Special Issue for Light Field Manipulation and Applications|26 Article(s)
Manipulation of Multimodal Vector Optical Fields in Three-dimensional Space(Invited)
Yuan GAO, Jianping DING, and Huitian WANG
Polarization, as an intrinsic property applying to optical waves, which can specify the geometrical orientation of the oscillation, has always been an important modulated parameter in optical fields. Compared with traditional scalar optical fields, Vector Optical Fields (VOFs) with non-uniform States of Polarization (SoPs) distributions denote that their geometrical orientations of the oscillation dependent on spatial locations are varying. The early research and manipulation on VOFs were limited to a single two-dimensional (2D) plane and mainly focused on the single modal modulation of SoPs. Later, researchers gradually brought to mind that the characteristics of VOFs, such as spatial geometries, polarization distributions, and the law of propagation, were also influenced by their amplitude and phase distributions. So the independent modulations of amplitude and phase based on the achieved polarization modulation caught people’s views and were accomplished after a short time, which means the generation of multimodal VOFs including these three fundamentally modulated degrees of freedom. More importantly, the deep applications related to multimodal VOFs in many realms, such as optical information transmission, manipulation of focal fields, optical micro-manipulation, have attracted researchers’ attention to the significant improvement of modulation efficiencies and the longitudinal extension of multi-dimensional modulation. Specifically, on the one hand, researchers selected the optical elements with high working efficiency and built reformative VOFs’ generators to reduce the unnecessary energy loss in the generation process. On the other hand, they studied the transmission of properties and modulation mechanism along the longitudinal direction for multimodal VOFs. Proposed active methods could modulate the distributions of different parameters, not only include three fundamental parameters amplitude, phase, SoPs but also other complex parameters such as energy flow, angular momentum, and optical singularities in three-dimensional (3D) space. In this review, we present an overview of the recent advances to spatially modulating multimodal 3D VOFs. Firstly, a brief introduction of three representations for a single SoP based on a polarization ellipse, Stokes parameters, and a classical Poincaré sphere respectively, are arranged. After that other three special representations of Cylindrically symmetric SoPs distributions with new types of Poincaré spheres are added. Secondly, we outline several different types of improved extra-cavity methods to generate VOFs, including highly efficient generators of arbitrary VOFs based on phase-only SLMs, compact polarization converters with high conversion efficiency, and sub-wavelength polarization modulators created by metasurfaces. Their advantages and limitations are comparatively demonstrated for readers. Thirdly, we highlight the principle of generating VOFs according to the superposition of two orthogonally polarized basic vectors and consider the applicable conditions of this principle in 3D space. And three relatively effective modulation methods of 3D multimodal VOFs are mentioned. The first utilizes on-axis modulations of non-diffractive Bessel beams to finish the polarization evolution along an optical axis. The second uses Fourier phase-shift principle to achieve independent modulations of polarization modes on multi-planes. The third develops a vector beam-shaping technique in focusing space. These methods are suitable to apply in different optical processes. Finally, the general application situation of VOFs in optical micromanipulation is illustrated to tell readers a great necessity and importance of modulating multimodal VOFs in 3D space.
Acta Photonica Sinica
  • Publication Date: Jan. 25, 2022
  • Vol. 51, Issue 1, 0151101 (2022)
Spatiotemporal Sculpturing of Light and Recent Development in Spatiotemporal Optical Vortices Wavepackets(Invited)
Qian CAO, and Qiwen ZHAN
This paper reviews Spatiotemporal Coupled (STc) optical fields including their theoretical background, the experimental configuration for generating STc optical fields, and the current research status of the newly-discovered Spataiotemporal Optical Vortices (STOV) wavepackets. Firstly, we review the origin and early study of STc optical fields, and introduce the theoretical model for describing STc optical fields. To give an example, we show the spatiotemporal evolution of a STOV wavepacket under normal dispersive, anomalous dispersive, and non-dispersive propagation. Under normal dispersive propagation, the STOV wavepacket maintains its ring-like field structure in the spatiotemporal domain. Under non-dispersive propagation, the STOV wavepacket evolves into a diagonal shape. Under anomalous dispersive propagation, the STOV wavepacket has the polarity reversal during the process. Secondly, we describe the typical experimental setup for generating STc optical fields. Although the experimental configuration for realizing STc optical fields shares almost exact the same experimental setup of a standard 4-f pulse shaper, the generating process is more complicated compared with conventional pulse shaping or beam shaping techniques as it involves a subtle interplay between the dispersion and diffraction effects for the generated STc optical fields. We categorize the operation of a STc optical field generator into three different regimes bythe distance between the exit plane of the generator and the observation plane for the generated STc optical fields: 1) the “far-field regime” where the observation plane is at several Rayleigh range after the generator; 2) the “near-field” regime where the observation plane is within one Rayleigh range; and 3) the “intermediate” regime where the observation plane is placed between “far-field” and “near-field”. For each operation regime, we give an example of the experimentally generated STOV wavepacket, namely, the first experimental realization of STOV wavepackets, STOV lattices, and Bessel STOV wavepackets. Thirdly, we give a detailed review about the state-of-art research status of the newly discovered STOV wavepackets including introducingwhy STOV wavepacket has become an interesting research topic for scientists, the demonstration of the conservation of transverse photonic Orbital Angular Momentum (OAM) proved by a nonlinear Second Harmonic Generation (SHG) experiment, and STOV wavepackets superposed with additional spatial photonic singularities. Compared with conventional vortex beam, the spatiotemporal spiral phase carried by a STOV wavepacket enables the photon within the wavepacket to have a pure transverse OAM, which makes STOV wavepacket an interesting tool in many research fields. Besides STOV wavepackets, other STc optical fields generated by this STc optical field generator setup also feature unique photonic properties such as achieving negative refraction, propagating free of diffraction, and propagation in a controllable group velocity. So far, the pulse shaper based STc optical field generation method has seen great success and received tremendous interests by the research community. Despite the great success already achieved, there are much more need to be studied, understood and developed in STc optical fields. With the unprecedented level of control of the spatiotemporal degree of freedoms of light, spatiotemporally sculptured optical fields will significantly enrich the photonics arsenal for scientistsin broad research fields ranging from quantum optics, nanophotonics, spin-photonics and spintronics, optical information transmission and processing, optical spectroscopy, laser driven particle acceleration, and much more beyond.
Acta Photonica Sinica
  • Publication Date: Jan. 25, 2022
  • Vol. 51, Issue 1, 0151102 (2022)
Research Advances of Optical Waveguides by Light-manipulation Based Femtosecond Laser Writing(Invited)
Bin ZHANG, Lei WANG, Yuechen JIA, and Feng CHEN
Integrated optical circuits play an essential role in the field of optical communication, by which the high-speed processing and transmitting of optical signals can be realized. Optical waveguide, in which the light will be confined into a micron or submicron volume for non-diffraction propagation, is one of the most importantly basic components in integrated optical circuits. The low-loss optical waveguides can be applied to fabricate high-performance photonic devices, e.g., beam splitters, frequency converters, and waveguide lasers. Hence, the fabrication of low-loss optical waveguides is of great significance to many applications in integrated optics and quantum photonics. The optical waveguides in transparent materials can be produced by ion exchange, ion implantation, and Ti-indiffusion. Nevertheless, these waveguides are limited to a 2D planar geometry. The 3D optical waveguides could be fabricated by femtosecond laser direct writing. Femtosecond laser direct writing is a maskless, efficient, and flexible 3D fabrication technique, which has become one of the most widely used techniques for precision machining of materials. The femtosecond laser possesses ultrashort pulse width and extremely high peak intensity, which could lead to the suppression of heat-affected zones and the appearance of nonlinear interactions (e.g., multiphoton absorption, tunneling ionization, and avalanche ionization), respectively. The microscopic objective is often utilized to focus NIR femtosecond laser into transparent materials, resulting in material modifications in focal regions. The material modifications can be classified into two types: Type-I modification and Type-II modification. The refractive index change is positive in the areas of Type-I modification, and the refractive index change is negative in the areas of Type-II modification. By using these two types of modifications, the single-line waveguide, dual-line waveguide, vertical-dual-line waveguide, multi-line waveguide, and depressed-cladding waveguide have been fabricated in transparent materials (e.g., glasses and crystals). In the past 20 years, a variety of photonic devices have been produced with femtosecond-laser-written optical waveguides, such as waveguide arrays, electro-optic modulators, and directional couplers. It can be anticipated that the novel, multi-functional, and high-efficient waveguide-based photonic devices will be created in succession with the in-depth study on laser-matter interactions. Although femtosecond laser direct writing has made a series of achievements in waveguide fabrication, there are still some challenges to rapidly produce low-loss optical waveguide with circular cross-section, due to spherical aberration at the interface caused by refractive index mismatch. In order to improve the waveguide quality and fabrication efficiency, the researchers are dedicated to develop the femtosecond laser writing technique based on light-manipulation. First, slit beam shaping. In this technique, a slit is inserted before the microscopic objective (slit orientation is parallel to laser-scanning direction), by which the aspect ratio of femtosecond-laser-induced track can be greatly reduced. It has been reported that the propagation loss of waveguide written by this processing technique can be reduced to less than 0.5 dB/cm, which is suitable to construct high-performance photonic devices. The slit beam shaping is an effective technique to improve the performance of femtosecond-laser-written waveguides. However, the existence of slit will inevitably result in a lot of loss of femtosecond laser energy, which is a disadvantage of slit beam shaping. Second, astigmatic beam shaping. As for this technique, an astigmatic cylindrical telescope is placed before the microscopic objective to reshape femtosecond laser, by which the waveguide with circular cross-section could be obtained as well. The minimum propagation loss of waveguide fabricated with this processing technique is less than 0.5 dB/cm, which is also applicable to constitute low-loss 3D waveguide configurations. It should be noted that, when fabricating 2D and 3D optical waveguides, the slit beam shaping and astigmatic beam shaping need to adjust slit orientation and cylindrical lens direction, respectively. It is this additional complexity that restricts the further applications of these two beam shaping techniques in integrated photonics. Third, deformable mirror beam shaping. In this technique, a 2D deformable mirror is utilized to reshape the spatial profile of femtosecond laser, by which the propagation loss of waveguide can also be reduced to some extent (~1.5 dB/cm). Fourth, simultaneous spatiotemporal focusing. This technique can strongly reduce nonlinear side effects, and have many potential applications for fabricating low-loss waveguides. However, the waveguide written by this processing technique has not been reported yet. Fifth, spatial light modulator beam shaping. It is a versatile and energy-efficient technique to control energy distribution of laser focus, which is promising to fabricate low-loss and high-quality optical waveguides. This paper, starting from the introduction of five beam shaping techniques, summarizes the latest research advances of waveguides fabricated by shaped femtosecond laser. An outlook is presented including several potential spotlights.
Acta Photonica Sinica
  • Publication Date: Jan. 25, 2022
  • Vol. 51, Issue 1, 0151106 (2022)
Research Progress of Generation of Partially Coherent Beams with Prescribed Correlation Structures(Invited)
Xinlei ZHU, Jiayi YU, and Yangjian CAI
Due to its brightness, directionality, and monochromaticity, coherent laser light has been widely used in military defense, medicine, industrial processing, optical communication systems and other fields, playing a vital role in human social progress and economic development. However, with the development of application of laser light, it is found that the one with high coherence will induce some negative effects. Fortunately, it is found that decreasing the coherence can not only keep the original features, but also reduce many negative effects caused by high coherence. Therefore, spatial coherence has gradually become fourth intrinsic property of light that can be optimized for particular tasks, in addition to amplitude, polarization and phase.Laser beams with decreased spatial coherence, called partially coherent beams, and they often have advantages over their coherent counterparts. In the past few decades, researchers only focused on the conventional partially coherent beams, i.e., Gaussian Schell-model beams, and such beams exhibit single and boring propagation property, which cannot meet the increasing demand for laser featrues. Therefore, how to manipulate the propagation features of laser fields to meet the actual demand is particularly important. From the perspective of manipulation of the coherence structures, a new class of light field with prescribed coherence structures can show novel propagation features. Until recently, only a few papers were devoted to partially coherent beams with non-conventional correlations (i.e. non-Gaussian correlated), such as J0-correlated Schell-model beams and vortex-carrying partially coherent beams. Investigations of such beams were limited due to the difficulty in proving that a given function is, in fact, a mathematically valid correlation function.But in 2007, a powerful new method for designing correlation functions of scalar partially coherent beams was introduced by Gori and Santarsiero, followed in 2009 by a more general method for vector partially coherent beams, allowing a wide variety of novel partially coherent beams to be investigated. Among the classes that have been studied since then are multi-Gaussian correlated Schell-model beams, Laguerre-Gaussian correlated Schell-model beams, Hermite-Gaussian Schell-model beams, and optical coherence lattices. Such beams display many extraordinary and potentially beneficial properties, such as flat-topped and ring-shaped intensity profiles in the far field, self-splitting properties, and lattice-like intensity patterns that form on propagation. And they have useful applications in many areas, such as free-space optical communication, particle trapping, image transmission and image encryption.In this review, we first outline the fundamental theories on constructing scaler and vector partially coherent beams with prescribed correlation structures, and then present detailed description of the beams models and their propagation properties of several typical examples. Finally, we review the methods of experimental generation of partially coherent beams with prescribed correlation structures. We hope our review will stimulate further efforts in this area of research.
Acta Photonica Sinica
  • Publication Date: Jan. 25, 2022
  • Vol. 51, Issue 1, 0151103 (2022)
Research Progress of Vortex Beam Laser(Invited)
Mingxin LV, Yipeng ZHANG, Jianlang HE, Xiaopeng HU, and Yong ZHANG
The vortex beam laser outputs a high-energy and high-quality vortex beam, which is one typical structured light field. Vortex beam has potential applications in many important fields, such as optical communications, optical manipulation, precision measurement, quantum information, and superresolution imaging. Therefore, how to efficiently generate high quality vortex beam has attracted considerable interests of research in recent years. In this paper, we first briefly introduce the generation principle and main applications of the vortex beam. There are two main ways of generating vortex beams, i.e., active method and passive method. Compared with active method, passive method generally suffers from low conversion efficiency and poor beam quality (especially for high order vortex beams). When a high quality vortex beam is needed, active method is a better choice. The active method generates vortex beams under laser configuration. For example, the high purity vortex beam can be generated by using the mode selection of the laser cavity. At present, the research focus on vortex beam laser is to improve the laser performance and the mode purity of the output vortex beam. In addition, the integration of vortex beam laser, which facilitates various commercial applications, is also a hot topic. Then, we review the recent progress of vortex beam lasers, including solid-state vortex laser, vortex-beam optical parametric oscillator, fiber vortex laser and on-chip integrated vortex lasers. Solid-state vortex laser is one of the most common methods to generate a vortex beam. By properly designing various types of resonators, one can generate the desired laser vortex mode while suppressing the unwanted ones. Taking Laguerre–Gaussian beams as an example, one can use the pump shaping technique to transform the pump beam from a Gaussian beam to a ring shaped intensity profile, which can effectively enhance the gain of the matched Laguerre–Gaussian cavity mode and decrease the gain of other modes. In addition, a tilted etalon can also be used in the resonant cavity to precisely control the gain and loss of different cavity modes. A recent method is to add spatial phase modulation elements (such as spiral phase plate, vortex half-wave plate, and so on) into the cavity. By satisfying the polarization and spatial mode self-reproduction condition of the cavity mode, the output beam can carry a specific spiral phase, i.e., one can obtain a desired vortex laser beam. Interestingly, the use of spatial light modulators and metasurface greatly enriches the types of output spatial light beams. By loading different holograms on the spatial light modulators or properly designing the structures of the metasurfaces, one can get various types of vortex modes, including those with large l and p indices that are difficult to be produced in the previous methods. Along with the foundation of solid state vortex laser, other forms of vortex beam lasers have also been rapidly developed in recent years. Vortex beam parametric oscillator can achieve the output of vortex beam with a tunable wavelength by controlling the phase matching conditions. Compared with the solid state vortex laser, the output wavelength band of vortex beam is greatly expanded. The fiber vortex laser uses the fiber configuration to output vortex beam. The low cost and high stability of the fiber laser can be effectively combined with the vortex beam output for practical applications in high capacity information transmission. This unique characteristic makes fiber vortex laser particularly useful in the field of optical fiber communication. The development of micro/nano fabrication techniques make it possible to integrate vortex lasers on a chip. Finally, we present the prospects of the future development of the vortex beam laser. High conversion efficiency and high mode purity are two critical requirements for high end applications of vortex beam lasers.
Acta Photonica Sinica
  • Publication Date: Jan. 25, 2022
  • Vol. 51, Issue 1, 0151105 (2022)
Progress on Longitudinal Modulation of Light Field(Invited)
Peng LI, Xinhao FAN, Yu LI, Sheng LIU, Bingyan WEI, and Jianlin ZHAO
Since the invention of the laser in 1960, the relevant basic scientific research, technological development, and engineering application supported by lasers have developed rapidly, making optics enter a new laser era. In last two decades, with the development of laser technology and the growth of its application demand, a series of spatially structured light fields that have elaborate in-plane distributions of amplitude, phase, and polarization have been proposed, such as vector and vortex beams, self-healing and self-accelerating beams, which have shown great potential in solving bottleneck problems such as the diffraction limit of light wave. These spatially structured light fields have been successfully used in realms of super-resolution imaging, particle manipulation, laser micromachining, classical and quantum communication and information restoration, etc. On the other hand, the relevant theories and technologies of light field modulation have further promoted the development of other physical fields. With the in-depth research and wide application of spatially structured light field, scientists gradually focus on the three-dimensional structure control of light field. In this special perspective, the first problem to be solved is the control of light field in the longitudinal dimension, which will provide a more adequate means for the further development of the application of light field in optical micro-manipulation, microscopic imaging, optical processing, angular momentum control, information storage, three-dimensional control of photonic state and so on.In this paper, we give an overview of recent progress on light field intensity and polarization modulation in the longitudinal dimension, and discuss the resulted spatially structured light fields from spatial properties to application potentials. Firstly, we briefly review the on-axis intensity modulation of light field based on the Bessel spectrum control and coherent superposition principles, which enable to create longitudinally modulated fields with discrete and continuous intensity profiles. Next, we describe the light fields with arbitrary propagation trajectories along the longitudinal direction based on two basic theories, i.e., caustic theory and Bessel spectrum mapping theory, which lead to the nontrivial propagation that has with large bending angle in non-paraxial condition. Then, we introduce a special kind of light field with arbitrary trajectory, i.e., the light fields that spirally propagate around the propagating axis, followed by the discussion of generation methods and self-accelerating characteristics of light fields with equidistant spiral, non-equidistant spiral, segmented spiral, and radially structured profiles. We also place particular emphasis on the recent development of polarization conversion during optical beam propagating in free space, and reveal this intriguing phenomenon in the view of spin-orbital angular momentum coupling. Wherein, in contrast to the polarization conversion with intensity profile variation, we introduce a special longitudinal polarization manipulation that exist in scalar and vector beams with non-diffraction intensity profiles. In addition, we trace the joint control of light field intensity and the polarization structure under the tightly focusing condition, which induces a strong longitudinal field component to engineer the longitudinal distribution of light field, and summarize the special three-dimensionally structured light fields constructed from this method.Longitudinally controlling light field not only enriches the spatially structured light field and its relevant theory, e.g., the topologically structured vortex and vector light fields, which have topologies as a new degree of freedom in photonics, it is noteworthy that it also spawns numerous new photonic devices. However, there are still many opportunities and challenges in this rapidly developing research realm. At last, we envision the possible challenges and prospects of longitudinal modulation of light fields, such as how to realize the subtly longitudinal modulation with wavelength-scale variation period, the combined control of multiple parameters along the longitudinal direction, and the construction of novel eigenmodes with three-dimensional spatial correlation, which will stimulate broader and more in-depth theoretical and application exploration.
Acta Photonica Sinica
  • Publication Date: Jan. 25, 2022
  • Vol. 51, Issue 1, 0151104 (2022)
Scale Transformation Properties of Small Light Fields with Flat Wave Front
Fuyuan GUO, Lianhuang LI, and Hua ZHENG
In the optical system, the conjugate transformation of the light field between object and its image is a common transformation. Based on the paraxial approximation and the Fresnel diffraction theory, the relation of light field between the object plane and its conjugate image plane of lens is presented in the book of Introduction of Fourier Optics. It is consistent with the scale transformation of the geometric optics prediction. The sine condition is a requirement for lens which carries out a perfect image for the object with vertical axis facet in an optical system. It was demonstrated by Fermat's principle in the field of geometrical optics, but it could not be explained by the classical diffraction integral formulae in the non-paraxial field.As the law of conservation of radiant energy in the traveling wave field was underappreciated in the classical diffraction theory, the classical diffraction integral formulae have limitations. As the Huygens' principle and the law of conservation of radiation energy in traveling wave field were engaged for analyzing the diffraction process of the non-paraxial light field in the cylindrical coordinate system, it was presented that the rationality of the normalized inclination factor was expressed by the square root of the cosine of the inclination angle in the side of diffraction source for analyzing the far field characteristics what diffracted from small light fields with flat wave front, and it was presented that the rationality of the normalized inclination factor was expressed by the square root of the cosine of the inclination angle in the observation side for analyzing the characteristics of the light field in the focal plane what diffracted from convergent hemispherical wave front also.Based on the normalized inclination factor expressed by the square root of the cosine of inclination angle, the non-paraxial integral formulas for analyzing the far-field characteristics of the diffraction source with the symmetric small flat wave front and the focal plane field characteristics of the diffraction source with the symmetric convergent wave front are suggested in the rectangular coordinate system respectively. As the light field of the diffraction source is a symmetric traveling wave field, and the Parseval's theorem and the calculation formula of the total power of the light field on the reference surface are employed, the total power of the light field on the observation reference surface which was expressed by the new normalized inclination factor equal to the total power of the light field on the diffraction source reference. It satisfies the law of conservation of energy. So the new normalized inclination factor is reasonable.As the Huygens' principle is engaged for analyzing the diffraction process of the symmetric traveling wave field, both the diffraction source reference surface and the observation reference surface coincide with the light wave fronts. The diffraction source reference surface is a small flat wave front, and the observation reference surface is a hemispherical wave front for analyzing the far field characteristics what diffracted from small light fields with a flat wave front. The diffraction source reference surface is a hemispherical wave front, and the observation reference surface is a small flat wave front for analyzing the characteristics of the light field in the focal plane what diffracted from the convergent hemispherical wave front. These two diffraction processes are the process of exchanging the diffraction source reference surface and the observation reference surface. If the diffraction source reference surface and the observation reference surface were reciprocated, the expression function of the traveling wave field of symmetric diffraction source and the expression function of the traveling wave field of observation surface in two diffraction integral formulae are reciprocated, and the functions of two inclination factors in two diffraction integral formulae are reciprocated also. Then, the reciprocity between the non-paraxial diffraction integral formula used to calculate the far field distribution of light wave what diffracted from symmetric small traveling wave field with a flat wave and the non-paraxial diffraction integral formula used to calculate the light field distribution of focal plane what diffracted from symmetric traveling wave field hemispherical wave front is clarified.As the normalized inclination factor of diffraction source side expressed by the square root of the cosine of inclination angle of diffraction source side and the normalized inclination factor of observation side expressed by the square root of the cosine of inclination angle of observation side are engaged, and the apodization function of the lens on the normal of light wave front is taken into consideration. The scale transformation characteristic of traveling wave fields with flat wavefront between the object-image conjugate planes of the lens which satisfied the sine condition is verified, and the applicable conditions of the relationship of scale transformation are expounded.
Acta Photonica Sinica
  • Publication Date: Jan. 25, 2022
  • Vol. 51, Issue 1, 0151114 (2022)
Coherent Raman Scattering Spectroscopy and Microscopy Based on Optical Field Engineering(Invited)
Runfeng LI, Dashan DONG, and Kebin SHI
The interaction of light and matter has always been a hot issue in research. As a non-intrusive detection method, light can efficiently and non-destructively obtain rich information inside sample. This information either reveals the chemical specificity of the sample and provides a basis for quantitative material composition analysis; or reflects the fine spatial structure of the sample, allowing people to use light as a medium to extract the morphological characteristics of microorganisms and microstructures; or open the time window to observe the sample, using ultra-short light pulses as information carriers to reveal transient dynamics.Spontaneous Raman scattering spectroscopy and imaging technology are important research directions in this field. Since its discovery in 1928, it has gradually become an important research tool in optics. On one hand, Raman scattered photons carry molecular vibration information, which makes up for the insufficient detection ability of infrared spectroscopy at the water absorption window, and provides an important tool for research in the biological and medical fields; on the other hand, as an important label-free detection method, Raman spectroscopy can achieve non-destructive and long-term observation while maintaining sample activity.Since the spontaneous Raman signal requires a long integration time, the imaging speed is greatly restricted when it involves some transient dynamic processes and dynamic observation of living organisms. In order to further improve the intensity of the Raman signal, the Coherent Raman Scattering technology realized by nonlinear optical processes has been developed vigorously. The main methods include Coherent Anti-stokes Raman Scattering and Stimulated Raman Scattering. Compared with spontaneous Raman, coherent Raman greatly improves the signal intensity, shortens the integration time of signal acquisition, and provides new possibilities for high-sensitivity spectroscopy technology and rapid in vivo biological imaging. Since the application of Coherent Raman Scattering, new requirements that have appeared in various chemical, biological, and medical applications are also constantly putting forward new challenges: how to achieve a higher signal-to-noise ratio, greater penetration depth, and faster detection speed, richer spectral information, and stronger resolving power have greatly promoted the rapid development of coherent Raman technology in the past two decades. By combining various optical field engineering methods, such as polarization, chirp, timing, phase and other dimensions of the beam in the non-linear process of coherent Raman, to meet the above challenges, the spectrum and imaging technology can be used in multiple dimensions and have more practical value.This article takes optical field engineering method as the main line, combing through the development and application of CRS spectroscopy and imaging, and mainly includes the principle of the nonlinear process of the coherent Raman method. The main control methods in coherent Raman scattering spectroscopy: incident angle, timing. Finally, there are more abundant control methods in coherent Raman scattering imaging technology: time & chirp, polarization, phase, and spatial frequency engineering.
Acta Photonica Sinica
  • Publication Date: Jan. 25, 2022
  • Vol. 51, Issue 1, 0151108 (2022)
Polarization State Variation Induced by the Circularly Polarized Light Scattering in LiNbO3:Fe Crystal(Invited)
Chunlei SHANG, Shi WANG, Chengzhen LU, Chunhao LIANG, Yuanmei GAO, Yangjian CAI, and Zengrun WEN
Doped LiNbO3 crystal is one kind of photorefractive mediums which are used for optical storage. When light passes through LiNbO3 crystal, it will cause local refractive index change and form noise grating. The noise grating diffracts the incident light, transferring part of the light energy to the scattered light, namely, light-induced scattering. In recent years, the modulation instability, generation process and energy transfer efficiency of light scattering in LiNbO3 crystals have been widely studied. Since a single component of linearly polarized light is used for the incident, no variation of the polarization state of the outgoing light is observed. However, considering the difference of energy transmission efficiency between o-light and e-light, it is reasonable to guess that the polarization state of the outgoing light may change when another light is incident. According to our investigation, there is no research in this field. In this work, the variation of the polarization state of LiNbO3:Fe crystal during the light-induced scattering process is reported. The circularly polarized light is focused into a thin light sheet and irradiated on the crystal with the c-axis along the vertical direction. The light scattering phenomenon at different incident angles is studied. In the tens of minutes from the generation of the scattered light to the steady state, the scattered light mainly grows in the direction parallel to the c-axis, and there is also a component in the direction parallel to the light sheet. The growth rate of scattered light decreases with the increase of the incident angle. Further observation manifests that the circularly polarized light becomes ordinary elliptically polarized light after passing through the crystal, and the ellipticity varies with the incident angle. Combined with the noise grating formed in the crystal, the scattering model of circularly polarized light is established, and the reason and mechanism of polarization state change are analyzed through calculation. Qualitative analysis shows that the ellipticity of scattered light is related to the transmission coefficient τ and azimuth angle, and the left-handed or right-handed polarization state is also related to the phase difference δ between o-light and e-light. This discovery proposes a method to generate elliptically polarized light using the light-induced light-scattering properties of photorefractive crystals, which can be applied to optical storage and photonic lattice based on photorefractive materials.
Acta Photonica Sinica
  • Publication Date: Jan. 25, 2022
  • Vol. 51, Issue 1, 0151113 (2022)
Sub-cycle Laser Field Shaping(Invited)
Yudong YANG, and Zhiyi WEI
Ultrashort laser pulses are powerful and important tools for scientific researches in many areas in that they allow studying ultrafast dynamics in materials with extreme time resolution. Different experiments across different research fields ask for laser pulses with very different characteristics. Ultrafast laser pulse shaping, where the amplitude, phase or polarization of laser pulses are modulated to fulfill various requirements of different experiments, is widely used. On the other hand, the pure quest of the technology development and the desires for studying even faster dynamics in materials jointly motivate the development of ultrafast laser technology. The record of the shortest pulse duration was continuously renewed. Eventually, ultrafast lasers step into the few cycle regime thanks to the introduction of Ti:Sapphire lasers. When the pulse duration approaches the oscillation period of the laser carrier wave, the differences between few cycle pulses and longer pulses emerge. One of the most notable differences is that even for two few cycle pulses with identical envelopes, the electric fields underneath can be utterly different. Hence, full control over few cycle pulses requires direct control over the electric field, which implies the technological leap from laser pulse shaping to sub-cycle laser field shaping. Sub-cycle laser field shaping technology not only enables full control over laser pulses, but also makes possible direct manipulation of strong-field physics process via tailored optical waveforms, which fundamentally enhances the toolbox for controlling light and matter interaction.Preliminary laser field shaping can be achieved via the Carrier Envelope Phase (CEP) of laser pulses, which is sufficient to significantly affect the electric field and alter the outcomes of light and matter interactions. Therefore, CEP stabilization is crucial for laser field shaping. Currently, CEP locking methods can be categorized into active stabilization and passive stabilization. Active CEP stabilization requires feedback loops which lock the CEP mostly by tuning the inter-cavity group velocity dispersion. In contrast, the passive CEP stabilization exploits the phase relation between different beams in nonlinear optics process, where the idler beam of OPA/DFG is naturally CEP stabilized if the signal beam and the pump beam shares identical CEP fluctuation. Additionally, controlling the spectral phase precisely further enhances the shaping capability that the electric field can be shaped to deviate notably from sinusoidal oscillation. Complete characterization of such few-cycle/single-cycle pulses is indispensable for utilizing them in experiments. Typical ultrashort pulse characterization methods measure the pulse envelopes but the exact shape of the electric fields. New methods which measure the electric field have to be developed. The field-sensitive methods are usually based on high harmonic generation, either by exploiting the process itself or by employing the XUV radiation from HHG.Laser field shaping targets extending the capability of direct electric field control in radio frequency to optical frequency. Customizing optical waveforms builds on the generation of extremely broadband spectrum and precise control of the spectral phase. Since laser pulses with broad bandwidth correspond to pulses which are temporally compressible to very short duration, sub-cycle laser field shaping and sub-cycle laser pulse generation share common technological ground. However, generating spectrum experimentally with bandwidth supporting sub-cycle laser pulses with a single light source is, if not impossible, extremely difficult. On the other hand, coherent combination, or synthesis, of several few-cycle pulses of different colors is the enabling technology for extremely broadband spectrum and intense sub-cycle laser pulses. Different approaches have been proposed along the development of optical waveform synthesis. The optical waveform synthesizer based on noble gas filled hollow-core fibers is one of the most successful attempts, which leads to fruitful results. However, the HCF approach has its own limits which are, e.g. the pulse energy and the bandwidth. To overcome such limits, OP(CP)As are introduced for the waveform synthesis. After conceptual demonstration with small OPAs, the signal beam, the idler beam and even the pump beam of more powerful OP(CP)As are employed for coherent synthesis, which takes advantage of the fact that the beams are inherently synchronized. The full potential of a parametric waveform synthesizer is however yet to explore. Hence, a waveform synthesizer consists of several different OP(CP)As was built, which outputs millijoule level sub-cycle pulses and the waveform can be varied by tuning the synthesis parameters. With the intense sub-cycle pulses, isolated attosecond pulses are directly generated without the assistance of additional gating methods. Moreover, tunable isolated attosecond pulses are conveniently delivered via varying the synthesis parameters. In the meantime, simulations are performed to illustrate the shaping of the generated attosecond pulses by tailored waveforms.
Acta Photonica Sinica
  • Publication Date: Jan. 25, 2022
  • Vol. 51, Issue 1, 0151109 (2022)