Sunflower seeds, recognized for their nutritional value and taste, are a well-loved snack. However, throughout their growth and storage, sunflower seeds can develop various defects that not only compromise their quality but also present potential health hazards. To address these issues and ensure adherence to safety standards, we investigate the use of THz spectroscopy and imaging techniques for non-destructive identification and classification of common defects in sunflower seeds. The study begins by analyzing spectroscopy features to identify defective seeds, particularly those affected by mildew. It establishes three qualitative discrimination models (support vector machine, random forest, and backpropagation neural networks), which achieve overall accuracies of 88.3%, 91.7%, and 95%, respectively. Furthermore, THz transmission imaging is employed as a quantitative method to visualize the internal structure of sunflower kernels and provide precise plumpness estimates. A noteworthy innovation is the analysis of time delays in reflected pulses at each pixel, enabling the extraction of valuable kernel thickness information. These data are then utilized to convert traditional two-dimensional scanning data into intricate three-dimensional (3D) images, facilitating direct measurements of both 3D plumpness and kernel weight. The findings have significant implications for improving the quality and safety of sunflower seeds and may extend to the assessment of other agricultural products, contributing to enhanced quality control in the food industry.

The distributive configuration of cooperative target is one of the important factors affecting the accuracy of pose measurement with monocular vision. In this paper, we propose a cooperative target configuration optimization method based on particle swarm optimization (PSO) to achieve a more accurate pose solution. First, the mathematical relationship between the distributive configuration of cooperative targets and measurement accuracy is derived based on the Perspective-n-Point (PnP) principle, meanwhile, the necessity for the distributive configuration optimization of cooperative targets is also demonstrated. Then, with the help of a pose solving algorithm based on angle parameterization, the PSO is adopted to construct the objective function and design the corresponding parameters. Next, the global optimal distributive configuration of cooperative target can be obtained by multiple reiterative methods, and the mathematical relationship is given between the cooperative target distributive configuration and the pose solution error. Finally, the feasibility and effectiveness of our method are verified by simulation and physical experiments. Compared to the random or artificially set cooperative target configurations, the proposed optimal configuration method improves the accuracy of pose measurement by 20%.

Image detail enhancement is critical to the performance of short-wave infrared (SWIR) imaging systems. Recently, the requirement for real-time processing of high-definition (HD) SWIR video has shown rapid growth. Nevertheless, the research on field programmable gate array (FPGA) implementation of HD SWIR streaming video processing architecture is relatively few. This work proposes a real-time FPGA architecture of SWIR video enhancement by combining the difference of Gaussian filter and plateau equalization. To accelerate the algorithm and reduce memory bandwidth, two efficient key architectures, namely edge information extraction and equalization and remapping architecture, are proposed to sharpen edges and improve dynamic range. The experimental results demonstrated that the proposed architecture achieved a real-time processing of 1280 × 1024@60Hz with 2.7K lookup tables, 2.5K Slice Reg, and about 350 kb of block RAM consumption, and their utilization reached 12.5%, 19.2%, and 12.5% for the XC7A200T FPGA board, respectively. Moreover, the proposed architecture is fully pipelined and synchronized to the pixel clock of output video, meaning that it can be seamlessly integrated into diverse real-time video processing systems.

We report the use of noncontact optical interferometric microscopy for surface topographic profiling. A Linnik optical configuration is used to create interferograms of an object’s surface. When the coherence condition between the two beams is met, the phase information of the object’s surface can be extracted to establish its topographic contour image. Using four π/4 phase-shifted images, with the synchronization of a piezoelectric actuator reference mirror positioning, the present single camera optical interferometric microscope can reconstruct a surface topography with a field of view of 312 μm×234 μm, axial resolution of 3.31 nm, lateral resolution of 488 nm, and maximum topography height of 270 nm.

Single-shot non-invasive imaging through a scattering medium has been an area of research focus for several years. Achieving an accurate autocorrelation distribution is crucial for effective image reconstruction. However, the obtained autocorrelation is always contaminated by various types of noise. In this study, we propose an effective method to extract the autocorrelation distribution of the sample from the noise-laden raw data before image retrieval. This is accomplished by incorporating an intensity threshold and a spatial Gaussian filter into the retrieval algorithm, which effectively eliminates the noise artifacts in the reconstructed image. The proposed algorithm exhibited remarkable robustness for both visible (laser, white light, and light-emitting diode light) and X-ray light sources, and its efficacy was verified through multiple experiments.

We present a method for precision intrinsic calibration of pinhole-model cameras. We refer to this method as “3-Axis.” The algorithm employs a target moving with known displacements along three perpendicular axes to decrease the number of unknown terms that must be determined. This method along with new evaluation strategies for comparing the accuracy of pinhole camera calibration algorithms is described in detail. The 3-Axis approach is then compared with the current state-of-the art algorithms in simulated and physical settings and is shown to exceed them in accuracy and computational performance assuming an equal number of samples acquired.

Optical coherence tomography (OCT) offers high-resolution, three-dimensional (3D) imaging of semi-transparent media, including biological samples, in a swift manner. Its operation in the near-infrared spectrum ensures non-invasive and non-destructive imaging, making OCT advantageous for diverse applications spanning biology, biomedical science, agriculture, and material science. We introduce a compact OCT microscope prototype tailored for field assessments in aquatic animal study. Specifically, it is used to evaluate pesticide exposure in freshwater fish. The prototype, comprising exclusively commercial optical components, achieves imaging depths up to 2 mm from a sample’s surface at a rate of 20 frames per second and boasts resolutions of 10 μm both laterally and in depth. We demonstrated its utility by imaging African catfish exposed to the herbicide 2,4-D dimethylammonium, comparing these with control samples. We performed in vivo 3D imaging, using the developed prototype, on each catfish throughout its hatching and embryonic development period, comparing the 2,4-D exposure with the control. The results show significant deviations in the exposed group from the typical developmental patterns observed in the control group. This confirms the capability of OCT to assess the effects of herbicides on aquatic organisms.

Raman spectroscopy has emerged as an essential technique for material composition analysis due to its noncontact, nondestructive, and rapid characteristics. Spatial heterodyne spectroscopy offers the advantages of high stability, high throughput, and ultra-high spectral resolution, making it particularly suitable for Raman spectroscopic detection. The main design parameters of a spatial heterodyne Raman detection system were determined under near-infrared wavelength excitation: a Raman shift detection range of 300 to 2000 cm−1 and a spectral resolution of 5 cm−1 at an excitation wavelength of 785 nm. The optical design of the dispersion module and imaging module was completed. Test results demonstrate that the spectrometer achieves a spectral resolution of 5.33 cm−1 and can detect Raman shifts in the range of 315 to 2131.6 cm−1. Verification tests on cyclohexane and lipstick samples confirm that the system exhibits excellent fluorescence suppression capability, with a signal-to-noise ratio of the cyclohexane Raman peak reaching 1600.8.

This paper proposes a colorimetric characterization method based on support vector machine (SVM) for computer-controlled liquid crystal displays (LCD). First, forward and inverse colorimetric characterization models based on SVM are established for LCD monitors. Second, the radial basis function (RBF) kernel function is introduced to deal with the nonlinear problem of color space conversion. Then, the parameters Gamma, penalty coefficient C, and ε are determined by 10-fold cross-validation grid search optimization. Next, the effectiveness of the method is validated by three brands of monitors; for the forward colorimetric characterization model, the average predicted color differences are 1.259, 1.091, and 1.005 CIELUV units, respectively; for the inverse colorimetric characterization model, the average goodness of fit are 0.985, 0.989, and 0.988, respectively. Finally, the method is compared with piecewise linear assuming variation in a chromaticity model, three-dimensional look-up-table, polynomial regression model, and artificial neural network model, and it is verified that the method is superior to the related colorimetric characterization models in color prediction accuracy. The method can better support the color management and high-fidelity reproduction of monitors.

The conventional two-dimensional (2D) digital image correlation (DIC) method often fails to work when there is a large rotation between the reference and deformed images. The objective of this study is to propose a 2D DIC method to tackle this issue and calculate the full-field rotation angle. In our method, the deformed subset is chosen along the direction of gradient orientation angle error (GOAE) between the pseudo point of interest and the searching point. In addition, the initial deformation parameters for the inverse compositional Gaussian–Newton (IC-GN) method are set based on the GOAE to guarantee its convergence. Furthermore, a best fit method is presented to calculate the rotation angle of each target deformed subset using the converged deformation parameters from IC-GN method. Moreover, due to the high computation intensity of the proposed method, a multi-thread parallel computation technique is used to speed up its computation. Simulation and bending experiment results show that the proposed method can measure both the full-field in-plane displacement and rotation angle with relatively high accuracy and efficiency in the case of a large rotation, in comparison with the conventional DIC method and a ring template-based method. The proposed method is also robust to the strained deformation and image noise to some degree.

The current measurement equipment, which adheres to the ANSI Z87.1 standard measurement method, features an extended path length of nearly 11 m. This extended length poses challenges for laboratories or testing units in terms of space utilization and significantly impacts the energy configuration required for accurate detection. The initial phase of this study focuses on automation, entailing the development of a measuring device equipped with full automation capabilities. This device ensures the precise measurement of diopter and astigmatism data at a consistent and efficient pace. The subsequent phase aims to minimize the distance between the objective lens and the target without altering the fundamental principles of the original diopter testing system. This reduction is achieved through the utilization of optometry automatic detection technology. Concurrently, to complement the scaled-down detection equipment, precision optical targets are manufactured using semiconductor standard photomask fabrication techniques. Results indicate a substantial reduction in equipment size to <1 m, retaining the same level of precision. This reduction significantly economizes the installation space required for the instrument. The implementation of automated measurements minimizes errors stemming from human judgment and allows the device to be employed in the assessment of various safety lenses.

A principle of a wavefront measurement sensor based on computer generated holograms is presented. In contrast to conventional Shack–Hartmann sensors, the wavefront is not sampled using a lens array but an array of subholograms. Each subhologram generates a multipoint pattern with unique point distribution on the camera fulfilling two tasks. First, by calculating the wavefront reconstruction for each of the multiple points, averaging improves the accuracy compared to a single point evaluation. Second, the dynamic range of the proposed sensor is enhanced due to the unique shape of each pattern. The assignment between pattern and subhologram can be achieved even for overlapping patterns by the use of digital image correlation. First results show that the averaged standard deviation is 12.5 nm, which corresponds to less than λ/50, and the maximum angle of the incident wavefront is 8.7 deg. The sensitivity related to the aperture size is better than λ/126.

We present a nondestructive sensor system to assess meat freshness. The quality of meat is affected by the microbial population, which is associated with CO2 gas emission and the color of the meat. The proposed system comprises nondispersive infrared (NDIR), fluorescence, and color sensory technologies for capturing the CO2 gas and color of the meat. The NDIR source and receiver are procured at 4.3 μm, and the fluorescence film is produced with the 8-hydroxypyrene-1,3,6-tri sulfonic acid trisodium salt dye for CO2 sensing. A TCS 3200 color sensor is employed to analyze the color changes during meat spoilage. The optoelectronic hand-held system is made with light sources, detectors, signal conditioning boards, and a display unit; it monitors the freshness of chicken, fish, and goat. The response characteristics of the thin film, such as the stability, response time, and reproducibility, are evaluated before they are applied for CO2 monitoring. The NDIR values and fluorescence sensor data along with the color information improve the quality inspection of the meat. These sensors offer several advantages, such as a high sensitivity, fast response time, minimal sample preparation, miniaturization, and ability to determine the meat spoilage level.

Dynamic goniometers (DGs) are used to calibrate optical angle encoders with high accuracy over a wide range of rotation speeds. The dynamic mode of operation of the goniometer causes an additional error. A current task is to research this dynamic error. The systematic component of the DG’s error can be eliminated by introducing corrections to the measurement result. Thus the main factor limiting the accuracy is a random error, including non-stationary components. For its analysis, the Allan variation method, Fourier, and wavelet analysis were used. Experimental studies of the DG with ball bearing and reference optical angle encoder at various rotation speeds have been carried out. Arrays of random variables characterizing the DG’s random error were studied; they are non-stationary in terms of the average value. Using the Allan variation, the sinusoidal and white noise were determined. Fourier analysis revealed the presence of frequency errors in the random error, which are caused by the processes of movement of the bearing components, as well as a defect in the separator. Wavelet analysis made it possible to clarify the nature of the processes, to trace the change in the frequency composition over time. When constructing a DG for calibrating encoders, special attention should be paid to the quality of ball bearings. It has been determined that dynamic errors make a significant impact on the measurement error. The ball bearing, namely the defect of the bearing cage, is the major reason that causes non-stationary effects at low frequencies. As studies have shown, ball bearings increase the random error of the DG by an order of magnitude or more compared to its potential accuracy, which is significantly <10−2″.

Accurately assessing the microvibration effects on the line-of-sight (LOS) stability for the space laser communication terminal (LCT) is crucial for ensuring the long-term stability of communication links. To address this challenge, an integrated modeling approach is proposed to analyze the microvibration effects on the LOS stability for the space LCT. First, the LCT’s finite-element model, consisting of the off-axis telescope antenna and the rear optical path, is built to extract the nodes’ data of each surface on the optical element under the microvibration. Then the dual quaternion, which could avoid the solution error and insufficient generalization ability of the traditional least squares method, is used to calculate the surface’s rigid body displacement on the optical element. Furthermore, combined with the LCT’s optical sensitivity matrix obtained by the regression analysis method, the LOS jitter of the terminal is calculated to evaluate the LCT’s stability and guide the structural optimization. Finally, a test is conducted to validate the reliability of the integrated modeling approach. Simulation and experimental results show that the method proposed can further improve the accuracy of analysis and guide the structural optimization of LCT.

We describe the state of the development of a coherence scanning interferometer to measure local changes in topology and local induced vibrations of a mirror at cryogenic temperatures. The metrology instrument incorporates an optical phase mask and a microlenses array, enabling the acquisition of complete white light interferograms within a single-camera frame. This stands in contrast to traditional temporal phase-shifting interferometers. We design the optical phase mask as a combination of steps of different thicknesses, so each step introduces a different optical path difference to the rays. The local interferograms for each camera frame provide us with information on the local topology of the mirror. The interferogram displacement between camera frames allows us to monitor the mirror’s local induced vibrations. In this work, we report the metrology instrument’s working principle through numerical simulations and present the latest results of a proof of concept developed at the laboratory. The metrology instrument shown is of extensive usability in diverse applications related to real-time measurements of various fast physical processes and real-time characterization of the optical components topology.

Small unmanned aerial systems provide a versatile platform for covering large areas quickly. By adding sensors to drones, imagery of large areas can be taken for a variety of applications. Traditionally, single fixed staring systems or gimballed sensors are used to take this imagery. Both options require a compromise between field of view (FOV), resolution, scanning speed, and flight path to properly perform the desired task. If more than one sensor is integrated onto the drone, a wide FOV can be covered without a scanning gimbal and obtain higher resolution than a traditional wide FOV staring system. Presented is a multi-camera design approach based on a constraining ground sample distance for a wide area coverage (WAC) system. This design approach can be used for any imaging bands. A figure of merit (FoM) is derived to quantify and compare the performance of the WAC systems in the visible (0.4 to 0.7 μm), short wave infrared (1.0 to 1.7 μm), and longwave infrared (8 to 14 μm) for both good and bad visibility conditions. The performance of three optimized and fabricated WAC systems is compared and tested through the FoM and flight testing.

Since the magneto-optical Kerr effect plays an important role in the technology of detecting thin film magnetism, and the current detection technology has problems such as poor magneto-optical Kerr signals as well as large frequency of change, how to enhance the Kerr response of magneto-optical materials has become an important issue in today’s scientific research. A three-layer plasma hybrid microstructure covered with “L”-shaped periodic aluminum blocks aiming for Kerr response enhancement is proposed. In the near-infrared wavelength range of 0.6 to 1.1 μm, by varying the size of the opening and width of the upper aluminum film, a maximum polar magneto-optical Kerr rotation angle of 11.08 deg was obtained in a bismuth iron garnet film with thin films and accompanied by a high reflectance. The effects of different opening sizes, widths, incidence angles, magnetic field sizes, metal materials, and magneto-optical materials on the structure were also investigated. The results show that the structure can both increase the Kerr rotation angle by adjusting the magnetic field and incidence angle as well as combine different plasma metals to enhance the magneto-optical Kerr effect, which improves the universality of engineering applications. In addition, the electric field contours at the intersection of different materials are given and the physical mechanism of the enhanced polar magneto-optical Kerr rotation effect is explained. The results of this work will provide some guidance and help for the amplification of experimental measurement signals of the Kerr effect and the design of tunable magneto-optical Kerr devices.

Precision in optical micromanipulation is critical for non-invasive, non-contact control of objects with light and for using light to measure forces and torques. We demonstrate the control of birefringent objects over three translational directions and three rotational degrees of freedom. Our work trapping and rotating calcite, a unique crystal with no axis of symmetry, extends the positioning abilities of optical tweezers. We trap and levitate a single crystal in linearly polarized light and position it in three rotational directions relative to the polarization. Our model of the torque on the crystals is based on determining the induced polarization and its interaction with the incident electric field. Crystals are shown to rest in equilibrium with their optic axis along the beam axis and one extraordinary axis along the direction of polarization. The calculated equilibrium positions align with our observations of stable trapped calcite.

An asymmetric waveform signal generation scheme based on dual polarization multiplexing-quadrature phase shift keying (PM-QPSK) modulator is proposed and analyzed. The key technique to generate periodic asymmetrical function waveform is how to control the amplitude and phase of the Fourier series of the target waveform simultaneously. In order to achieve this function in this proposal, a continuous-wave laser is modulated by radio-frequency signals with various phases, using a PM-QPSK modulator. The orthogonal sine and cosine harmonic signals are output through a polarization beam combiner. A polarizer is then employed to balance the power of these two orthogonal signals. By properly adjusting the variables, both the amplitude and phase of the Fourier series of the target waveform can be simultaneously set. When the root mean square error is <5%, a half duty cycle ramp wave signal with adjustable slope (20%≤δ≤80%) can be obtained. In addition, the influence of variable drift on the target waveform is analyzed in detail.

A fast interrogation scheme for fiber Bragg grating (FBG) using Kalman filter (KF) algorithm is presented. The resolution of the system using KF algorithm is tested, and the dynamic interrogation of the system is verified using vibration signals. Meanwhile, different degrees of static pressure were applied to the FBG, and the static interrogation of the system is verified. Experimental verification shows that the Bragg wavelength resolution of the proposed algorithm can reach 0.3 pm and is relatively stable as the wavelength resolution of the system decreases. In addition, the antinoise performance of KF algorithm is compared with that of direct peak search algorithm. Compared with other commonly used FBG peak search algorithms, the innovative algorithm has the characteristics of low computational cost, high sensitivity, and stability.

With the development of optical phased array (OPA) chips, it is urgent to design an optical system for detecting OPA chips. We propose a near- and far-field common optical path detection system adapted to commercial cameras. By changing the focal length of the rear set of lenses, the optical system can be used to realize the near-field detection, far-field field of view (FOV) detection, and far-field resolution detection of OPA chips. The object spatial resolution of the near-field system is 1.11 μm, and the far-field system can achieve ±45 deg FOV detection and 0.0185 deg angular resolution. Simulation results show that it can be used to characterize the far-field spot with 0.1 deg resolution. The optical system is designed with spherical lenses, and its imaging performance approaches the diffraction limit.

The maximal output fluence of high-power laser systems remains limited by KH2PO4 (KDP)-based optical components used to convert the infrared pulses to 3ω pulses. The correlation between optical defects (scales of a few micrometers) on KDP crystal surfaces and damage initiation was systematically investigated by combining laser-induced fluorescence (FL) and light-scattering microscopies incorporated an in-situ damage testing facility (wavelength at 355 nm and pulse width of 7 ns). We demonstrated that the surface defects featuring both FL and light-scattering have a high propensity to damage. The scattering feature of the FL precursors was found to be a key factor for damage initiation, whose physical mechanism was thoroughly revealed. The results further implied that the removal of such damage precursors can greatly improve the damage resistance of KDP-based crystals. Considering that this combined imaging technique is noninvasive, the results establish this methodology as an attractive and non-destructive tool for evaluating and improving the surface damage performance of KDP-based crystals used in high-power lasers.

A highly sensitive flow rate sensor based on high-order whispering gallery mode (WGM) in an optofluidic microcavity is proposed and experimentally demonstrated. The microfluidic flow rate can be measured by detecting the shift of the resonance wavelength of the high-order WGM, which is generated by coupling the ultrafine fiber cone with the microfluidic microcavity. The detectable flow rate range is 4 to 100 μL/min and is segmented into two parts according to the sign of the resonance wavelength shift. The resonance wavelength for the flow rate of 4 to 16 μL/min is blue-shifted with a sensitivity of 48.8 pm/(μL/min), and the resonance wavelength for the flow rate of 16 to 100 μL/min is red-shifted with a sensitivity of 26.3 pm/(μL/min) with the heating temperature set to 55°C.

We report the high repetition frequency dual-wavelength pulse laser output of Pr3+:YLF at 604 and 639 nm. We discuss the relationship between the threshold powers of the dual-wavelength laser with the insertion of a Fabry-Perot etalon and numerically solve the corresponding four-level rate equations. Based on the results, we designed and conducted the experimental research. Then we obtained dual-wavelength pulse laser output at different repetition frequencies. When the repetition frequency is 40 kHz and the maximum absorbed pump power is 2.723 W, the output power at 604 and 639 nm are 27 and 25 mW, respectively. This study provides a simplest method for direct output of dual-wavelength visible pulsed laser.

A tapered-optical fiber tip coupler for the excitation of capillary whispering gallery modes (WGMs) is proposed. The phase matching theory and spectral features are analyzed. The light transmission characteristics and cross-sectional mode fields of the coupler are simulated. The impact of fabrication tolerance on excited WGM is studied, and the structure is fabricated based on the simulation results. It has been experimentally verified that this structure can excite the WGM resonance of capillaries of various sizes, with the highest quality factor (Q factor) of 3.6×103 and refractive index sensitivity of 22.28 nm/RIU. The resonant coupler is not fragile, large fabrication tolerance, and can excite WGM in capillaries of different sizes, which can provide a new means of detection in the field of capillary biochemical sensing.

In a coherent Doppler wind lidar detection system, heterodyne efficiency is an extremely important parameter that reflects the matching degree between local oscillator (LO) light and signal light. More generally, assuming that both the LO light and signal light are partially coherent beams based on the Gaussian–Schell model, we have derived an analytical expression for the heterodyne efficiency in the presence of tilt aberration. The expression indicates that the heterodyne efficiency not only depends on the tilt aberration but also is influenced by the beam parameters, misalignment angle, detector diameter, and wavelength. Specific examples of the variation in heterodyne efficiency with these factors are given in the simulation section. The numerical results show that if high heterodyne efficiency is required, the intensity width of the LO light does not have to be large when the intensity width of the signal light is determined; besides, setting ws/wLO=0.5 is appropriate, where ws and wLO are the intensity width of signal and LO, respectively. Even if LO is a partially coherent beam, the heterodyne efficiency can still reach the global maximum of 0.86, and setting δs/δLO=0.6 is suitable, where δs and δLO are the coherence width of the signal and LO, respectively. As expected, the bigger the δs is, the higher the heterodyne efficiency is. For the same heterodyne efficiency, the tilt aberration can be larger slightly when the misalignment angle is small. We also find that the detector diameter plays a significant role in enhancing the heterodyne efficiency, and it is beneficial to choose a small detector diameter. Besides, the tilt aberration in the system should be less than 0.5λ. Moreover, we show that the rate of decrease in heterodyne efficiency with the tilt aberration is different for different misalignment angles. We also have demonstrated that the longer the wavelength, the higher the heterodyne efficiency for system with the same tilt aberration.

The chikungunya virus (CHIKV), transmitted by mosquitos, induces severe symptoms such as joint pain, headaches, and exhaustion, necessitating swift diagnosis and intervention. In this investigation, the transfer matrix method is employed to enhance virus detection using a ternary photonic crystal (PC) biosensor across normal and infected blood constituents—plasma, platelets, red blood cells, and uric acid. These components possess distinctive refractive indices crucial for the biosensor’s selective CHIKV detection. A peak sensitivity of 800 nm/RIU at N=4 is achieved by analyzing PC transmittance spectrum curves in the presence of diverse blood elements to emulate real-world conditions. Optimal average sensitivity at 1037.5 nm/RIU is observed at incidence angles of 30 deg, 55 deg, and 80 deg, with 30 deg yielding the highest quality factor of 1015.94. This PC-based biosensor exhibits promise for compact, highly sensitive, and precise virus detection, potentially advancing viral detection biosensor technology and enhancing diagnostic tools for infectious diseases.

We introduce an approach for creating optical filters with narrow bandwidth and high transmittance within the visible light spectrum. By combining the interferometric properties of the film system and the diffraction properties of the sub-wavelength grating, we have designed a bandpass filter based with a two-dimensional subwavelength metal grating structure and an Al₂O₃-air bilayer coating. The Al₂O₃-air bilayer coating not only suppresses random scattering leakage in the short-wavelength range but also maintains the relative stability of the central wavelength position, which lead to reduced sidebands and increased transmittance of resonance peaks. Moreover, to reduce electromagnetic energy loss in the waveguide layer, we optimized the thickness of the buffer layer and introduced a bottom Al₂O₃ layer, enabling the filter to achieve a low full width at half-magnitude. By adjusting the grating period, the filter achieves narrow bandwidth and high transmittance resonant peaks across the entire visible spectrum. The proposed approach presents a metal grating-based guided-mode resonance transmissive filter, exhibiting a low sideband of 23%, high transmittance of 85%, a narrow bandwidth of less than 5 nm, and tunability across the entire visible light spectrum. The proposed design is experimentally validated and the results demonstrate that this method is effective in achieving excellent filtering performance in the visible spectrum.

Aluminum-rich p-AlGaN electron-blocking layers (p-EBL) usually block the hole injection in the AlGaN-based deep ultraviolet (DUV) light-emitting diode (LED). Meanwhile, the ionization energy of the magnesium receptors in the aluminum-rich p-AlGaN layer is considerably high, and the conductivity is low, leading to a reduction in the hole injection capability. Consequently, AlGaN-based DUV LEDs suffer from a low internal quantum efficiency (IQE). A structure of p-EBL and hole source layer (p-HSL) using AlN composition gradient AlGaN is proposed. The polarization bulk charges are introduced in both p-EBL and p-HSL (the p-type dual polarization) to shield the polarization-induced electric field and increase the hole energy. Although the single-polarized structure of p-EBL or p-HSL can also effectively enhance the performance of the device, calculation results indicate that the performance of the p-type dual-polarized structure can be doubled. This is because optimizing a single p-EBL or p-HSL can only introduce polarization bulk charge in the monolayer but not in all p-type regions. Therefore, in LEDs with shorter wavelengths, to maximize the hole energy, a dual-polarization structure should be adopted for the p-type region. Based on this, a maximum IQE of 55.65% and an efficiency droop of 4.84% are obtained at a wavelength of 237 nm. Findings of this study provide insights to solve the problem of low IQE in high-performance DUV LEDs.

Based on the ITO-HfO₂-graphene stacked layers, a modulator that can achieve independent modulation of TE mode and TM mode within a single silicon waveguide is proposed. Graphene and ITO are employed as electrodes of graphene-HfO₂-ITO capacitor. Under applied voltage, carriers accumulate within the 5 nm thick ITO film, leading to a modification of the optical dielectric constant. Using ITO’s epsilon-near-zero behavior and its sensitivity to optical polarization, a modulation depth (MD) of no <2.5 dB/μm and an insertion loss of less than 0.054 dB/μm are achieved at the 1.55 μm. Furthermore, when modulating TE and TM modes independently, the mode crosstalk between the two modes is <0.02 dB/μm, demonstrating the high performance of the polarization-multiplexing (PM) modulator. At near-infrared band of 1.35 to 1.6 μm, the broadband properties of ITO and graphene are used to achieve an MD of no <2.2 dB/μm while maintaining mode crosstalk and insertion loss of less than 0.06 dB/μm. This modulator has robustness to top Si waveguide sidewall angle and compatibility with complementary metal oxide semiconductor processes. It is expected to be used in highly integrated polarization de-multiplexing and polarization switching systems.

This study investigated MXene ink as the absorbing material in a wideband millimeter wave (MMW), metasurface absorber and applied the ink in a periodic array. The MXene ink was characterized in the Ka-band, and a Debye permittivity model was found that accurately described the highly frequency-dependent properties. A 1-bit coding-metasurface algorithm was used to discover an optimized design that minimized the average reflectance at normal incidence, utilizing the unique properties of the materials. The absorber was fabricated using hybrid processing techniques and measured at multiple incident angles to compare against the simulated design. The absorber demonstrated a wideband response (i.e., 14 GHz bandwidth) at normal incidence with a greater than 93% absorptance from 26 GHz to 40 GHz and an average reflectance of 2% over the entire band. Furthermore, the absorber tested at a 10-deg incident angle demonstrated a greater than 91% absorptance at the Ka-band and an average reflectance of 3.4% across the band; Tested at a 20-deg incident angle, the absorber demonstrated a greater than 88.5% absorptance and an average reflectance of 3.9% across the band. The study established the value of using MXene ink in metasurface absorbers for wideband applications in MMW frequencies.

Drawing upon the principles of digital coding metasurfaces, we propose the double-split ring resonators coding metasurface that can switch between different frequency bands with varying amplitudes. Under transverse-magnetic (TM) polarized terahertz wave incidence, the same metasurface meta-atom structure utilizes amplitude coding to achieve spatial imaging functionality. Due to limitations in device size and fabrication techniques, actively controlled coding metasurfaces in the terahertz frequency range are rare. In order to enhance the flexibility of terahertz coding metasurfaces, a phase-change material, vanadium dioxide (VO2), is introduced for active control within the metasurface structure. The imaging functionality can be switched by a single switching metasurface, achieved by the phase transition characteristics between the insulating and metallic states of VO2. Therefore, the designed coding metasurface, which is based on the influence of VO2 on amplitude, provides a new approach for flexible control of terahertz waves. It holds great prospects for significant applications in terahertz transmission, communication, and other emerging terahertz-related fields.

In optical wireless communication (OWC), visible light communication (VLC) has shown great potential and attractive performance in indoor environments. However, the limited modulation bandwidth of VLC technologies poses performance restrictions. We propose a new approach for downlink communication utilizing multiple-input multiple-output (MIMO) technology. Our approach integrates non-orthogonal multiple access (NOMA) with orthogonal time frequency space modulation (OTFS) to improve transmission performance, particularly for low mobility when using NOMA-OTFS. Our proposed work focuses on the crucial stage of NOMA with successive interference cancellation (SIC), which involves equalization using efficient schemes, such as decision feedback equalizer (DFE), frequency-domain zero-forcing linear equalizer, and minimum mean square error-SIC at the receiver. Through extensive experimentation, it is observed that the DFE with SIC outperforms other equalizers, demonstrating a lower outage probability and a better BER. The effectiveness of the optimized analytical algorithm for ML-based downlink NOMA-OTFS modulation is confirmed through theoretical BER validation. Moreover, the simulation findings indicate that, in a multi-user (MU) scenario, the proposed NOMA-OTFS exhibits a high performance compared with traditional NOMA combined with orthogonal frequency division multiplexing in downlink MU-MIMO VLC systems. This performance advantage is observed for both MIMO and multiple-input single-output multiplexing techniques with power allocation of the users, specifically in terms of BER improvement and peak-to-average-power ratio reduction.

Erratum corrects the sign of the coefficient in Eq. 5.