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 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)
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)
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)
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)
Broadband Epsilon-near-zero Metamaterials and Its Application in Optical Field Manipulation(Invited)
Lei SUN, and Guoping WANG
Since the beginning of metamaterial research, epsilon-near-zero metamaterials have long been a research hotspot in the field of electromagnetic metamaterials because of the electromagnetic features granted by their near-zero effective permittivity, making them interesting in both theoretical studies and technical applications. To achieve the near-zero effective permittivity response, in short, the current epsilon-near-zero metamaterial research follows three fundamental theories, the waveguide theory, the band structure theory for photonic crystals, and the effective medium theory. In general, the waveguide theory makes the epsilon-near-zero metamaterials operate as a waveguide at the cut-off frequency, while the band structure theory makes the epsilon-near-zero metamaterials perform as a photonic crystal at the frequency of the Dirac point. On the other hand, the effective medium theory follows another principle of the cancellation of the positive permittivity and the negative permittivity of different components of the epsilon-near-zero metamaterials at a specific frequency. All three theories strongly depend on the electromagnetic properties and the microstructures of the components in the epsilon-near-zero metamaterials, which also limits the near-zero permittivity response frequency. In brief, the single operating frequency and operating mode caused by the inherent limitation of these theories is always a bottleneck preventing further applications of the epsilon-near-zero metamaterials. Therefore, how to break through the limitations, to realize the broadband near-zero permittivity response of metamaterials under multi-stimulation modes, to master its physical principles, and to establish a new theoretical framework are of great significance in theoretical research and application development.In this work, we present a systematic review of our research on broadband epsilon-near-zero metamaterials to address the single operating frequency issue of current epsilon-near-zero metamaterials. In our research, we establish a spectral representation theory for the broadband epsilon-near-zero metamaterials based on the Bergman-Milton spectral representation theory of effective medium theory concerning the characteristics of the effective permittivity spectral representation of typical microstructures of metamaterials and theoretically demonstrate the application of the spectral representation theory in various broadband epsilon-near-zero metamaterials construction. To be specific, the spectral representation theory for the broadband epsilon-near-zero metamaterials firstly abstracts the material and the microstructural properties of the metamaterials into a simple algebra model, in which the broadband near-zero permittivity response can be constructed as will for different probing electromagnetic waves. After that, the spectral representation theory exactly maps the spectral representation of the broadband epsilon-near-zero metamaterials into the physical structures via an analytical inverse algorithm. Through the spectral representation theory, we successfully realize broadband epsilon-near-zero metamaterials with different superlattice configurations in theory, such as the multilayer and the Hashin-Shtrikman structures. Meanwhile, we clearly explain the physical principles of the broadband near-zero permittivity response by numerical simulations. The broadband epsilon-near-zero metamaterials successfully extend the near-zero permittivity property into a wide operating frequency band without changing the near-zero permittivity property. Therefore, they expand the application potential of the epsilon-near-zero metamaterials in broadband optical field manipulation. Through the numerical simulation, we demonstrate several applications of the designed broadband epsilon-near-zero metamaterials, such as the broadband electromagnetic tunneling and focusing, the broadband electromagnetic wave directional emission, and the broadband electromagnetic wavefront modulation. In the end, we wish our achievement can enrich the fundamental theory and application prospects of electromagnetic metamaterials.
Acta Photonica Sinica
  • Publication Date: Jan. 25, 2022
  • Vol. 51 Issue 1 0151107 (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)
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)
Effect on the Optical Properties of Planar Microlens Arrays with Different Ion-exchange Time
Yimunan XIE, Xiaoping JIANG, Sumei ZHOU, and Zhe YUAN
The plane microlens array is a plane array that is buried under the surface of the flat glass substrate,which consists of several microlens elements with uniform geometric dimensions and a three-dimensional gradient of refractive index according to a certain rule. There are many methods for making GRIN planar microlens arrays, including hot embossing technology, ion beam etching, the melted photoresist method, and so on. Each of these methods has certain advantages and limitations. The hot embossing technology has high pattern reproduction accuracy, but the demolding process is complicated. Using ion beam etching to fabricate microlens has good surface microstructure, but the production cost is high and production efficiency is low. The melted photoresist method has a simple manufacturing process, but the planar microlens array made by this method has poor thermal stability and is prone to aging. The fabrication process of GRIN planar microlens array using ion-exchanging method is relatively simple. The size of the lens element fabricated by ion-exchanging method can be less than 10 μm, the focal length can be less than 0.1 mm, and the thickness can be less than 0.1 mm. Moreover, the array structure and imaging uniformity are good. It has been widely used in artificial intelligence, portable equipment, integrated imaging, three-dimensional imaging, beam homogenization, beam shaping and other fields that require small-sized array optical elements. In this paper, circular aperture gradient index plane microlens arrays with different diameters are fabricated by ion-exchange technology and photolithography. By sampling in 6 time intervals during the ion-exchange process, the ion-exchanging depth and width of microlenses with different aperture sizes, the focal length, distortion and numerical aperture of lens elements were measured. The ratio of ion-exchange width to depth decreases with the increase of ion-exchange time, and the average diffusion rate of ions in the z-direction and r-direction gradually decreases. Moreover, that decreases faster for the flat microlens array with small opening diameter. As the ion-exchanging time increases, the focal lengths of the microlens arrays with two apertures gradually become shorter, and the focal lengths of the smaller aperture diameters are relatively shorter. The numerical aperture of the planar microlens arrays with different aperture diameters increases while the ion-exchange time increases, and its distortion decreases with the increase of ion exchange time. The rule that the optical characteristics of the gradient index planar microlens array changes with the ion-exchange time provides a reference for the production of the planar microlens array required in different optical systems.
Acta Photonica Sinica
  • Publication Date: Jan. 25, 2022
  • Vol. 51 Issue 1 0151121 (2022)