Special Issue for the 60th Anniversary of XIOPM of CAS, and the 50th Anniversary of the Acta Photonica Sinica|17 Article(s)
Review of the Development of Interferometric Spectral Imaging Technology(Invited)
Bingliang HU
Interferometric spectroscopic imaging technology has undergone decades of experience accumulation and technological development. At present, it has extensive and mature applications in the fields of astronomical exploration, atmospheric pollution, water environment monitoring, surface geological mineral exploration, vegetation survey and other fields. Compared with dispersive and filter-type spectral imaging techniques, interferometric spectroscopic imaging technology has the advantages of high resolution, high sensitivity, and high wave number accuracy. According to the type of optical path acquisition method, this paper summarizes the research status of interferometric spectroscopic imaging technology at domestic and abroad from three aspects: time modulation, spatial modulation, and spatio-temporal joint modulation. Then introduces and reviews their representative research results. For time-modulated interferometric spectroscopy imaging, the interferogram acquired by the detector can be seen as a collection of time series. Interference maps are acquired one by one over time. The main advantages of this technique are the high spectral resolution and detection sensitivity. However, this type of optical system requires a continuously moving precision part to produce an optical path difference that changes over time. The main technical difficulty of time-modulated interferometer spectroscopy is how to develop a set of stable, reliable and long-working high-precision moving mirror scanning system. Spatial modulation interferometric spectroscopy records interference information at different cell positions of the detector for different optical path differences of the measured target. A complete interference map of the target can be obtained with a single exposure. This technology fundamentally overcomes the problem of precision moving mirror scanning system in time-modulated spectrometers, and also improves the real-time performance of obtaining spectral information. The essence of spatiotemporal combined modulation interferometer spectroscopy is to insert a transverse shear interferometer into the camera system. Since there is no slit in the front optical system, this type of instrument not only has the characteristics of high detection sensitivity, high stability and high signal-to-noise ratio, but also has the advantages of high throughput. It can be seen that different modulation methods use different optical path structures. Of course, each spectroscopic principle also has its advantages and disadvantages, which can be applied to different application areas. The research of interferometric spectroscopy imaging technology has always attracted much attention. There is no doubt that the emergence of relevant new technologies is often very eye-catching. Over the past three decades, interferometric imaging spectroscopy technology has been rapidly developed in the field of remote sensing, and has gradually become an effective tool for high-resolution remote sensing detection.With the rapid development of detector focal plane array, precision machinery, high-speed data transmission and storage compression, data quantification and computer image processing, the future of spectral imagers will have a large field of view, wide spectral range, high sensitivity, high spectral resolution, high spatial resolution and other performance. Moreover, the technology gradually tends to develop in the direction of new principles, integration, automation, wireless, intelligence, single cylinder and miniaturization.
Acta Photonica Sinica
  • Publication Date: Jul. 25, 2022
  • Vol. 51, Issue 7, 0751401 (2022)
Progress in Rotating Mirror Ultra-high Speed Imaging Technology(Invited)
Jingzhen LI
The ZDF-24 half-circle waiting rotary camera, making outstanding achievements in the first atomic bomb explosion test in China, is the ground breaking work of Xi'an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences to enter the world field of vision, and the cornerstone of the development of ultra-high speed rotary camera. As you know, 80% of the rotating mirror ultra-high speed cameras in China are originated from Xi'an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, which is the hometown of developing the rotating mirror ultra-high speed cameras in China. The rotating mirror ultra-high speed imaging technology has the advantages of large picture, large number of pictures, high spatial resolution, wide spectral band, wide frequency and convenient operation, so rotating mirror cameras have been the mainstay of mega-frame per second imaging for decades. There is still no electronic camera that can match a film based rotary mirror camera for the combination of frame count, speed, resolution and dynamic range. This paper discusses the research progress of the topology of its information theory, an exploration of rotary mirror dynamics, the modern design theory of rotary camera and the waiting type simultaneous framing and streak imaging system, so called “the pearl in the crown”. It is expected to explain why China can become a powerful country in rotary mirror ultra-high speed photography.
Acta Photonica Sinica
  • Publication Date: Jul. 25, 2022
  • Vol. 51, Issue 7, 0751402 (2022)
Ultrafast Terahertz Characteristic Spectroscopy Based on Femtosecond Laser and Its Application(Invited)
Wenhui FAN, Hui YAN, Xiaoqiang JIANG, Longchao CHEN, Zhuanping ZHENG, Jia LIU, Hui LI, Ling DING, and Chao SONG
Terahertz (THz) waves (0.1 THz ~ 10 THz, 1 THz = 1012 Hz) locate in the transitional region of the electromagnetic spectrum, between the classical electronics (radio, microwave and millimeter wave) and the photonics (infrared, visible, ultraviolet and x-ray). As a kind of coherent measurement technology in THz frequency range, THz characteristic spectroscopy, with high sensitivity, rapidness and nondestructive testing as well as other unique advantages, has shown an attractive promising application prospect in detection, analysis and identification of biochemical molecules and materials. As the widely used broadband THz wave source, THz Photoconductive Antenna (THz-PCA) can emit broadband THz radiation. Therefore, as one of the promising THz emitters and detectors, THz-PCA has the advantages to overcome the defects confronted by other devices (e.g., low operation frequency, strict working condition and bulk size) and these unique advantages have made THz-PCA become the most commonly utilized THz sources in THz Time-Domain Spectroscopy (THz-TDS). Although a variety of THz-PCAs are commercially available and become indispensable in many practical applications currently, the insufficient radiation THz power still hinder the further development of THz technologies based on THz-PCA. In order to further promote the research interests of THz-PCA, the working mechanism and some new research progress, technical challenges in the process of practical application and strategies of THz-PCA have to be discussed and analyzed. The underlying physical mechanism of the transient response in THz-PCA emitter and detector are investigated, as well as the influence of several parameters including the power intensity of femtosecond pump laser, the laser pulse duration and the carrier lifetime of the substrate material, are also analyzed based on theoretical models, which provide the technical foundation for designing the efficient THz-PCA. Moreover, a plenty of valuable research schemes have been proposed to develop the THz technologies based on THz-PCA in the past decades, including photoconductive materials and structure design of THz-PCA. To be specific, the sub-picosecond carrier life time of photoconductor can be realized by creating a massive density of defects, dislocations and scattering centers in the substrate material. As for structure design of THz-PCA, the previous researches on THz-PCA was mainly focused on the saturation effect at high pump power and the large aperture dipoles, dipole arrays and interdigitated electrodes structures have been investigated during the early stage. In the recent years, as the quick development of micro-nano fabrication technologies, the THz-PCA incorporated with plasmonic nanostructures and all-dielectric nanostructures have also been widely investigated for improving its performances.In this paper, the working principle and development status of THz-PCAs based on ultrashort pulsed laser are introduced, including theoretical models, substrate materials and different structures of photoconductive antennas. Furthermore, with the dramatic development of source and detector components, THz spectroscopy technology has been utilized in various fields such as chemical detection and substance identification, biomedical application and pharmaceutical industry. THz-TDS is the most commonly used technique in current commercial THz spectroscopy, which has attracted wide attention for its spectral fingerprint, high temporal-spatial resolution, noninvasive and nonionizing properties. Various important biomolecules, such as amino acids, nucleobases and saccharides reveal rich absorption features in THz range. It is verified that THz spectral features originate from the collective molecules of low frequency vibration, rotation and weak interaction with the surrounding molecules (hydrogen bonding, van der Waals force, etc.), so they are very sensitive to the molecular structure and surrounding environment. It is a powerful tool to investigate molecular conformation, positional isomerism of functional groups, intermolecular interactions of organic acids and their salts, optical isomerism, etc. However, it is worth noting that the investigated targets are usually in the form of multi-component mixtures in actual scenario. When the spectral features became more complicated, the much broader THz features would be severely overlapped and accompanied by baseline drift in THz spectra. Identification and quantitative analysis of complex multi-component mixtures will become a great challenge for THz spectral analysis. To overcome such problem, a practical strategy has been proposed by combining machine learning methods with THz-TDS for implementation of practical applications. Moreover, another issue worth noting is conventional free-standing spectroscopy measurement devices are hardly adequate for the detection of microgram level or trace substance. Combination of metamaterials and conventional free-standing THz spectroscopy to enhance the sensing signal is a feasible and effective method, which is crucial for the practicability of clinical adoption. Furthermore, some recent progress we have achieved in THz characteristic spectral technology and its applications are also summarized and discussed.
Acta Photonica Sinica
  • Publication Date: Jul. 25, 2022
  • Vol. 51, Issue 7, 0751403 (2022)
Development Trends and Prospects of Snapshot Spectral Imaging Technology(Invited)
Yan YUAN, Anqi LIU, and Lijuan SU
Snapshot spectral imaging technology can obtain the target's two-dimensional (2D) spatial information and one-dimensional (1D) spectral information within a single exposure. Compared with the scanning spectral imaging technology, it reduces the requirement on the stability of the platform and can capture the temporal-spatial-spectral datacube of dynamic targets. Therefore, it has broad application prospects in biological imaging, medical diagnosis, food production monitoring, dangerous substance leakage warning, and dynamic target monitoring. This paper summarizes the data acquisition schemes of typical snapshot spectral imaging techniques, which are categorized into direct and indirect measurement. The direct measurement transfers the multidimensional radiation to the grayscale response directly and builds the one-to-one correspondence between the datacube voxels and detector pixels. Therefore, the number of datacube voxels should be less than the number of detector pixels. The indirect measurement modulates the multidimensional radiation and measures the coupled spatial-spectral information. The datacube voxels have to be calculated based on the indirect measurements. The number of datacube voxels is no longer limited by the number of detector pixels. However, the reconstruction algorithms have high computation costs and render limited performance in real scenarios. There are two main development trends of snapshot spectral imaging technology. On the one hand, extending the dimension of detection can provide a more comprehensive analysis of the target. For example, the snapshot spectral volumetric imaging technology can detect the volumetric target in real time and obtain four-dimensional (4D) data, including three-dimensional (3D) spatial and 1D spectral information. There are two main methodologies to realize snapshot spectral volumetric imaging. One methodology integrates a spectral imaging system with a volumetric imaging system. The two imaging systems work independently and their detection data are merged by post-processing. Common volumetric imaging modalities can be divided into active systems and passive systems. The active systems use auxiliary light sources and are suitable for indoor scenarios, such as structured light imager, laser scanning imager, and Time-of-Flight (ToF) imager. The passive systems, such as stereo imager and light field imager, utilize the parallax to estimate depth, which are less sensitive to ambient light. The other method to perform snapshot spectral volumetric imaging is to modulate the spectral-volumetric data and capture the coupled spatial-spectral measurement in a single exposure. Compared with the independent measurement scheme, the coupled measurement scheme has an advantage in system compactness and robustness. However, with the detection dimension increasing, the resolution of the reconstructed datacube acquired by the direct measurement is limited by the resolution of the detector. Therefore, the indirect measurement of spectral volumetric information is becoming a research hotspot. On the other hand, the snapshot spectral imaging technologies based on direct measurement sacrifice the spatial resolution for the spectral resolution. To improve target detection ability, a lot of algorithms are proposed to enrich the spatial details, which are called spectral image Super-Resolution (SR). There are two solutions to perform spectral image SR. One solution is to capture an additional RGB/panchromatic image with a higher spatial resolution of the same target at the same time, such as using a beam splitter. Then the RGB/panchromatic measurements are fused with the Low-Resolution (LR) spectral image to generate the High-Resolution (HR) spectral image. The fusion method requires additional hardware implementation and the result is sensitive to the spatial alignment error. On the contrary, the single spectral image SR uses the LR spectral image as the input, and the HR spectral image is obtained with no need of auxiliary HR images. Benefiting from the development of deep learning in single image SR, the methods of distributing channel attention in both spatial and spectral dimensions are exploited to increase spatial resolution and keep spectral fidelity at the same time. The spectral image SR networks are expected to directly learn the end-to-end mapping relationship between LR and HR spectral images and have great prospects in the future. In summary, for snapshot spectral imaging technology, the development of technical principles has cutting-edge research significance, and the development of data processing technologies has practical significance for promoting its application in real scenarios.
Acta Photonica Sinica
  • Publication Date: Jul. 25, 2022
  • Vol. 51, Issue 7, 0751404 (2022)
High-resolution Terahertz Computed Tomography Using 3D Printed Diffractive Elements(Invited)
Dayong WANG, Ran NING, Xiaoyu JIN, Lu RONG, Jie ZHAO, Yunxin WANG, and Shufeng LIN
Terahertz waves, located between the millimeter waves and infrared radiation in the electromagnetic spectrum, have unique penetration and imaging properties for diverse application scenarios. Terahertz computed tomography, derived from its ancestor in the X-ray domain, is known as a major transmissive three-dimensional terahertz imaging method to obtain the inner geometrical structure of the sample as well as its three-dimensional distribution of either absorption coefficient or refractive index. In this study, a three-dimensional printing binary diffraction lens is proposed to generate a convergent terahertz beam with a focal depth of 25 mm and a waist diameter of 1 mm, which enhances the resolution and reconstruction fidelity of a 0.3 terahertz computed tomographic system.For this terahertz binary diffractive lens, its focal depth is modulated by sets of concentric rings with standard step height and different integer multiples of step width. The phase delays caused by steps of binary heights are 0 and π, respectively. A simulated annealing algorithm is applied to find the premium structure of this annular phase modulator, in which the full width at half maximum of the focal spot as well as the maximum available depth-of-focus are selected as the constraints. This diffractive lens is constituted by 20 sets of concentric rings with minimum width of 0.5 mm, with a total effective diameter of 25 mm. It is fabricated by a three-dimensional printer with a precision of 0.1 mm using a photosensitive resin material. Its refractive index and transmissive ratio are 1.66 and 82%, respectively. The step height is therefore set to 0.81 mm, corresponding to an optical path difference of half wavelength. In terms of depth-of-focus, both theoretical value and experimental value are approximately 25 mm, the latter of which is measured by an array microbolometer. The theoretical full width at half maximum of the focus spot is 1 mm while experimental one is approximately 0.8 mm. The disparity comes from incomplete coverage of the incident beam on the effective region of the lens. For comparison, the minimum diameter of the focal spot achieves about 2 mm with a very limited depth-of-focus by using a regular polymethylpentene lens to converge the terahertz beam.A terahertz computed tomographic setup is built based on a continuous-wave avalanche diode source and a golay cell detector. Two different samples are fabricated by three-dimensional printing as well using white resin. Either sample can be mounted on a combination of a manually controlled rotation stage and an electric controlled translation stage, in which the annular and lateral interval are 5° and 0.5 mm, respectively. It is assumed that the terahertz beam propagates in a straight line through the sample, and the sinogram is recorded over a rotation of the sample from 0° to 180°. According to Fourier slice theorem, two-dimensional cross-sectional image can be obtained from the sinogram using the filtered back projection algorithm. Three-dimensional reconstruction can be obtained by stacking the cross-sections at different heights. The experimental results validate that a resin pipe with a wall thickness of 1.2 mm can be reconstructed with an average error of 4% by using the proposed binary diffractive lens, meanwhile solid resin cylinders with diameters of 1 mm, 1.5 mm and 2 mm can be clearly recognized inside a chamber made of the same opaque material. Quantitative comparison is conducted using a more conventional terahertz computed tomographic geometry with the regular lens. The experimental results validate that both resolution and fidelity of the reconstructed images, i.e., two-dimensional sectional images and three-dimensional reconstruction, can be significantly improved by using this convergence method. Considering the flexibility and relatively low cost of three-dimensional printing, the proposed method would promote terahertz computed tomography into real applications of non-destructive testing.
Acta Photonica Sinica
  • Publication Date: Jul. 25, 2022
  • Vol. 51, Issue 7, 0751405 (2022)
Position-sensitive Micro-channel-plate Having Cross-delay-line Detector Based Photon-counting Three-dimensional Super-resolution Laser Imaging(Invited)
Hui ZHAO, Haomeng YIN, Yongan LIU, Lizhi SHEN, Xianghui YANG, Gangyi ZOU, Siyu XIA, Mingyang YANG, and Xuewu FAN
Laser-based active detection and imaging is where the space situational awareness heading in the future. The photon-sensitive detector is the core of the photon-counting-based three-dimensional laser imaging system whose working range is much longer and which is capable of realizing both high-precision distance measurement and three-dimensional surface reconstruction at the same time. Therefore, the photon-counting-based three-dimensional laser imaging is especially suitable for realizing ranging and three-dimensional imaging of distant space targets in a deep space background. In this manuscript, the position-sensitive Micro-channel-plate with Cross-delay-line (MCP/CDL) detector-based photon-counting three-dimensional imaging technique is proposed. First of all, the research background of this manuscript is systematically introduced and the contribution of the work reported to the field of photon-counting-based three-dimensional laser imaging is summarized. After that, starting from the characteristics of the MCP/CDL detector itself, the photon-counting three-dimensional laser imaging system by using the position-sensitive MCP/CDL detector is designed and the corresponding operating mode is discussed simply, which gives a reference for real application in future. After that, the principle of the proposed position-sensitive MCP/CDL detector-based photon-counting three-dimensional laser imaging technique is introduced from two aspects. In the one hand, how to use MCP/CDL detector to realize the synchronous timing and positioning of arriving photon is qualitatively explained. On the other hand, with the help of the classical LIDAR equation, the echo power model and the echo photon model generated from the echo power model are given respectively. Based on the two basic models, three important models including the signal to noise ratio model, detection probability model, and ranging accuracy model are derived one by one, based on which the performance of the proposed technique is investigated numerically. After establishing the all-chain imaging model, the potentials of this technique in space-borne space targets monitoring are investigated through end-to-end simulated imaging and performance analysis. By using the Monte Carlo simulation method, the simulated photon-counting three-dimensional laser imaging is carried out with different conditions and in this way, the potential performance of this system in space situational awareness is demonstrated vividly. However, nowadays, most photon-counting detectors including the MCP/CDL detector have a low spatial resolution. In this case, the ranging imaging has a strong mosaic effect which is hard to resolve finer details. Besides that, the ranging accuracy is mainly determined by timing electronics. Considering these two situations, how to improve the spatial and temporal resolution is also discussed. Taking the finer details provided by the higher resolution intensity image as reference, the spatial resolution of ranging images could be improved prominently. At the mean time, a controllable time delay is introduced to realize super-sample in ranging direction and the higher ranging accuracy could be obtained by fusing multiple range images. According to numerical simulations, the potential of this system in realizing super-resolution both in the spatial domain and in the temporal domain is demonstrated. Finally, the prototype camera using MCP/CDL detector is designed, tested and fabricated. By using the prototype camera, two groups of three-dimensional laser imaging experiments are carried out. The results demonstrate that this technique has the capability in resolving small distance variation of being less than 5 mm when the imaging distance is about 6.8 m. Therefore, the position-sensitive micro-channel-plate having cross-delay-line based photon-counting three-dimensional laser imaging is proven to be effective.
Acta Photonica Sinica
  • Publication Date: Jul. 25, 2022
  • Vol. 51, Issue 7, 0751407 (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.
Acta Photonica Sinica
  • Publication Date: Jul. 25, 2022
  • Vol. 51, Issue 7, 0751412 (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.
Acta Photonica Sinica
  • Publication Date: Jul. 25, 2022
  • Vol. 51, Issue 7, 0751415 (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.
Acta Photonica Sinica
  • Publication Date: Jul. 25, 2022
  • Vol. 51, Issue 7, 0751408 (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.
Acta Photonica Sinica
  • Publication Date: Jul. 25, 2022
  • Vol. 51, Issue 7, 0751410 (2022)