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Special Issue for the 60th Anniversary of XIOPM of CAS, and the 50th Anniversary of the Acta Photonica Sinica|17 Article(s)
Main-chain Chiral Liquid Crystal Elastomers（Invited）
Jiazhe MA, Yanzhao YANG, Ling WANG, and Wei FENG
Main chain chiral liquid crystal elastomers, with periodic nanostructures, are a kind of soft photonic crystals, including cholesteric liquid crystal elastomers and blue phase liquid crystal elastomers. They can not only reflect circularly polarized light selectively, but also dynamically adjust their structural color responding to changes in the environment, which could find wide applications in adaptive optics, bionic camouflage, information encryption, intelligent soft robots and so on. With the development of new material systems and advanced preparation techniques, researchers have explored various ways to design and synthesize chiral liquid crystal elastomers for emerging applications. Main-chain cholesteric liquid crystal elastomers are mainly prepared by parallel orientation, anisotropic deswelling, bar coating, and 3D printing. The parallel orientation method is a traditional method, which usually occurs within liquid crystal cells with planar-aligned alignment. Due to the interaction of liquid crystalline molecules with alignment coatings, helical nanostructures’ self-assembly can occur. The parallel orientation method has been used to design and develop photonic liquid crystal elastomers, 4D photonic actuators, inflating chiral nematic liquid crystalline elastomers with broadband and pixelated camouflage, chameleon skin-like cholesteric liquid crystal elastomers and other advanced functional materials. Based on the two-stage thiol-acrylate Michael addition and photopolymerization reaction, a facial and easily scalable method has been proposed to prepare uniformly colored large-area cholesteric liquid crystal elastomer films, which benefits from anisotropic deswelling. The gel formation from the Michael addition adheres strongly to the substrate, restricting the deswelling to take place only in the vertical direction and causing an in-plane orientation of the liquid crystal director. The resulted film with helical nanostructures shows a robust, rapid, and reversible mechanochromic response to strain, resulting in a broad color shift across the full visible spectrum. During the bar coating method, the precursor was applied to a bare substrate at a certain temperature via an applicator. Because of the shear forces during the bar coating process, the mesogenic molecules are able to align along with a common director, resulting in the formation of cholesteric liquid crystal elastomers with brilliant reflection color. The thickness of the film was controlled by changing the distance between the applicator and the substrate. This bar coating method has important significance for the application of advanced photonic crystal coatings. 3D printing is an emerging and advanced fabrication technique, and current 3D printing technologies for liquid crystal elastomers include direct ink writing and digital light processing. During the direct ink writing process of cholesteric liquid crystal elastomers, a viscous ink composed of uncrosslinked liquid crystal oligomers is often extruded from the printing nozzle. The shear force generated during the extrusion process causes the mesogens to spontaneously align along with the printing path, forming periodic helical nanostructures that selectively reflect circularly polarized light. Unlike cholesteric liquid crystal elastomers, blue phase liquid crystal elastomers are generally prepared through injecting chiral liquid crystal materials into liquid crystal cells, heating to isotropic phase, and then slowly cooling down. The liquid crystal precursors can self-assemble into a three-dimensional cubic lattice without the assistant of the surface alignment. The resulting blue phase liquid crystal elastomers have narrow photonic band gaps, which could enable functional implementations in nonlinear optics, lasing, spectral imaging and energy applications. In this review, different preparation methods of main-chain chiral liquid crystal elastomers are synthetically introduced. The state-of-the-art advancement and future perspective of main-chain chiral liquid crystal elastomers are discussed. It is expected that this review could shine new light on the development of advanced chiral liquid crystal elastomers and their emerging applications in diverse fields such as adaptive photonics, soft robotics, electronic skins and beyond. Main chain chiral liquid crystal elastomers, with periodic nanostructures, are a kind of soft photonic crystals, including cholesteric liquid crystal elastomers and blue phase liquid crystal elastomers. They can not only reflect circularly polarized light selectively, but also dynamically adjust their structural color responding to changes in the environment, which could find wide applications in adaptive optics, bionic camouflage, information encryption, intelligent soft robots and so on. With the development of new material systems and advanced preparation techniques, researchers have explored various ways to design and synthesize chiral liquid crystal elastomers for emerging applications. Main-chain cholesteric liquid crystal elastomers are mainly prepared by parallel orientation, anisotropic deswelling, bar coating, and 3D printing. The parallel orientation method is a traditional method, which usually occurs within liquid crystal cells with planar-aligned alignment. Due to the interaction of liquid crystalline molecules with alignment coatings, helical nanostructures’ self-assembly can occur. The parallel orientation method has been used to design and develop photonic liquid crystal elastomers, 4D photonic actuators, inflating chiral nematic liquid crystalline elastomers with broadband and pixelated camouflage, chameleon skin-like cholesteric liquid crystal elastomers and other advanced functional materials. Based on the two-stage thiol-acrylate Michael addition and photopolymerization reaction, a facial and easily scalable method has been proposed to prepare uniformly colored large-area cholesteric liquid crystal elastomer films, which benefits from anisotropic deswelling. The gel formation from the Michael addition adheres strongly to the substrate, restricting the deswelling to take place only in the vertical direction and causing an in-plane orientation of the liquid crystal director. The resulted film with helical nanostructures shows a robust, rapid, and reversible mechanochromic response to strain, resulting in a broad color shift across the full visible spectrum. During the bar coating method, the precursor was applied to a bare substrate at a certain temperature via an applicator. Because of the shear forces during the bar coating process, the mesogenic molecules are able to align along with a common director, resulting in the formation of cholesteric liquid crystal elastomers with brilliant reflection color. The thickness of the film was controlled by changing the distance between the applicator and the substrate. This bar coating method has important significance for the application of advanced photonic crystal coatings. 3D printing is an emerging and advanced fabrication technique, and current 3D printing technologies for liquid crystal elastomers include direct ink writing and digital light processing. During the direct ink writing process of cholesteric liquid crystal elastomers, a viscous ink composed of uncrosslinked liquid crystal oligomers is often extruded from the printing nozzle. The shear force generated during the extrusion process causes the mesogens to spontaneously align along with the printing path, forming periodic helical nanostructures that selectively reflect circularly polarized light. Unlike cholesteric liquid crystal elastomers, blue phase liquid crystal elastomers are generally prepared through injecting chiral liquid crystal materials into liquid crystal cells, heating to isotropic phase, and then slowly cooling down. The liquid crystal precursors can self-assemble into a three-dimensional cubic lattice without the assistant of the surface alignment. The resulting blue phase liquid crystal elastomers have narrow photonic band gaps, which could enable functional implementations in nonlinear optics, lasing, spectral imaging and energy applications. In this review, different preparation methods of main-chain chiral liquid crystal elastomers are synthetically introduced. The state-of-the-art advancement and future perspective of main-chain chiral liquid crystal elastomers are discussed. It is expected that this review could shine new light on the development of advanced chiral liquid crystal elastomers and their emerging applications in diverse fields such as adaptive photonics, soft robotics, electronic skins and beyond.
Acta Photonica Sinica
- Publication Date: Jul. 25, 2022
- Vol. 51, Issue 7, 0751417 (2022)
Progress and Challenge of 3D Photonic Integrated Circuit（Invited）
Yuexin YIN, Xinru XU, Yingzhi DING, Mengke YAO, Guoyan ZENG, and Daming ZHANG
The ever-increasing demand for high bandwidth is continued to grow in the forthcoming era of the Internet of Things (IoT) and 5G. Photonic Integrated Circuit (PIC), especially Silicon Photonics (SiPh), compatible with Complementary Metal-Oxide-Semiconductor (CMOS) technologies, is a solution for a high capacity and low power consumption communication. However, the scale of PIC is limited by optical loss and reticle size of Ultraviolet (UV) lithography, which is far from meeting the requirements of high-speed data communication and networking in Data Centers (DCs) and High Performance Computers (HPCs). Three dimensional integration Photonic Integrated Circuits (3D PICs), like the 3D integrated electronics, overcome many limitations of 2D photonic devices where all photonic components are on the same plane. Till now, several 3D PICs have been designed, proposed and experimentally demonstrated on different materials platforms, including SOI, SiN-on-SOI and polymer platforms. Wafer bonding and low temperature deposition are two general methods to fabricate 3D PICs. Wafer bonding is an effective way to achieve multilayer SOI devices for 3D PICs and photonic electronic integrated circuits. Also, bonding offers a method for 3D heterogeneous integration between Ⅲ-Ⅴ on silicon, which is an on-chip source solution. However, the cost of bonding equipment and fabrication is too expensive. Besides, several materials, including silicon nitride (SiN), amorphous silicon (a-Si) and polycrystalline silicon (poly-Si), have been deposited on SOI wafer to conduct 3D PICs. Although SiN has some unique advantages, such as extremely low propagation loss, the large fabrication tolerance, SiN itself has no active effect (low thermal tuning). Therefore, SiN-on-SOI platform has been provided by many CMOS pilot lines, leveraging on both advantages of Si and SiN platform. However, multi active layers are still required in many applications such as optical switch and Optical Phase Array (OPA). A-Si is a similar material like SiN. And A-Si is possible turned to poly-Si with an order higher mobility by high temperature annealing or laser crystallizing. Laser crystallizing is a potential method for low loss and high mobility poly-Si, but lots of efforts are still needed to achieve wafer scale fabrication. Benefiting from low cost and simple fabrication, several optical devices have been experimentally demonstrated on polymer-based Planar Lightwave Circuits (PLCs) platform. The low index difference between the cores and cladding leads the core size of polymer to several micrometers, which is hard to achieve high dense PICs. The 3D integration polymer PLC could improve the integration degree effectively, and lots of 3D polymer-based devices have been proposed, including wavelength division multiplexer/demutiplexers, mode division multiplexer/demutiplexers and OPAs.In summary, to develop 3D devices, this study widely researched the design, fabrication method and measurement methods. Till now, several 3D devices based on different materials platforms have been carefully analyzed and demonstrated experimentally. In this review, we overview the origin, development, recent progress of 3D PICs along with the applications of these devices. In our opinion, the 3D integration offers a platform not only a denser integration but also a multi-functional or multi-materials integration platform, which contains sources, modulators, optical routers, photodetectors and their drivers on one chip. The ever-increasing demand for high bandwidth is continued to grow in the forthcoming era of the Internet of Things (IoT) and 5G. Photonic Integrated Circuit (PIC), especially Silicon Photonics (SiPh), compatible with Complementary Metal-Oxide-Semiconductor (CMOS) technologies, is a solution for a high capacity and low power consumption communication. However, the scale of PIC is limited by optical loss and reticle size of Ultraviolet (UV) lithography, which is far from meeting the requirements of high-speed data communication and networking in Data Centers (DCs) and High Performance Computers (HPCs). Three dimensional integration Photonic Integrated Circuits (3D PICs), like the 3D integrated electronics, overcome many limitations of 2D photonic devices where all photonic components are on the same plane. Till now, several 3D PICs have been designed, proposed and experimentally demonstrated on different materials platforms, including SOI, SiN-on-SOI and polymer platforms. Wafer bonding and low temperature deposition are two general methods to fabricate 3D PICs. Wafer bonding is an effective way to achieve multilayer SOI devices for 3D PICs and photonic electronic integrated circuits. Also, bonding offers a method for 3D heterogeneous integration between Ⅲ-Ⅴ on silicon, which is an on-chip source solution. However, the cost of bonding equipment and fabrication is too expensive. Besides, several materials, including silicon nitride (SiN), amorphous silicon (a-Si) and polycrystalline silicon (poly-Si), have been deposited on SOI wafer to conduct 3D PICs. Although SiN has some unique advantages, such as extremely low propagation loss, the large fabrication tolerance, SiN itself has no active effect (low thermal tuning). Therefore, SiN-on-SOI platform has been provided by many CMOS pilot lines, leveraging on both advantages of Si and SiN platform. However, multi active layers are still required in many applications such as optical switch and Optical Phase Array (OPA). A-Si is a similar material like SiN. And A-Si is possible turned to poly-Si with an order higher mobility by high temperature annealing or laser crystallizing. Laser crystallizing is a potential method for low loss and high mobility poly-Si, but lots of efforts are still needed to achieve wafer scale fabrication. Benefiting from low cost and simple fabrication, several optical devices have been experimentally demonstrated on polymer-based Planar Lightwave Circuits (PLCs) platform. The low index difference between the cores and cladding leads the core size of polymer to several micrometers, which is hard to achieve high dense PICs. The 3D integration polymer PLC could improve the integration degree effectively, and lots of 3D polymer-based devices have been proposed, including wavelength division multiplexer/demutiplexers, mode division multiplexer/demutiplexers and OPAs.In summary, to develop 3D devices, this study widely researched the design, fabrication method and measurement methods. Till now, several 3D devices based on different materials platforms have been carefully analyzed and demonstrated experimentally. In this review, we overview the origin, development, recent progress of 3D PICs along with the applications of these devices. In our opinion, the 3D integration offers a platform not only a denser integration but also a multi-functional or multi-materials integration platform, which contains sources, modulators, optical routers, photodetectors and their drivers on one chip.
Acta Photonica Sinica
- Publication Date: Jul. 25, 2022
- Vol. 51, Issue 7, 0751416 (2022)
Nonlinear Amplification Techniques for Ultrafast Fiber Lasers（Invited）
Runzhi CHEN, Yuting XING, Yao ZHANG, Dongliang WANG, Junli WANG, Zhiyi WEI, and Guoqing CHANG
High-power and high-energy femtosecond fiber laser usually consists of a master oscillator followed by a power amplifier. Nonlinear effects are the main factor restricting the amplified pulse energy. Although traditional chirped pulse amplification can generate femtosecond pulses with an average power of ~1 kW and a 1 mJ level pulse energy in a single large-mode-area Yb-doped fiber, the obtained pulse duration is limited to >200 fs. However, various applications require modest pulse energy (1~100 µJ) but a much shorter pulse duration (100 μJ, 100 W average power and >1 GW peak power. Furthermore, PCM-DPA combined with coherent beam combining may produce ~1 mJ, 1 kW average power. Such kilowatt-level, high repetition rate and high-energy femtosecond laser sources hold great promise in various fields such as basic science, laser processing and national defense, and certainly open a series of new research fields. High-power and high-energy femtosecond fiber laser usually consists of a master oscillator followed by a power amplifier. Nonlinear effects are the main factor restricting the amplified pulse energy. Although traditional chirped pulse amplification can generate femtosecond pulses with an average power of ~1 kW and a 1 mJ level pulse energy in a single large-mode-area Yb-doped fiber, the obtained pulse duration is limited to >200 fs. However, various applications require modest pulse energy (1~100 µJ) but a much shorter pulse duration (100 μJ, 100 W average power and >1 GW peak power. Furthermore, PCM-DPA combined with coherent beam combining may produce ~1 mJ, 1 kW average power. Such kilowatt-level, high repetition rate and high-energy femtosecond laser sources hold great promise in various fields such as basic science, laser processing and national defense, and certainly open a series of new research fields.
Acta Photonica Sinica
- Publication Date: Jul. 25, 2022
- Vol. 51, Issue 7, 0751415 (2022)
Research Progress of Femtosecond Laser Preparation of Durable Superhydrophobic Surface and Its Application（Invited）
Jialiang ZHANG, Yang CHENG, Qing YANG, Jie LIANG, Zheng FANG, Xun HOU, and Feng CHEN
Although superhydrophobic surfaces have shown great potential in oil-water separation, anti-icing and self-cleaning, the practical applications were often limited by their brittle durability. The fragility of superhydrophobic surface structure is the main reason that hinders its practical application. Due to the further study of superhydrophobic surface, the failure of superhydrophobic surface has also been explored. This paper reviews the research progress of durable superhydrophobic surface prepared femtosecond laser and its application.In the section of background, firstly, several basic wettability models are introduced, and the characteristics of durable superhydrophobic surfaces are analyzed. Then, the advantages of femtosecond laser micro-nano processing in the preparation of superhydrophobic surfaces are discussed. The research progress of femtosecond laser preparation of the durable superhydrophobic surface for different materials such as polymer, metal and glass were concluded. The application fields of durable superhydrophobic surface fabricated by femtosecond laser were summarized.There are several methods to improve the durability of the superhydrophobic surface according to the application requirements. Firstly, the mechanical stability of the superhydrophobic surface with multi-scale micro-nanocomposite structure is much higher than that of the single micron or nano-scale rough structure. Secondly, the durability of superhydrophobic surface can also be improved by strengthening the surface microstructure on the material surface through the bonding layer. This form of adhesive can not only strengthen the bonding strength between microstructure and surface, but also play a buffer role when the surface is impacted by external force, protecting the micro-nano rough structure, and effectively improving the mechanical durability of superhydrophobic surface. Thirdly, the physical protection of micro-nano structures is carried out by constructing a surface protective layer, thereby improving their mechanical durability. Finally, the construction of self-healing superhydrophobic surface has become a powerful mean to improve the durability. Different from the previous strategies, self-healing strategies mainly focus on the recovery of damaged superhydrophobic function, thereby extending its service life. Finally, the matrix material with good intrinsic durability can be selected. Combined with femtosecond laser, a durable superhydrophobic surface can be prepared. Considering the failure reason of superhydrophobic surface and the advantages of femtosecond laser processing, durable superhydrophobic surfaces were prepared on different durable matrix materials, which is of great significance to the practical application of superhydrophobic surfaces. The variety of application requirements of superhydrophobic surfaces are different, and the corresponding durability requirements also show a great variety. Regarding the matrix materials, the superhydrophobic surface matrix material can be roughly divided into polymer, glass and metal. In terms of polymers, the superhydrophobic surface with excellent durability in PTFE can be easily prepared by femtosecond laser, as PTFE is in great stability. Besides, femtosecond laser can also prepare light responsive superhydrophobic surface on shape memory polymer. After the surface structure is deformed by an external force, it can recover to the initial state under a sunlight, and then restore to the superhydrophobic state. For metal materials, femtosecond laser combined with low surface energy material modification can directly prepare superhydrophobic surfaces on stainless steel, aluminum, zinc and other metal surfaces, and some of the superhydrophobic properties will change interestingly. For example, superhydrophobic surfaces fabricated on zinc can change the superhydrophobic properties in UV irradiation and dark storage. In terms of glass, the prepared superhydrophobic surface has very good mechanical stability and thermal stability. Based on the properties of the substrate material, adjusting the femtosecond laser processing strategy and parameters, and then implementing appropriate chemical modification according to the needs, can give the surface of the substrate material superhydrophobic properties.Generally speaking, the durability of superhydrophobic surface includes mechanical durability and chemical durability. To improve the mechanical durability, multi-scale micro-nano structures need to be constructed. The femtosecond laser has great advantages in constructing multi-scale micro-nano structures, which can be applied to almost all hard materials. In terms of chemical durability, femtosecond laser also achieves very stable superhydrophobic surfaces on some chemically stable and difficult-to-machine materials. Based on many different matrix materials, superhydrophobic materials have been increasingly applied in many fields such as self-cleaning, oil-water separation and anti-icing. Superhydrophobic materials have been applied in real life in the field of self-cleaning, chemical stability, as well as anti-ultraviolet. In oil-water separation, due to the excellent durability of matrix materials, the durable superhydrophobic materials play a huge role in solving oil pollution and industrial wastewater treatment. As for anti-icing, due to the low ice adhesion and mechanical durability in superhydrophobic materials, the key problems of transportation and aviation are hopeful to be solved.With the advantages of high machining accuracy, good universality and strong controllability of femtosecond laser in the field of micro-nano processing, supplemented by other methods such as chemical modification, and combined with the failure characteristics of superhydrophobic surfaces, it is expected to prepare durable superhydrophobic surfaces on many materials, which also greatly expands the application prospect of durable superhydrophobic surfaces in the fields of self-cleaning, oil-water separation and antiicing. Although superhydrophobic surfaces have shown great potential in oil-water separation, anti-icing and self-cleaning, the practical applications were often limited by their brittle durability. The fragility of superhydrophobic surface structure is the main reason that hinders its practical application. Due to the further study of superhydrophobic surface, the failure of superhydrophobic surface has also been explored. This paper reviews the research progress of durable superhydrophobic surface prepared femtosecond laser and its application.In the section of background, firstly, several basic wettability models are introduced, and the characteristics of durable superhydrophobic surfaces are analyzed. Then, the advantages of femtosecond laser micro-nano processing in the preparation of superhydrophobic surfaces are discussed. The research progress of femtosecond laser preparation of the durable superhydrophobic surface for different materials such as polymer, metal and glass were concluded. The application fields of durable superhydrophobic surface fabricated by femtosecond laser were summarized.There are several methods to improve the durability of the superhydrophobic surface according to the application requirements. Firstly, the mechanical stability of the superhydrophobic surface with multi-scale micro-nanocomposite structure is much higher than that of the single micron or nano-scale rough structure. Secondly, the durability of superhydrophobic surface can also be improved by strengthening the surface microstructure on the material surface through the bonding layer. This form of adhesive can not only strengthen the bonding strength between microstructure and surface, but also play a buffer role when the surface is impacted by external force, protecting the micro-nano rough structure, and effectively improving the mechanical durability of superhydrophobic surface. Thirdly, the physical protection of micro-nano structures is carried out by constructing a surface protective layer, thereby improving their mechanical durability. Finally, the construction of self-healing superhydrophobic surface has become a powerful mean to improve the durability. Different from the previous strategies, self-healing strategies mainly focus on the recovery of damaged superhydrophobic function, thereby extending its service life. Finally, the matrix material with good intrinsic durability can be selected. Combined with femtosecond laser, a durable superhydrophobic surface can be prepared. Considering the failure reason of superhydrophobic surface and the advantages of femtosecond laser processing, durable superhydrophobic surfaces were prepared on different durable matrix materials, which is of great significance to the practical application of superhydrophobic surfaces. The variety of application requirements of superhydrophobic surfaces are different, and the corresponding durability requirements also show a great variety. Regarding the matrix materials, the superhydrophobic surface matrix material can be roughly divided into polymer, glass and metal. In terms of polymers, the superhydrophobic surface with excellent durability in PTFE can be easily prepared by femtosecond laser, as PTFE is in great stability. Besides, femtosecond laser can also prepare light responsive superhydrophobic surface on shape memory polymer. After the surface structure is deformed by an external force, it can recover to the initial state under a sunlight, and then restore to the superhydrophobic state. For metal materials, femtosecond laser combined with low surface energy material modification can directly prepare superhydrophobic surfaces on stainless steel, aluminum, zinc and other metal surfaces, and some of the superhydrophobic properties will change interestingly. For example, superhydrophobic surfaces fabricated on zinc can change the superhydrophobic properties in UV irradiation and dark storage. In terms of glass, the prepared superhydrophobic surface has very good mechanical stability and thermal stability. Based on the properties of the substrate material, adjusting the femtosecond laser processing strategy and parameters, and then implementing appropriate chemical modification according to the needs, can give the surface of the substrate material superhydrophobic properties.Generally speaking, the durability of superhydrophobic surface includes mechanical durability and chemical durability. To improve the mechanical durability, multi-scale micro-nano structures need to be constructed. The femtosecond laser has great advantages in constructing multi-scale micro-nano structures, which can be applied to almost all hard materials. In terms of chemical durability, femtosecond laser also achieves very stable superhydrophobic surfaces on some chemically stable and difficult-to-machine materials. Based on many different matrix materials, superhydrophobic materials have been increasingly applied in many fields such as self-cleaning, oil-water separation and anti-icing. Superhydrophobic materials have been applied in real life in the field of self-cleaning, chemical stability, as well as anti-ultraviolet. In oil-water separation, due to the excellent durability of matrix materials, the durable superhydrophobic materials play a huge role in solving oil pollution and industrial wastewater treatment. As for anti-icing, due to the low ice adhesion and mechanical durability in superhydrophobic materials, the key problems of transportation and aviation are hopeful to be solved.With the advantages of high machining accuracy, good universality and strong controllability of femtosecond laser in the field of micro-nano processing, supplemented by other methods such as chemical modification, and combined with the failure characteristics of superhydrophobic surfaces, it is expected to prepare durable superhydrophobic surfaces on many materials, which also greatly expands the application prospect of durable superhydrophobic surfaces in the fields of self-cleaning, oil-water separation and antiicing.
Acta Photonica Sinica
- Publication Date: Jul. 25, 2022
- Vol. 51, Issue 7, 0751414 (2022)
Research Progress on Viewing Angle-related Performance of Liquid Crystal Display（Invited）
Yuqiang GUO, and Qionghua WANG
At present, Liquid Crystal Display (LCD) has become an important display technology, especially in the large-size display field. Since the liquid crystal is optically anisotropic materials, LCD has inherent viewing angle-related problems, which is increasingly becoming a bottleneck restricting its further development. In this case, LCD needs to constantly innovate to cope with the fierce competition with other display technologies and the increasing performance demands from consumers. In recent years, some technologies that can improve the LCD’s viewing angle problem have been proposed. In order to make the researchers quickly find out the relevant technical progress, we summarize some research on improving the viewing angle-related performance in recent years. This overview can be divided into the following three parts: LCD structure and display mode, the research progress of viewing angle-related performance, and special viewing angle control technology. 1) LCD structure and display mode. The basic structure and display principle of LCD are first introduced, and four common display modes, including twisted nematic, vertical alignment, in-plane switching, and fringe-field switching, are described in the order in which they were proposed. We then describe the spatial positions of the electrode structures and the initial liquid crystal orientation in the four display modes. Besides, the specific display principles of the different display modes are explained in detail. Then, the advantages and disadvantages of the different display modes and their suitable applicable fields are briefly introduced. As we know, the viewing angle problems of different display modes are different, and thus the corresponding improvement measures are also different. 2) Research progress of viewing angle-related performance. Among the many display performances, some performances have the dependence on viewing angle. Here, we introduce the properties related to viewing angle, such as brightness, contrast, grayscale, color, and color gamut. Representative improvement methods are pointed out at these performances, and the advantages and disadvantages of different methods are analyzed. In terms of brightness, several methods that can improve the brightness at the full viewing angle are introduced, such as high brightness backlight, narrow electrode technology, and field sequential color technology. We introduce the wide viewing angle compensation film, regional dimming technology, dual-cell display technology, and surface anti-reflection structure to improve the contrast. In terms of the grayscale and color performance, we introduce the use of light scattering films, single-domain and multi-domain electrode structures, and optimized driving methods to reduce the gamma shift and color difference. In addition, the methods to improve the color gamut of LCD are pointed out, such as high color gamut backlights and broadband optical filters. Each of the above methods has an important reference value for improving the viewing angle-related performance of LCD, but each approach focuses on a different viewing angle problem, and each method has its advantages and disadvantages. Therefore, researchers need to choose the appropriate methods to solve their specific problems. 3) Special viewing angle control technology. In some special application fields, the wide viewing angle technology is no longer applicable, such as business mobile phones, bank automated teller machines, ciphers, and aviation display that require privacy protection. In order to address the needs of the above fields, several special viewing angle control technologies are introduced, including narrow viewing angle, specified viewing angle, and viewing angle controllable technologies. In the aspect of narrow viewing angle technology, two commonly used methods of shading privacy film and viewing angle compensation film are introduced. In terms of the more special non-face-to-view display field, the specified viewing angle display technology based on the viewing angle deflection film is introduced. Besides, several viewing angle controllable technologies are introduced, such as dual-pixel structure, dual-cell device structure, electrode bias method. These methods can make LCD exhibit the viewing angle performance different from the common LCD with wide viewing angle, and they have the application value for some application fields with special viewing angle requirements. In different practical situations, relevant researchers should select out the appropriate technical solutions according to the actual needs, so as to solve the specific viewing angle problems in the design and manufacturing processes.Due to the limited space of this overview, the research on the various viewing angle performances cannot be summed up in all. And thus, only some representative research works are reviewed. It should be noted that, in addition to the viewing angle-related performance introduced in this overview, LCD needs to be continuously optimized in terms of flexible display, reducing the motion picture response time, and reducing the power consumption, etc. Driven by market competition, consumers pay more attention to the comprehensive performances of display technology, thus we should not sacrifice other performances just to improve one performance of LCD. In fact, how to realize the “multi-parameter linkage optimization” has become an important task to improve the comprehensive performance of LCD, which also poses a greater challenge to future research works. At present, LCD is still a relatively important display technology. From the perspective of development trends, the overall display performance of LCD based on the Mini LED backlight and regional dimming is excellent. It has the advantages of a million-level dynamic contrast ratio, more than 2 000 nits peak brightness, and ultra-high color gamut. Therefore, Mini LED LCD is an important display technology in the future, the methods mentioned in the paper are also effective in improving the viewing angle-related performance of LCD. At present, Liquid Crystal Display (LCD) has become an important display technology, especially in the large-size display field. Since the liquid crystal is optically anisotropic materials, LCD has inherent viewing angle-related problems, which is increasingly becoming a bottleneck restricting its further development. In this case, LCD needs to constantly innovate to cope with the fierce competition with other display technologies and the increasing performance demands from consumers. In recent years, some technologies that can improve the LCD’s viewing angle problem have been proposed. In order to make the researchers quickly find out the relevant technical progress, we summarize some research on improving the viewing angle-related performance in recent years. This overview can be divided into the following three parts: LCD structure and display mode, the research progress of viewing angle-related performance, and special viewing angle control technology. 1) LCD structure and display mode. The basic structure and display principle of LCD are first introduced, and four common display modes, including twisted nematic, vertical alignment, in-plane switching, and fringe-field switching, are described in the order in which they were proposed. We then describe the spatial positions of the electrode structures and the initial liquid crystal orientation in the four display modes. Besides, the specific display principles of the different display modes are explained in detail. Then, the advantages and disadvantages of the different display modes and their suitable applicable fields are briefly introduced. As we know, the viewing angle problems of different display modes are different, and thus the corresponding improvement measures are also different. 2) Research progress of viewing angle-related performance. Among the many display performances, some performances have the dependence on viewing angle. Here, we introduce the properties related to viewing angle, such as brightness, contrast, grayscale, color, and color gamut. Representative improvement methods are pointed out at these performances, and the advantages and disadvantages of different methods are analyzed. In terms of brightness, several methods that can improve the brightness at the full viewing angle are introduced, such as high brightness backlight, narrow electrode technology, and field sequential color technology. We introduce the wide viewing angle compensation film, regional dimming technology, dual-cell display technology, and surface anti-reflection structure to improve the contrast. In terms of the grayscale and color performance, we introduce the use of light scattering films, single-domain and multi-domain electrode structures, and optimized driving methods to reduce the gamma shift and color difference. In addition, the methods to improve the color gamut of LCD are pointed out, such as high color gamut backlights and broadband optical filters. Each of the above methods has an important reference value for improving the viewing angle-related performance of LCD, but each approach focuses on a different viewing angle problem, and each method has its advantages and disadvantages. Therefore, researchers need to choose the appropriate methods to solve their specific problems. 3) Special viewing angle control technology. In some special application fields, the wide viewing angle technology is no longer applicable, such as business mobile phones, bank automated teller machines, ciphers, and aviation display that require privacy protection. In order to address the needs of the above fields, several special viewing angle control technologies are introduced, including narrow viewing angle, specified viewing angle, and viewing angle controllable technologies. In the aspect of narrow viewing angle technology, two commonly used methods of shading privacy film and viewing angle compensation film are introduced. In terms of the more special non-face-to-view display field, the specified viewing angle display technology based on the viewing angle deflection film is introduced. Besides, several viewing angle controllable technologies are introduced, such as dual-pixel structure, dual-cell device structure, electrode bias method. These methods can make LCD exhibit the viewing angle performance different from the common LCD with wide viewing angle, and they have the application value for some application fields with special viewing angle requirements. In different practical situations, relevant researchers should select out the appropriate technical solutions according to the actual needs, so as to solve the specific viewing angle problems in the design and manufacturing processes.Due to the limited space of this overview, the research on the various viewing angle performances cannot be summed up in all. And thus, only some representative research works are reviewed. It should be noted that, in addition to the viewing angle-related performance introduced in this overview, LCD needs to be continuously optimized in terms of flexible display, reducing the motion picture response time, and reducing the power consumption, etc. Driven by market competition, consumers pay more attention to the comprehensive performances of display technology, thus we should not sacrifice other performances just to improve one performance of LCD. In fact, how to realize the “multi-parameter linkage optimization” has become an important task to improve the comprehensive performance of LCD, which also poses a greater challenge to future research works. At present, LCD is still a relatively important display technology. From the perspective of development trends, the overall display performance of LCD based on the Mini LED backlight and regional dimming is excellent. It has the advantages of a million-level dynamic contrast ratio, more than 2 000 nits peak brightness, and ultra-high color gamut. Therefore, Mini LED LCD is an important display technology in the future, the methods mentioned in the paper are also effective in improving the viewing angle-related performance of LCD.
Acta Photonica Sinica
- Publication Date: Jul. 25, 2022
- Vol. 51, Issue 7, 0751413 (2022)
Fresnel Zone Aperture Lensless Imaging by Compressive Sensing（Invited）
Jiachen WU, and Liangcai CAO
The Fresnel Zone Aperture (FZA) lensless imaging utilizes a Fresnel zone plate to encode the incident light as a holographic pattern. The image could be reconstructed by using holographic imaging methods. Compared with other mask-based lensless imaging methods, FZA imaging does not need any calibration. However, the inherent twin-image effect in in-line holography degrades the reconstructed image quality. In addition, the frequency of recorded fringes becomes higher with the increase of the FZA radius. Thus, a large size of the sensor could record fine fringes and obtain a high-resolution image. Because of the expensive cost of a large-size sensor, using several separate small-size sensors instead of a large-size sensor is an alternate scheme to realize high-resolution imaging. Since only partial measurements could be obtained by multiple sensors, the compressive sensing technique should be utilized for image reconstruction. The restricted isometry property is the sufficient condition of compressive sensing, unfortunately, this property is difficult to verify for a given matrix. Since the Gaussian random measurement matrix is proved to be a universal compressive sensing matrix, it is used as a reference to test the signal recovery ability of the FZA sensing matrix. The results show the reconstruction error decreases with the shrinking of the FZA constant. When the FZA constant is equal to 0.5 mm, the reconstruction performance is almost consistent with the Gaussian random matrix. Thus, compressive reconstruction for FZA imaging is feasible. Since the twin image, the original image and the sum of the two satisfy the forward model, image reconstruction belongs to an ill-posed problem because of multiple solutions. The regularization method is necessary to keep the solutions unique and stable. According to the sparsity difference between the twin image and the original image in the gradient domain, Total Variation (TV) regularization is introduced to suppress the twin image. The objective function of image reconstruction consists of an error term evaluated on sampling area and a TV regularization term. In particular, the error term calculates the first-order difference of the residual between prediction and measurements, and it can effectively eliminate the interference of the constant term in the coded image and improve the image quality. The objective function is solved by the Alternating Direction Multiplier Method (ADMM). ADMM decomposes the complex problem into several subproblems which are easy to solve, and reduce the scale of the problem and the difficulty of solving. In simulation test, the sizes of the original image and coded image both are 256×256 pixels, and the pixel pitch is 10 μm. In combination with the energy distribution of the coded image and the realizability of sampling mode, rectangle sampling and radiation sampling are tested, and the quality of the reconstructed image under different sampling ratios are analyzed. Since the image sensor is not sensitive to oblique incident light, the actual field of view is limited to a small range, and the light intensity received by each pixel of the image sensor only comes from the superposition of the local small area corresponding to the projection of the FZA. The coded image presents a frequency distribution similar to that of the FZA, that is the frequency increases gradually from the center of the image to the edge. Since the spectral energy of most natural images is concentrated at low frequencies, the center of the image should be more densely sampled than the edges to match the energy distribution. The results show that the radiation sampling mode has higher image sampling efficiency than the rectangular sampling mode, and only 7.3% of the experimental measurement data can obtain good quality images. The proposed method provides a theoretical basis for the stitching imaging of multiple small image sensors, which is beneficial to expanding the application field of lensless imaging with a coded mask. The Fresnel Zone Aperture (FZA) lensless imaging utilizes a Fresnel zone plate to encode the incident light as a holographic pattern. The image could be reconstructed by using holographic imaging methods. Compared with other mask-based lensless imaging methods, FZA imaging does not need any calibration. However, the inherent twin-image effect in in-line holography degrades the reconstructed image quality. In addition, the frequency of recorded fringes becomes higher with the increase of the FZA radius. Thus, a large size of the sensor could record fine fringes and obtain a high-resolution image. Because of the expensive cost of a large-size sensor, using several separate small-size sensors instead of a large-size sensor is an alternate scheme to realize high-resolution imaging. Since only partial measurements could be obtained by multiple sensors, the compressive sensing technique should be utilized for image reconstruction. The restricted isometry property is the sufficient condition of compressive sensing, unfortunately, this property is difficult to verify for a given matrix. Since the Gaussian random measurement matrix is proved to be a universal compressive sensing matrix, it is used as a reference to test the signal recovery ability of the FZA sensing matrix. The results show the reconstruction error decreases with the shrinking of the FZA constant. When the FZA constant is equal to 0.5 mm, the reconstruction performance is almost consistent with the Gaussian random matrix. Thus, compressive reconstruction for FZA imaging is feasible. Since the twin image, the original image and the sum of the two satisfy the forward model, image reconstruction belongs to an ill-posed problem because of multiple solutions. The regularization method is necessary to keep the solutions unique and stable. According to the sparsity difference between the twin image and the original image in the gradient domain, Total Variation (TV) regularization is introduced to suppress the twin image. The objective function of image reconstruction consists of an error term evaluated on sampling area and a TV regularization term. In particular, the error term calculates the first-order difference of the residual between prediction and measurements, and it can effectively eliminate the interference of the constant term in the coded image and improve the image quality. The objective function is solved by the Alternating Direction Multiplier Method (ADMM). ADMM decomposes the complex problem into several subproblems which are easy to solve, and reduce the scale of the problem and the difficulty of solving. In simulation test, the sizes of the original image and coded image both are 256×256 pixels, and the pixel pitch is 10 μm. In combination with the energy distribution of the coded image and the realizability of sampling mode, rectangle sampling and radiation sampling are tested, and the quality of the reconstructed image under different sampling ratios are analyzed. Since the image sensor is not sensitive to oblique incident light, the actual field of view is limited to a small range, and the light intensity received by each pixel of the image sensor only comes from the superposition of the local small area corresponding to the projection of the FZA. The coded image presents a frequency distribution similar to that of the FZA, that is the frequency increases gradually from the center of the image to the edge. Since the spectral energy of most natural images is concentrated at low frequencies, the center of the image should be more densely sampled than the edges to match the energy distribution. The results show that the radiation sampling mode has higher image sampling efficiency than the rectangular sampling mode, and only 7.3% of the experimental measurement data can obtain good quality images. The proposed method provides a theoretical basis for the stitching imaging of multiple small image sensors, which is beneficial to expanding the application field of lensless imaging with a coded mask.
Acta Photonica Sinica
- Publication Date: Jul. 25, 2022
- Vol. 51, Issue 7, 0751412 (2022)
Hohmann Transfer Structure Beam and Particle Manipulation（Invited）
Xinzhong LI, Liuhao ZHU, Haihao FAN, Wenjun WEI, Xin MA, Xueyun QIN, Huajie HU, and Yuping TAI
Since the invention of the laser in the 1960s, the higher power of the laser has led to a better understanding of the interaction between light and matter because of its monochromaticity, directionality, and coherence. Optical tweezers, which won the 2018 Nobel Prize in Physics, are one of the best applications of lasers. In 1976 ASHKIN A discovered that a single beam of light dependent on a gradient force could capture particles. The single beam optical tweezers is widely used in the biological field. In 1992, Orbital Angular Momentum (OAM) was discovered, structural beams carrying OAM have been widely used in the field of particle manipulation and it adds the degree of freedom of lateral manipulation for optical tweezers and has more abundant manipulation modes. However, in the case of the existing structural beams, no matter how the structure of light is changed, due to the nature of OAM, its structure beam will always make particles move along a given orbit in specific applications. The real-time orbital movement of particles is not considered. Therefore, there is an urgent need for a beam with a more abundant mode than the previous single OAM, which can simultaneously exist a variety of different OAMs and control the motion of particles in real-time. The Hohmann transfer was derived by the German engineer Dr. Walter Hohmann in 1925. It is a method to transfer the minimum energy of a satellite between two circular orbits with the same inclination and different altitudes. It’s widely used in the aerospace field. Although particles move in solution, they are also affected by buoyancy and other external forces in addition to the gravity of particles themselves, and the environment they live in is relatively complex, so their motion cannot be compared with the law of planetary motion. However, the orbital switching of Hohmann transfer can still solve the existing problems of structured optical tweezers.To solve this situation, in this paper, the corresponding orbit beam is generated by beam shaping technology, the orbit is transformed into an elliptical orbit by coordinate transformation technology, and finally, the orbit is combined by the Fourier phase shift theorem. A kind of Hohmann Transfer Structured Beam (HTSB) has been proposed via combining beam shaping technology, coordinate transformation technology and Fourier phase shift theorem. This beam has a very abundant mode of regulation and the phase gradient distribution can transfer the particles from the parking orbit to the synchronous orbit. Also, the size, structure, and phase gradient, can be arbitrarily adjusted, in the application can be based on the actual needs of the corresponding adjustment of the beam. Firstly, we analyze the relationship between each orbit of Hohmann transfer, and give the relationship between parameters corresponding to each orbit of HTSB, and the control method. The intensity and phase distribution of the HTSB with increasing radius are simulated. Secondly, we extend the HTSB according to the principle of Hohmann transfer, and discuss the parameter setting and generation method of HTSB with more orbits. The intensity and phase distribution of HTSB with more orbits are simulated. Finally, an optical tweezer experiment is set up, and the HTSB is used to manipulate the polystyrene particles. We have designed two experiments, the first experiment is using a fixed HTSB to make the particles from the parking orbit transfer to synchronous orbit, and the second experiment is using dynamic switch cover template on spatial light modulator particles in a week and a half after parking orbits respectively, choose an appropriate time to transfer orbit switch mask template, rotating again after a week and a half, Transfer to synchronous orbit and rotate once in synchronous orbit. Moreover, the beam can do a lot more. It can reverse the fixed orbit and the synchronous orbit by changing the topological charges; the velocity of particles moving between orbits can be controlled by adjusting the topological charge of each orbit; it can control the way the particles are transferred by rotating the beam as a whole. Therefore, this research proves the feasibility of Hohmann transfer in the microscopic field and has great significance in the field of optical micromanipulation. Since the invention of the laser in the 1960s, the higher power of the laser has led to a better understanding of the interaction between light and matter because of its monochromaticity, directionality, and coherence. Optical tweezers, which won the 2018 Nobel Prize in Physics, are one of the best applications of lasers. In 1976 ASHKIN A discovered that a single beam of light dependent on a gradient force could capture particles. The single beam optical tweezers is widely used in the biological field. In 1992, Orbital Angular Momentum (OAM) was discovered, structural beams carrying OAM have been widely used in the field of particle manipulation and it adds the degree of freedom of lateral manipulation for optical tweezers and has more abundant manipulation modes. However, in the case of the existing structural beams, no matter how the structure of light is changed, due to the nature of OAM, its structure beam will always make particles move along a given orbit in specific applications. The real-time orbital movement of particles is not considered. Therefore, there is an urgent need for a beam with a more abundant mode than the previous single OAM, which can simultaneously exist a variety of different OAMs and control the motion of particles in real-time. The Hohmann transfer was derived by the German engineer Dr. Walter Hohmann in 1925. It is a method to transfer the minimum energy of a satellite between two circular orbits with the same inclination and different altitudes. It’s widely used in the aerospace field. Although particles move in solution, they are also affected by buoyancy and other external forces in addition to the gravity of particles themselves, and the environment they live in is relatively complex, so their motion cannot be compared with the law of planetary motion. However, the orbital switching of Hohmann transfer can still solve the existing problems of structured optical tweezers.To solve this situation, in this paper, the corresponding orbit beam is generated by beam shaping technology, the orbit is transformed into an elliptical orbit by coordinate transformation technology, and finally, the orbit is combined by the Fourier phase shift theorem. A kind of Hohmann Transfer Structured Beam (HTSB) has been proposed via combining beam shaping technology, coordinate transformation technology and Fourier phase shift theorem. This beam has a very abundant mode of regulation and the phase gradient distribution can transfer the particles from the parking orbit to the synchronous orbit. Also, the size, structure, and phase gradient, can be arbitrarily adjusted, in the application can be based on the actual needs of the corresponding adjustment of the beam. Firstly, we analyze the relationship between each orbit of Hohmann transfer, and give the relationship between parameters corresponding to each orbit of HTSB, and the control method. The intensity and phase distribution of the HTSB with increasing radius are simulated. Secondly, we extend the HTSB according to the principle of Hohmann transfer, and discuss the parameter setting and generation method of HTSB with more orbits. The intensity and phase distribution of HTSB with more orbits are simulated. Finally, an optical tweezer experiment is set up, and the HTSB is used to manipulate the polystyrene particles. We have designed two experiments, the first experiment is using a fixed HTSB to make the particles from the parking orbit transfer to synchronous orbit, and the second experiment is using dynamic switch cover template on spatial light modulator particles in a week and a half after parking orbits respectively, choose an appropriate time to transfer orbit switch mask template, rotating again after a week and a half, Transfer to synchronous orbit and rotate once in synchronous orbit. Moreover, the beam can do a lot more. It can reverse the fixed orbit and the synchronous orbit by changing the topological charges; the velocity of particles moving between orbits can be controlled by adjusting the topological charge of each orbit; it can control the way the particles are transferred by rotating the beam as a whole. Therefore, this research proves the feasibility of Hohmann transfer in the microscopic field and has great significance in the field of optical micromanipulation.
Acta Photonica Sinica
- Publication Date: Jul. 25, 2022
- Vol. 51, Issue 7, 0751411 (2022)
Development of Ultrafast Spin-based Terahertz Photonics（Invited）
Zuanming JIN, Yingyu GUO, Bingyu JI, Zhangshun LI, Guohong MA, Shixun CAO, Yan PENG, Yiming ZHU, and Songlin ZHUANG
Terahertz (THz) radiation is generally defined as the region of the electromagnetic spectrum in the range of 0.1 to 10 THz, between the millimeter and infrared frequencies. THz radiation is important from both scientific and application point of view. THz science and technology has been an active research area for a wide variety of applications: such as spectroscopy, imaging and sensing, biology and medical sciences, and security evaluation. The development of efficient, ultra-broadband, and low-cost THz photonic devices requires new materials and mechanisms, which is the key challenge for the field of THz science and technology. The discovery of THz electromagnetic pulse emission from ultrafast demagnetization by femtosecond laser pulses gave insight into the microscopic interactions that connect the ultrafast spintronics and the THz photonics.Based on our experimental observations, this paper reviews the recent developments and applications, the current understanding of the physical processes, and the perspectives of ultrafast spin-based THz photonics.Firstly, ultrashort THz pulses have been demonstrated as a promising tool to investigate the ultrafast spintronics. We review the fundamental physical processes and properties including THz-driven spin waves, THz spin transport probing, and ultrafast THz magnetometry. 1) The THz pulses are used to excite and control the antiferromagnetic spin waves in rare-earth orthoferrites with the THz time-domain spectroscopy. In addition, we observe the magnon-polariton, magnon-spin coupling, and magnon-magnon coupling in the condensed matter systems. 2) We demonstrate the magnetic modulation of THz waves, along with heat- and contact-free giant magnetoresistance, tunneling magnetoresistance and anisotropic magnetoresistance readout using ultrafast THz signals. We directly determine the spin-dependent densities and momentum scattering times of conduction electrons. The various magnetic configurations between the parallel state and antiparallel state of the magnetizations of the ferromagnetic layers in the magnetic tunnel junctions have the effect of changing the conductivity, making a functional modulation of the propagating THz electromagnetic fields. 3) We demonstrate a method of ultrafast THz magnetometry, which indicates the sub-picosecond demagnetization dynamics in a laser-excited iron film. The measurements reveal the contributions originating from magnetization quenching and acoustically-driven modulation of the exchange interaction. In addition, the ultrafast photoinduced spin transport can be extracted from the THz emission signals. We observe the transition of laser-induced THz spin currents from torque-mediated to conduction-electron-mediated transport in ferromagnetic/non-magnetic heterostructures.Secondly, by exploring the ultrafast THz spintronic effects, new applications in THz photonic devices emerge, including spintronic THz emitters, THz modulators and THz detectors. 1) The ferromagnetic/non-magnetic heterostructure under the excitation of femtosecond laser has proved to be a potential candidate for high-efficiency THz emission. The ultrafast spin-charge conversion based on the Inverse Spin Hall Effect (ISHE) is used to generate broadband THz radiation. We summarize the efforts that have been made to improve the performance of spintronics-based THz emitters. Up to date, the efficiency of spintronics-based THz emission has been enhanced to reach the same level of millimeter-thick ZnTe crystal. 2) The combined spintronic and photonic heterostructures are exploited to realize active modulation of THz radiation. In addition, it is demonstrated that the THz radiation can be mediated coherently through the charge current induced by the ISHE and the built-in transient current quasi-simultaneously created within the patterned heterostructures. 3) Using the ISHE, an antiferromagnet/heavy metal bilayer is theoretically promising for the realization of a resonant, compact, and tunable THz detector. In addition, a coherent and phase-locked coupling between a single-cycle THz transient and the magnetization of cobalt films suggests new opportunities for THz pulse detection.Finally, a brief summary and outlook are given. Looking to the future, we introduce the applications of ultrafast spin-based THz photonics, such as ultra-broadband measurements, magnetic structure detection and imaging, and THz near-field microscopy. In addition, topological materials bear a large potential for efficient spin-to-charge conversion due to the inherent spin-momentum locking. The topological insulator/ferromagnetic heterostructures are expected to present a high-performance THz radiation. In addition, the topological spintronic THz emitter will show a potential to generate arbitrary THz waveforms. One can anticipate that the research scope of ultrafast spin-based THz photonics will successfully be used to understand the fundamental physics in new materials and give rise to high-efficient THz photonic devices and spectroscopy applications. We hope that our work will stimulate more fundamental and technological developments in this new research field. Terahertz (THz) radiation is generally defined as the region of the electromagnetic spectrum in the range of 0.1 to 10 THz, between the millimeter and infrared frequencies. THz radiation is important from both scientific and application point of view. THz science and technology has been an active research area for a wide variety of applications: such as spectroscopy, imaging and sensing, biology and medical sciences, and security evaluation. The development of efficient, ultra-broadband, and low-cost THz photonic devices requires new materials and mechanisms, which is the key challenge for the field of THz science and technology. The discovery of THz electromagnetic pulse emission from ultrafast demagnetization by femtosecond laser pulses gave insight into the microscopic interactions that connect the ultrafast spintronics and the THz photonics.Based on our experimental observations, this paper reviews the recent developments and applications, the current understanding of the physical processes, and the perspectives of ultrafast spin-based THz photonics.Firstly, ultrashort THz pulses have been demonstrated as a promising tool to investigate the ultrafast spintronics. We review the fundamental physical processes and properties including THz-driven spin waves, THz spin transport probing, and ultrafast THz magnetometry. 1) The THz pulses are used to excite and control the antiferromagnetic spin waves in rare-earth orthoferrites with the THz time-domain spectroscopy. In addition, we observe the magnon-polariton, magnon-spin coupling, and magnon-magnon coupling in the condensed matter systems. 2) We demonstrate the magnetic modulation of THz waves, along with heat- and contact-free giant magnetoresistance, tunneling magnetoresistance and anisotropic magnetoresistance readout using ultrafast THz signals. We directly determine the spin-dependent densities and momentum scattering times of conduction electrons. The various magnetic configurations between the parallel state and antiparallel state of the magnetizations of the ferromagnetic layers in the magnetic tunnel junctions have the effect of changing the conductivity, making a functional modulation of the propagating THz electromagnetic fields. 3) We demonstrate a method of ultrafast THz magnetometry, which indicates the sub-picosecond demagnetization dynamics in a laser-excited iron film. The measurements reveal the contributions originating from magnetization quenching and acoustically-driven modulation of the exchange interaction. In addition, the ultrafast photoinduced spin transport can be extracted from the THz emission signals. We observe the transition of laser-induced THz spin currents from torque-mediated to conduction-electron-mediated transport in ferromagnetic/non-magnetic heterostructures.Secondly, by exploring the ultrafast THz spintronic effects, new applications in THz photonic devices emerge, including spintronic THz emitters, THz modulators and THz detectors. 1) The ferromagnetic/non-magnetic heterostructure under the excitation of femtosecond laser has proved to be a potential candidate for high-efficiency THz emission. The ultrafast spin-charge conversion based on the Inverse Spin Hall Effect (ISHE) is used to generate broadband THz radiation. We summarize the efforts that have been made to improve the performance of spintronics-based THz emitters. Up to date, the efficiency of spintronics-based THz emission has been enhanced to reach the same level of millimeter-thick ZnTe crystal. 2) The combined spintronic and photonic heterostructures are exploited to realize active modulation of THz radiation. In addition, it is demonstrated that the THz radiation can be mediated coherently through the charge current induced by the ISHE and the built-in transient current quasi-simultaneously created within the patterned heterostructures. 3) Using the ISHE, an antiferromagnet/heavy metal bilayer is theoretically promising for the realization of a resonant, compact, and tunable THz detector. In addition, a coherent and phase-locked coupling between a single-cycle THz transient and the magnetization of cobalt films suggests new opportunities for THz pulse detection.Finally, a brief summary and outlook are given. Looking to the future, we introduce the applications of ultrafast spin-based THz photonics, such as ultra-broadband measurements, magnetic structure detection and imaging, and THz near-field microscopy. In addition, topological materials bear a large potential for efficient spin-to-charge conversion due to the inherent spin-momentum locking. The topological insulator/ferromagnetic heterostructures are expected to present a high-performance THz radiation. In addition, the topological spintronic THz emitter will show a potential to generate arbitrary THz waveforms. One can anticipate that the research scope of ultrafast spin-based THz photonics will successfully be used to understand the fundamental physics in new materials and give rise to high-efficient THz photonic devices and spectroscopy applications. We hope that our work will stimulate more fundamental and technological developments in this new research field.
Acta Photonica Sinica
- Publication Date: Jul. 25, 2022
- Vol. 51, Issue 7, 0751410 (2022)
Research Progress on Low-noise Laser for Space-based Gravitational Wave Detector（Invited）
Qiang LIU, Zaiyuan WANG, Jiehao WANG, and Yuhang LI
Gravitational Waves (GWs) are ripples of space-time that propagate across the universe at the speed of light. GWs detection is one of the most important frontiers of physics today. Ground-based laser interferometer gravitational wave detectors, such as LIGO, Virgo and KAGRA, have successfully confirmed the existence of GWs. GWs have become a new window for a human to observe the universe. Due to the influence of ground vibration noise, terrestrial gravity gradient noise and other factors, ground-based detectors are not sensitive to GWs below 1 Hz. Space-based detectors are free from such noises and can be made very large, thereby expanding the frequency range downwards to 10-4 Hz, where exciting GW sources are waiting to be explored. Space-based GW detectors are expected to bring even more information about the universe through low-frequency GWs. A highly stable and long-lifetime laser system is a key component of the space-based GW detector, and the output power, intensity noise, frequency noise and other properties of the laser directly affect the sensitivity of the space-based GW detector. Thus, space-based GW detectors put forward much higher and stricter requirements for the laser. After widely experimental and industrial surveys, the baseline architecture for the laser for space-based GW detector consists of a low-power, low-noise master oscillator followed by a power amplifier with 2~10 W continuous-wave output. So, lasers consisting of a Master Oscillator and a Power Amplifier (MOPA) have been identified as the most-promising architecture for the space-based GW detector. In this review, we focused on the research progress of low-noise MOPA lasers in LISA, TianQin and Taiji missions. As in other applications of precision interferometry, 1064 nm was chosen as the laser wavelength, due to the availability of high-quality bulk optics and the traditional low-noise Nd:YAG laser source represented by the Non-Planar Ring Oscillator (NPRO).The performance of different MOPA lasers for LISA injected by NPRO(m-NPRO), fiber laser and External Cavity Diode Laser (ECDL) were compared, and the m-NPRO has been identified as the most-promising MO architecture for the LISA laser, and the baseline architecture consists of a low-power, low-noise m-NPRO followed by a diode-pumped Yb-fiber amplifier with ~2 W output. As for the Relative Intensity Noise (RIN), the m-NPRO meets the LISA requirement. As to the Chinese space-based GW detection projects, the research progress and spatial test results of lasers were elaborated. A DBR laser was designed for the TianQin-1 mission as the space laser in the laser interferometer, and was successfully verified in the TianQin-1 mission. The laser passed the environment performance verification under the aerospace standard. Meanwhile, a high stability laser source at 1064 nm for Taiji-1 satellite was reported. The key component of the laser source was a NonPlanar Ring Oscillator (NPRO) solid laser with linewidth of 260 Hz. The frequency noise and power noise of Laser source were greatly improved by applying precision driving current control and temperature control. Furthermore, around the requirements of the noise performance of lasers by space-based GW detectors, the principle and progress of intensity noise suppression and frequency noise suppression were described, and the main achievements of photoelectric feedback suppression technology for intensity noise suppression and PDH technology for frequency noise suppression were discussed respectively. We designed a laser system with the characteristics of narrow-linewidth, low-noise and polarization-maintaining. In our experiment setup, optoelectronic feedback suppression technology was used to suppress pump LD fluctuations using low-noise photodetectors and PID controller. In the frequency domain, the relative intensity noise is reduced at the level of 10-3 Hz-1/2@1 mHz. Finally, the research on low noise lasers for space-based GW detection in China is prospected, and the development direction of low noise lasers is proposed. Gravitational Waves (GWs) are ripples of space-time that propagate across the universe at the speed of light. GWs detection is one of the most important frontiers of physics today. Ground-based laser interferometer gravitational wave detectors, such as LIGO, Virgo and KAGRA, have successfully confirmed the existence of GWs. GWs have become a new window for a human to observe the universe. Due to the influence of ground vibration noise, terrestrial gravity gradient noise and other factors, ground-based detectors are not sensitive to GWs below 1 Hz. Space-based detectors are free from such noises and can be made very large, thereby expanding the frequency range downwards to 10-4 Hz, where exciting GW sources are waiting to be explored. Space-based GW detectors are expected to bring even more information about the universe through low-frequency GWs. A highly stable and long-lifetime laser system is a key component of the space-based GW detector, and the output power, intensity noise, frequency noise and other properties of the laser directly affect the sensitivity of the space-based GW detector. Thus, space-based GW detectors put forward much higher and stricter requirements for the laser. After widely experimental and industrial surveys, the baseline architecture for the laser for space-based GW detector consists of a low-power, low-noise master oscillator followed by a power amplifier with 2~10 W continuous-wave output. So, lasers consisting of a Master Oscillator and a Power Amplifier (MOPA) have been identified as the most-promising architecture for the space-based GW detector. In this review, we focused on the research progress of low-noise MOPA lasers in LISA, TianQin and Taiji missions. As in other applications of precision interferometry, 1064 nm was chosen as the laser wavelength, due to the availability of high-quality bulk optics and the traditional low-noise Nd:YAG laser source represented by the Non-Planar Ring Oscillator (NPRO).The performance of different MOPA lasers for LISA injected by NPRO(m-NPRO), fiber laser and External Cavity Diode Laser (ECDL) were compared, and the m-NPRO has been identified as the most-promising MO architecture for the LISA laser, and the baseline architecture consists of a low-power, low-noise m-NPRO followed by a diode-pumped Yb-fiber amplifier with ~2 W output. As for the Relative Intensity Noise (RIN), the m-NPRO meets the LISA requirement. As to the Chinese space-based GW detection projects, the research progress and spatial test results of lasers were elaborated. A DBR laser was designed for the TianQin-1 mission as the space laser in the laser interferometer, and was successfully verified in the TianQin-1 mission. The laser passed the environment performance verification under the aerospace standard. Meanwhile, a high stability laser source at 1064 nm for Taiji-1 satellite was reported. The key component of the laser source was a NonPlanar Ring Oscillator (NPRO) solid laser with linewidth of 260 Hz. The frequency noise and power noise of Laser source were greatly improved by applying precision driving current control and temperature control. Furthermore, around the requirements of the noise performance of lasers by space-based GW detectors, the principle and progress of intensity noise suppression and frequency noise suppression were described, and the main achievements of photoelectric feedback suppression technology for intensity noise suppression and PDH technology for frequency noise suppression were discussed respectively. We designed a laser system with the characteristics of narrow-linewidth, low-noise and polarization-maintaining. In our experiment setup, optoelectronic feedback suppression technology was used to suppress pump LD fluctuations using low-noise photodetectors and PID controller. In the frequency domain, the relative intensity noise is reduced at the level of 10-3 Hz-1/2@1 mHz. Finally, the research on low noise lasers for space-based GW detection in China is prospected, and the development direction of low noise lasers is proposed.
Acta Photonica Sinica
- Publication Date: Jul. 25, 2022
- Vol. 51, Issue 7, 0751409 (2022)
High-throughput Full-color Fourier Ptychographic Microscopy for the Next Generation of Digital Pathologic Imager and Analyser（Invited）
An PAN, Yuting GAO, Aiye WANG, Huiqin GAO, Caiwen MA, and Baoli YAO
Fourier Ptychographic Microscopy (FPM) is a promising computational imaging technique, which tackles the intrinsic trade-off between high resolution and large wide Field Of View (FOV) with a combination of Synthetic Aperture Radar (SAR) and optical phase retrieval. In brief, an LED array beneath the microscope provides illumination of the object from different incident angles. The range of light that can be collected is determined by the Numerical Aperture (NA) of the objective, while parts of the scattering light with a high-angle illumination can also be collected because of light-matter interaction. The low resolution intensity images recorded at each illumination angle are then synthesized in the Fourier domain, thus the object’s high-frequency information can be modulated into the passband of the objective. After an iterative phase reconstruction process, the synthesized information generates a high resolution object image including both intensity and phase properties. Additionally, it preserves the original large FOV as a low-NA objective is used to stitch low resolution images together. Given its flexible setup without mechanical scanning and interferometric measurement, FPM has developed rapidly, which not only acts as a tool to obtain both HR and large FOV but is also regarded as a paradigm to solve a series of trade-off problems, say, the trade-off between angular resolution and spatial resolution in light field imaging. And it may inspire to solve the trade-off between spectral resolution and spatial resolution in imaging spectrometer in the future.In this paper, we comprehensively summarized the development trend of FPM technique in 9 aspects, including high-precision imaging, high-throughput imaging, high-speed or single shot imaging, 3D or tomography imaging, mixed state decoupling, spectral dimension (color imaging to hyperspectral imaging), high dynamic range, system extension, and typical applications. Among them, digital pathology is one of the earliest and the most successful applications of FPM. Distinguished from other reviews, we focused on introducing the development process and recent advances in the direction of digital pathology, and divided it into “0-1”, “1-10”, and “10-100” three periods and several stages. Several typical results are also provided. Specifically, the “0-1” refers to the birth of FPM, which breaks the mutual restrictions between FOV and spatial resolution. The “1-10” refers to the exploration period, where the accuracy and stability, limits and bottlenecks, and the efficiency of FPM have been successively discussed and improved. The stage of “10-100” refers to the industrialization period. During this period, researchers focus on market-oriented requirements including acquisition and analysis of color, since full-color imaging is of critical importance for analyzing labeled tissue sections.We point out that FPM has entered the industrialization stage of “10-100” in this application direction.The current task is to build a prototype or product based on FPM. We expect that the product can obtain a spatial resolution of around 200 nm~1 000 nm, a FOV of around 10 mm (2× objective) or 5 mm (4× objective) diameter full-color FPM reconstructed image within 4 s at the DOF of around 0.3~0.5 mm stably and efficiently. We estimate that it can be capable of automation and batch scanning within the next 1~2 years. We analyzed the industry development situations of digital pathology and related market requirements, and discussed the potential of FPM for large-scale socio-economic benefits. We demonstrated that the full-color images with high quality and content and quantitative phase images produced by FPM may play a role of promotion in wide fields, including intraoperative pathology, quantitative Artificial Intelligence (AI) diagnosis, three-dimensional reconstruction, telepathology, teaching and standardized industry criteria. It should also be clarified that as a typical interdisciplinary field, even if the instrument is successfully invented, it only solves issues in the imaging section of the whole process of digital pathology, and there still remain a series of tough tasks to complete. We discussed and classified related scientific problems, technical problems, engineering problems, and industrial problems in detail, whose successful and perfect resolution relies on joint efforts of various parties and constructive introduction of several potential approaches. By combining the FPM solution with the upstream and downstream advanced methods, including the virtual staining, multimodal fusion imaging, label-free observation in situ, non-destructive three-dimensional reconstruction, preliminary screening, and recognition with AI, etc., we believe that the industry problems will eventually be overcome or alleviated. Fourier Ptychographic Microscopy (FPM) is a promising computational imaging technique, which tackles the intrinsic trade-off between high resolution and large wide Field Of View (FOV) with a combination of Synthetic Aperture Radar (SAR) and optical phase retrieval. In brief, an LED array beneath the microscope provides illumination of the object from different incident angles. The range of light that can be collected is determined by the Numerical Aperture (NA) of the objective, while parts of the scattering light with a high-angle illumination can also be collected because of light-matter interaction. The low resolution intensity images recorded at each illumination angle are then synthesized in the Fourier domain, thus the object’s high-frequency information can be modulated into the passband of the objective. After an iterative phase reconstruction process, the synthesized information generates a high resolution object image including both intensity and phase properties. Additionally, it preserves the original large FOV as a low-NA objective is used to stitch low resolution images together. Given its flexible setup without mechanical scanning and interferometric measurement, FPM has developed rapidly, which not only acts as a tool to obtain both HR and large FOV but is also regarded as a paradigm to solve a series of trade-off problems, say, the trade-off between angular resolution and spatial resolution in light field imaging. And it may inspire to solve the trade-off between spectral resolution and spatial resolution in imaging spectrometer in the future.In this paper, we comprehensively summarized the development trend of FPM technique in 9 aspects, including high-precision imaging, high-throughput imaging, high-speed or single shot imaging, 3D or tomography imaging, mixed state decoupling, spectral dimension (color imaging to hyperspectral imaging), high dynamic range, system extension, and typical applications. Among them, digital pathology is one of the earliest and the most successful applications of FPM. Distinguished from other reviews, we focused on introducing the development process and recent advances in the direction of digital pathology, and divided it into “0-1”, “1-10”, and “10-100” three periods and several stages. Several typical results are also provided. Specifically, the “0-1” refers to the birth of FPM, which breaks the mutual restrictions between FOV and spatial resolution. The “1-10” refers to the exploration period, where the accuracy and stability, limits and bottlenecks, and the efficiency of FPM have been successively discussed and improved. The stage of “10-100” refers to the industrialization period. During this period, researchers focus on market-oriented requirements including acquisition and analysis of color, since full-color imaging is of critical importance for analyzing labeled tissue sections.We point out that FPM has entered the industrialization stage of “10-100” in this application direction.The current task is to build a prototype or product based on FPM. We expect that the product can obtain a spatial resolution of around 200 nm~1 000 nm, a FOV of around 10 mm (2× objective) or 5 mm (4× objective) diameter full-color FPM reconstructed image within 4 s at the DOF of around 0.3~0.5 mm stably and efficiently. We estimate that it can be capable of automation and batch scanning within the next 1~2 years. We analyzed the industry development situations of digital pathology and related market requirements, and discussed the potential of FPM for large-scale socio-economic benefits. We demonstrated that the full-color images with high quality and content and quantitative phase images produced by FPM may play a role of promotion in wide fields, including intraoperative pathology, quantitative Artificial Intelligence (AI) diagnosis, three-dimensional reconstruction, telepathology, teaching and standardized industry criteria. It should also be clarified that as a typical interdisciplinary field, even if the instrument is successfully invented, it only solves issues in the imaging section of the whole process of digital pathology, and there still remain a series of tough tasks to complete. We discussed and classified related scientific problems, technical problems, engineering problems, and industrial problems in detail, whose successful and perfect resolution relies on joint efforts of various parties and constructive introduction of several potential approaches. By combining the FPM solution with the upstream and downstream advanced methods, including the virtual staining, multimodal fusion imaging, label-free observation in situ, non-destructive three-dimensional reconstruction, preliminary screening, and recognition with AI, etc., we believe that the industry problems will eventually be overcome or alleviated.
Acta Photonica Sinica
- Publication Date: Jul. 25, 2022
- Vol. 51, Issue 7, 0751408 (2022)
Topics
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