal nonlinear optical microscopy, further advancements are needed to address challenges such as imaging speed and data analysis. By developing faster imaging schemes and exploring new analytical methods, the clinical applications of multimodal nonlinear optical imaging can be greatly enhanced. SignificanceNonlinear optical microscopy (NLOM) is a technology that combines nonlinear optical effect with optical microscopy to generate contrast images by nonlinear light-matter interactions. Additionally, NLOM differs from conventional microscopy, which is typically based on linear interactions such as absorption, scattering, refraction, and fluorescence. In the past few decades, nonlinear optical imaging techniques have become important tools for detecting biomolecules, cells, and tissues at the micrometer and nanometer levels. The NLOM advancements promote and enhance the basic research on biology, pharmacy, and medicine. The nonlinear imaging techniques mainly include second harmonic generation (SHG), third harmonic generation (THG), two-photon excited fluorescence (TPEF), three-photon excited fluorescence (3PEF), coherent anti-Stokes Raman scattering (CARS) microscopy, and stimulated Raman scattering (SRS) microscopy. These techniques rely on tight focusing of ultrashort pulses with high photon density to excite nonlinear processes, which feature diffraction-limited spatial resolution and optical sectioning. Additionally, nonlinear optical microscopes employ near-infrared light sources that provide strong penetration power and cause minimal photodamage to tissues, allowing label-free imaging at the subcellular level. The nonlinear optical properties of different molecules in biological tissues enable molecular specificity and selectivity, making nonlinear optical imaging techniques widely applicable in biomedical imaging.With the advances in biology, the applications of nonlinear optical imaging technology are expanding, and the complex structures and functions of living organisms pose new challenges to optical imaging. Biomedical research requires super-composite optical imaging technology to achieve multidimensional optical characterization of biological tissues and obtain comprehensive information about their microstructure and molecular metabolism. Multiple nonlinear contrastive imaging technologies eliminate the need for tedious tissue preparation and enable analysis of unlabeled tissue samples, which provides rich structural and functional information about complex organisms. Finally, the multimodal nonlinear optical imaging technology which integrates multiple optical characterization methods has emerged as a new direction in optical microscopy in recent years.It is necessary to summarize and explore the existing research progress and future development trends to further promote the development of multimodal nonlinear optical imaging technology and contribute to relevant biomedical research. This will provide references for researchers in related fields.ProgressThe generation of nonlinear optical effects relies on focusing ultrashort pulse lasers to achieve extremely high peak intensity. When multiple photons simultaneously interact with excited fluorophores or specific structures, nonlinear optical signals are generated by light-matter interactions. A deep understanding about the generation process of various nonlinear effects is necessary to obtain optical images with high signal contrast and signal-to-noise ratio (SNR). Furthermore, selecting appropriate excitation conditions and detection methods is crucial for effective nonlinear optical imaging. We introduce the generation process of different nonlinear optical signals and their imaging mechanisms, mainly including multiphoton excitation fluorescence (MPEF), SHG/THG, coherent Raman scattering (CRS), and two-photon fluorescence lifetime microscope (TP-FLIM).Multimodal nonlinear optical imaging technology allows for accurate and comprehensive multi-parameter optical physical information. It serves as an important tool in studying complex organisms and multi-threaded dynamic processes from a multi-dimensional perspective. This technology has extensive applications in biological research fields such as physiology, neurobiology, embryology, and tissue engineering. However, different nonlinear optical imaging systems have distinct requirements for optics and hardware in excitation conditions and detection methods. Therefore, the key to integrating multiple nonlinear optical imaging technologies lies in coordinating the synchronous excitation of multiple nonlinear effects and the simultaneous detection of multi-dimensional signals. Meanwhile, we elaborate on the technical challenges and solutions related to multimodal coupling in nonlinear optical imaging and introduce the research progress and biological applications of multimodal imaging with multiple coupling mechanisms.Additionally, we review the optimization schemes for multimodal nonlinear optical imaging from three aspects of imaging speed, spatial resolution, and SNR to further improve the performance of multimodal optical imaging system. System miniaturization is discussed, and multimodal nonlinear optical endoscopy is extended to enable dynamic monitoring of the epidermis and internal organs of living organisms. Furthermore, nonlinear optical imaging microscopes can visualize the tissue structure and molecules in organism specificity. The imaging results require combined image processing methods for the quantitative detection of biological molecules and tissue structures. Therefore, we further introduce quantitative analysis methods for different nonlinear optical images.Conclusions and ProspectsMultimodal nonlinear optical microscopy, along with corresponding quantitative analysis methods, can conduct imaging and characterize the structure and physiological dynamic processes of biological tissues from multiple information dimensions. It represents an important branch of nonlinear optical microscopy development, with extensive applications in biomedical fields such as cell detection, cancer diagnosis, and brain imaging. Additionally, it holds significant potential, particularly in clinical pathological diagnosis. However, there are still several aspects of this technology to be further developed and improved. Firstly, in multimodal imaging, TP-FLIM imaging based on time-correlated single photon counting (TCSPC) requires a longer accumulation time for photons to obtain the lifetime decay curve. Simultaneously, spectral scanning in stimulated Raman scattering (SRS) imaging necessitates changing the position of time delay displacement tables. The two imaging methods still limit the imaging speed of the system and hinder the multi-parameter optical characterization for certain dynamic physiological processes. Therefore, there is still a room for further research on fast multimodal nonlinear optical imaging schemes.Meanwhile, in practical applications, the images obtained from multi-parameter nonlinear optical imaging systems should be combined with corresponding analysis methods to extract relevant biochemical information. This requires extensive data processing and statistical analysis, particularly in the context of clinical pathological analysis. Exploring new analytical methods that enable rapid conversion from optical images to biological information will significantly enhance the clinical applicability of multimodal nonlinear optical imaging. In summary, despite the potential and utility in biomedical research presented by multimodal nonlinear optical microscopy, further advancements are needed to address challenges such as imaging speed and data analysis. By developing faster imaging schemes and exploring new analytical methods, the clinical applications of multimodal nonlinear optical imaging can be greatly enhanced.
Acta Optica Sinica
- Publication Date: Feb. 25, 2024
- Vol. 44, Issue 4, 0400002 (2024)
Underwater Orbital Angular Momentum Optical Communications
Jian Wang, and Zhongyang Wang
SignificanceThe ocean occupies more than 70% of the earth's surface, which has vast area and rich resources. Research and exploration of the ocean have never ended. Due to the complexity and variability of the underwater environment, the ocean has not yet been fully explored and utilized. Further exploration of the underwater environment plays an important role in climate change, oil and gas detection, disaster early-warning, biological research, and other fields. Underwater wireless communication ensures information transmission and interconnection between unmanned devices in the underwater environment during ocean exploration. As the demand for underwater data transmission increases, high-bandwidth and low-latency underwater communication has become a key technology for exploring and utilizing the ocean at a deeper level.Commonly used carriers for underwater wireless communications include sound waves, electromagnetic waves (e.g. radio frequencies), and light waves. Each of the three carriers has its own characteristics. Although sound waves, as a traditional underwater communication method, have the advantage of a wide transmission range and have been widely used, the problems of relatively narrow bandwidth and longer delay in the medium limit their applications. Electromagnetic waves are difficult to be widely used in underwater environments as they require complicated equipment and short transmission distances. As a new type of underwater communication technology, underwater wireless optical communication has gained widespread attention due to its advantages such as larger transmission bandwidth, better anti-interference ability, lower latency, and lower costs. Underwater wireless optical communication refers to an underwater communication system that uses light waves as the transmission carrier. In recent years, underwater wireless optical communication has made considerable progress in the transmission capacity through the expansion and utilization of multiple physical dimensions of light waves, such as wavelength, time, amplitude, phase, and polarization. However, there are challenges in further improving the transmission capacity. The exploration of the spatial dimension of light waves has become a feasible way for capacity scaling.Structured light refers to a special light field that exploits the spatial dimension by tailoring the spatial amplitude, phase, and polarization distribution of light waves to obtain the required characteristics. Especially, structured light with a spiral phase front carrying orbital angular momentum (OAM) has attracted interest in many applications such as optical manipulation, tweezers, sensors, metrology, microscopy, imaging, and quantum science. OAM-carrying structured light appears spatially as an annular intensity distribution due to phase singularity at the beam center. Since OAM-carrying structured light can accommodate multiple orthogonal spatial modes, it has important advantages in expanding the capacity of underwater wireless optical communication. We comprehensively reviewed the advances in underwater OAM optical communications.ProgressWe first introduced the development history of three types of underwater wireless communication technology, including underwater acoustic communication, underwater electromagnetic (radio frequency) communication, and underwater optical communication, and summarized their respective advantages and disadvantages. Then, we focused on underwater wireless optical communication using OAM modes, with their basic principle, generation, and measurement methods introduced. The research progress of underwater OAM mode wireless optical communication was comprehensively reviewed, including underwater OAM mode encoding and decoding communication, underwater OAM mode multiplexing communication, and underwater OAM mode broadcasting communication. Moreover, OAM mode optical communications involving air-water interface ("water-air-water" crossing air-water medium, total reflection by "air-water" interface) and fast auto-alignment assisted OAM mode optical communications were presented. In addition to the OAM mode, other underwater structured light (e.g. Bessel beam and Ince-Gaussian beam) communications were also introduced. Additionally, complex medium optical communications using OAM modes assisted by adaptive turbulence compensation and fast auto-alignment were presented.Conclusions and ProspectsOAM mode exploits the spatial dimension of light waves, providing a new way for the sustainable capacity expansion of underwater wireless optical communication. The future development trend of underwater wireless optical communication is as follows. From the spatial mode point of view, more flexible and powerful spatial light manipulation, a large number of OAM modes, more general structured light accessing the full spatial dimension (spatial amplitude, spatial phase, and spatial polarization), and full use of multiple dimensions are highly desired. From the underwater communication point of view, complex channel modeling, high capacity, long distance, and high robustness are highly expected. Key devices [lasers, modulators, detectors, converters, and (de)multiplexers] and techniques (high speed, high power, high sensitivity, high efficiency, high scalability, and high integration) are of great importance. Meanwhile, from the perspective of future underwater wireless optical communication, on the one hand, it is expected to be combined with electromagnetic (e.g. RF) communication and acoustic communication. According to different application scenarios and different capacity and distance requirements, one or more suitable communication methods and their combinations can be selected. On the other hand, the integration of underwater wireless optical communication technology and underwater perception technology (i.e. integrated communication and perception) is also an important research direction in the future, which is of great significance for improving the development capacity of marine resources, developing the marine economy, protecting the marine ecological environment, and serving the strategy of becoming a powerful marine country. SignificanceThe ocean occupies more than 70% of the earth's surface, which has vast area and rich resources. Research and exploration of the ocean have never ended. Due to the complexity and variability of the underwater environment, the ocean has not yet been fully explored and utilized. Further exploration of the underwater environment plays an important role in climate change, oil and gas detection, disaster early-warning, biological research, and other fields. Underwater wireless communication ensures information transmission and interconnection between unmanned devices in the underwater environment during ocean exploration. As the demand for underwater data transmission increases, high-bandwidth and low-latency underwater communication has become a key technology for exploring and utilizing the ocean at a deeper level.Commonly used carriers for underwater wireless communications include sound waves, electromagnetic waves (e.g. radio frequencies), and light waves. Each of the three carriers has its own characteristics. Although sound waves, as a traditional underwater communication method, have the advantage of a wide transmission range and have been widely used, the problems of relatively narrow bandwidth and longer delay in the medium limit their applications. Electromagnetic waves are difficult to be widely used in underwater environments as they require complicated equipment and short transmission distances. As a new type of underwater communication technology, underwater wireless optical communication has gained widespread attention due to its advantages such as larger transmission bandwidth, better anti-interference ability, lower latency, and lower costs. Underwater wireless optical communication refers to an underwater communication system that uses light waves as the transmission carrier. In recent years, underwater wireless optical communication has made considerable progress in the transmission capacity through the expansion and utilization of multiple physical dimensions of light waves, such as wavelength, time, amplitude, phase, and polarization. However, there are challenges in further improving the transmission capacity. The exploration of the spatial dimension of light waves has become a feasible way for capacity scaling.Structured light refers to a special light field that exploits the spatial dimension by tailoring the spatial amplitude, phase, and polarization distribution of light waves to obtain the required characteristics. Especially, structured light with a spiral phase front carrying orbital angular momentum (OAM) has attracted interest in many applications such as optical manipulation, tweezers, sensors, metrology, microscopy, imaging, and quantum science. OAM-carrying structured light appears spatially as an annular intensity distribution due to phase singularity at the beam center. Since OAM-carrying structured light can accommodate multiple orthogonal spatial modes, it has important advantages in expanding the capacity of underwater wireless optical communication. We comprehensively reviewed the advances in underwater OAM optical communications.ProgressWe first introduced the development history of three types of underwater wireless communication technology, including underwater acoustic communication, underwater electromagnetic (radio frequency) communication, and underwater optical communication, and summarized their respective advantages and disadvantages. Then, we focused on underwater wireless optical communication using OAM modes, with their basic principle, generation, and measurement methods introduced. The research progress of underwater OAM mode wireless optical communication was comprehensively reviewed, including underwater OAM mode encoding and decoding communication, underwater OAM mode multiplexing communication, and underwater OAM mode broadcasting communication. Moreover, OAM mode optical communications involving air-water interface ("water-air-water" crossing air-water medium, total reflection by "air-water" interface) and fast auto-alignment assisted OAM mode optical communications were presented. In addition to the OAM mode, other underwater structured light (e.g. Bessel beam and Ince-Gaussian beam) communications were also introduced. Additionally, complex medium optical communications using OAM modes assisted by adaptive turbulence compensation and fast auto-alignment were presented.Conclusions and ProspectsOAM mode exploits the spatial dimension of light waves, providing a new way for the sustainable capacity expansion of underwater wireless optical communication. The future development trend of underwater wireless optical communication is as follows. From the spatial mode point of view, more flexible and powerful spatial light manipulation, a large number of OAM modes, more general structured light accessing the full spatial dimension (spatial amplitude, spatial phase, and spatial polarization), and full use of multiple dimensions are highly desired. From the underwater communication point of view, complex channel modeling, high capacity, long distance, and high robustness are highly expected. Key devices [lasers, modulators, detectors, converters, and (de)multiplexers] and techniques (high speed, high power, high sensitivity, high efficiency, high scalability, and high integration) are of great importance. Meanwhile, from the perspective of future underwater wireless optical communication, on the one hand, it is expected to be combined with electromagnetic (e.g. RF) communication and acoustic communication. According to different application scenarios and different capacity and distance requirements, one or more suitable communication methods and their combinations can be selected. On the other hand, the integration of underwater wireless optical communication technology and underwater perception technology (i.e. integrated communication and perception) is also an important research direction in the future, which is of great significance for improving the development capacity of marine resources, developing the marine economy, protecting the marine ecological environment, and serving the strategy of becoming a powerful marine country.
Acta Optica Sinica
- Publication Date: Feb. 25, 2024
- Vol. 44, Issue 4, 0400001 (2024)
Review of Real-Time Space-Based Optical Detection Technology for Global Total Lightning
Shulong Bao, Huan Li, Fan Sun, Feng Lu, Zhiqing Zhang, Xiaojie Chen, Shaofan Tang, Hua Liang, and Yanhua Zhao
SignificanceSevere convective disasters are the most frequent and widely affected meteorological disasters, causing huge economic losses and posing a serious threat to people and social security. They are also a major threat to new technological fields such as aerospace and information communication. Lightning, as a typical element of global severe convective weather, plays an important role in indicating and warning strong convection. Therefore, lightning detection and warning of severe convective disasters have become one of the important tasks of space remote sensing.Lightning detection systems mainly include ground-based and space-based detection systems. The ground-based lightning detection system mainly detects and locates broadband electromagnetic radiation signals emitted by lightning strikes, with detection spectral bands mainly including very low frequency (VLF), low frequency (LF), and very high frequency (VHF) bands. The ground-based lightning detection system has developed early and matured in technology, forming a relatively complete business system that plays an important role in lightning warning and forecasting. However, due to the discontinuous station layout of the ground-based lightning detection system and the barrier in mountainous areas, it is unable to effectively carry out uninterrupted lightning detection globally, especially in marine and mountainous areas. In order to overcome the limitations of ground-based lightning detection, space-based lightning detection technology has rapidly developed. The space-based lightning detection system has advantages such as large coverage range and is not limited by ground conditions. Among them, geostationary orbit lightning detection has unique advantages such as 24-hour uninterrupted and high real-time performance, and has become the main direction of international research on space-based lightning detection. It is a priority for the development of space-based lightning detection methods. The ground-based lightning detection system, low orbit and high orbit space-based lightning detection systems, and other lightning detection methods complement each other, achieving 24-hour uninterrupted, high-precision, and real-time detection of lightning, jointly serving strong convective disaster warning and prediction and climate research.ProgressIn the research of space-based lightning optical detection, the United States was the earliest to conduct research, with a leading position in depth and breadth. Through the development of low orbit space-based lightning detection cameras optical transient detecter (OTD) and lightning imaging sensor (LIS), the United States ultimately achieved the development of a geostationary orbit lightning detection camera GOES-16 GLM (geostationary lightning mapper), which was launched in November 2016. At the same time, Europe and China directly conducted research and development on geostationary orbit lightning detection cameras. China launched FY-4A LMI (lightning mapping imager) in December 2016, and Europe launched MTG LI (lightning imager) in December 2022. Currently, all three geostationary orbit lightning detection cameras are in orbit.Due to the fact that lightning usually occurs in strong convective cloud systems, the background formed by reflected sunlight on land, oceans, and clouds has complex, gradual changes, and high-intensity characteristics. Lightning often occurs in areas with clouds, and its intensity and location are random, with short duration and significant differences in intensity. These characteristics make space-based lightning detection cameras significantly different from traditional imaging cameras and point target warning cameras. It has extremely development difficulty (Fig. 9 and 10).FY-4A LMI is a geostationary orbit FY-4A LMI with independent intellectual property rights, developed by combining the spectral characteristics of background, lightning and its noise (Fig. 13), spatiotemporal characteristics (Fig. 11 and 12), and their variation patterns. It adopts multiple core technologies such as time filtering, spatial filtering, ultra narrowband spectral filtering (Fig. 15), and multi-dimensional fusion point target detection in spacetime and space (Fig. 16). It was launched in December 2016 and applied in orbit meteorological applications. Domestic meteorological departments, numerous research institutes, and universities have utilized the lightning detection results of FY-4A LMI to conduct research and applications on lightning generation and development mechanisms, typhoon monitoring and forecasting, severe convective disaster forecasting, and lightning data assimilation. Accurate prediction and early warning of lightning, severe convective disasters, and their secondary disasters have been achieved (Fig. 18), resulting in huge social and economic benefits and broad application prospects.Conclusions and ProspectChina has already achieved the detection and meteorological application of lightning below the troposphere in geostationary orbit, but there is still a significant gap in high-precision positioning, refined detection, intelligent detection, and real-time application of lightning below the troposphere. At the same time, China has not yet established an effective atmospheric lightning in the stratosphere, mesosphere and thermosphere (TLEs, transient luminous events) detection system, especially a space-based detection system that has not been planned. Therefore, in the field of lightning below the troposphere detection, we should gradually develop towards three-dimensional high-precision detection, intelligent detection, on-demand independent planning and application closed-loop. In the detection of lightning in the stratosphere, mesosphere and thermosphere (TLEs), research on detection methods and cameras should be carried out in the future to achieve real-time detection and early warning, serving the safety guarantee of China's entry and exiting into the atmosphere and space-based spacecraft. SignificanceSevere convective disasters are the most frequent and widely affected meteorological disasters, causing huge economic losses and posing a serious threat to people and social security. They are also a major threat to new technological fields such as aerospace and information communication. Lightning, as a typical element of global severe convective weather, plays an important role in indicating and warning strong convection. Therefore, lightning detection and warning of severe convective disasters have become one of the important tasks of space remote sensing.Lightning detection systems mainly include ground-based and space-based detection systems. The ground-based lightning detection system mainly detects and locates broadband electromagnetic radiation signals emitted by lightning strikes, with detection spectral bands mainly including very low frequency (VLF), low frequency (LF), and very high frequency (VHF) bands. The ground-based lightning detection system has developed early and matured in technology, forming a relatively complete business system that plays an important role in lightning warning and forecasting. However, due to the discontinuous station layout of the ground-based lightning detection system and the barrier in mountainous areas, it is unable to effectively carry out uninterrupted lightning detection globally, especially in marine and mountainous areas. In order to overcome the limitations of ground-based lightning detection, space-based lightning detection technology has rapidly developed. The space-based lightning detection system has advantages such as large coverage range and is not limited by ground conditions. Among them, geostationary orbit lightning detection has unique advantages such as 24-hour uninterrupted and high real-time performance, and has become the main direction of international research on space-based lightning detection. It is a priority for the development of space-based lightning detection methods. The ground-based lightning detection system, low orbit and high orbit space-based lightning detection systems, and other lightning detection methods complement each other, achieving 24-hour uninterrupted, high-precision, and real-time detection of lightning, jointly serving strong convective disaster warning and prediction and climate research.ProgressIn the research of space-based lightning optical detection, the United States was the earliest to conduct research, with a leading position in depth and breadth. Through the development of low orbit space-based lightning detection cameras optical transient detecter (OTD) and lightning imaging sensor (LIS), the United States ultimately achieved the development of a geostationary orbit lightning detection camera GOES-16 GLM (geostationary lightning mapper), which was launched in November 2016. At the same time, Europe and China directly conducted research and development on geostationary orbit lightning detection cameras. China launched FY-4A LMI (lightning mapping imager) in December 2016, and Europe launched MTG LI (lightning imager) in December 2022. Currently, all three geostationary orbit lightning detection cameras are in orbit.Due to the fact that lightning usually occurs in strong convective cloud systems, the background formed by reflected sunlight on land, oceans, and clouds has complex, gradual changes, and high-intensity characteristics. Lightning often occurs in areas with clouds, and its intensity and location are random, with short duration and significant differences in intensity. These characteristics make space-based lightning detection cameras significantly different from traditional imaging cameras and point target warning cameras. It has extremely development difficulty (Fig. 9 and 10).FY-4A LMI is a geostationary orbit FY-4A LMI with independent intellectual property rights, developed by combining the spectral characteristics of background, lightning and its noise (Fig. 13), spatiotemporal characteristics (Fig. 11 and 12), and their variation patterns. It adopts multiple core technologies such as time filtering, spatial filtering, ultra narrowband spectral filtering (Fig. 15), and multi-dimensional fusion point target detection in spacetime and space (Fig. 16). It was launched in December 2016 and applied in orbit meteorological applications. Domestic meteorological departments, numerous research institutes, and universities have utilized the lightning detection results of FY-4A LMI to conduct research and applications on lightning generation and development mechanisms, typhoon monitoring and forecasting, severe convective disaster forecasting, and lightning data assimilation. Accurate prediction and early warning of lightning, severe convective disasters, and their secondary disasters have been achieved (Fig. 18), resulting in huge social and economic benefits and broad application prospects.Conclusions and ProspectChina has already achieved the detection and meteorological application of lightning below the troposphere in geostationary orbit, but there is still a significant gap in high-precision positioning, refined detection, intelligent detection, and real-time application of lightning below the troposphere. At the same time, China has not yet established an effective atmospheric lightning in the stratosphere, mesosphere and thermosphere (TLEs, transient luminous events) detection system, especially a space-based detection system that has not been planned. Therefore, in the field of lightning below the troposphere detection, we should gradually develop towards three-dimensional high-precision detection, intelligent detection, on-demand independent planning and application closed-loop. In the detection of lightning in the stratosphere, mesosphere and thermosphere (TLEs), research on detection methods and cameras should be carried out in the future to achieve real-time detection and early warning, serving the safety guarantee of China's entry and exiting into the atmosphere and space-based spacecraft.
Acta Optica Sinica
- Publication Date: Jan. 25, 2024
- Vol. 44, Issue 2, 0200006 (2024)
Topics
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