The guest editors summarize the Special Section on Advances in Terahertz and Infrared Optoelectronics.

We propose far-infrared photodetectors with the graphene nanoribbon (GNR) array as the photosensitive element and the black phosphorus (bP) base layer (BL). The operation of these GNR infrared photodetectors (GNR-IPs) is associated with the interband photogeneration of the electron–hole pairs in the GNR array followed by the tunneling injection of either electrons or holes into a wide gap bP BL. The GNR-IP operating principle is akin to that of the unitraveling-carrier photodiodes based on the standard semiconductors. Due to a narrow energy gap in the GNRs, the proposed GNR-IPs can operate in the far-, mid-, and near-infrared spectral ranges. The cut-off photon energy, which is specified by the GNR energy gap (i.e., is dictated by the GNR width), can be in the far-infrared range, being smaller that the energy gap of the bP BL of Δ_G  ≃  300  meV. Using the developed device models of the GNR-IPs and the GNR-IP terahertz photomixers, we evaluate their characteristics and predict their potential performance. The speed of the GNR-IP response is determined by rather short times: the photocarrier try-to-escape time and the photocarrier transit time across the BL. Therefore, the GNR-IPs could operate as terahertz photomixers. The excitation of the plasma oscillations in the GNR array might result in a strong resonant photomixing.

The aim of this investigation is to search for the possibility of terahertz (THz) spectroscopy in combination with dielectrophoresis for studying erythrocytes from patients with diffuse liver diseases for diagnostics and differentiation of liver fibrosis degrees. Seventy-nine men aged 33 to 67 years with diffuse liver pathology, mainly alcohol, viral, and mixed genesis with varying degrees of liver fibrosis, were included in the study with 30 men (31- to 64-year-old) without signs of pathology of internal organs and liver fibrosis (F0, first group). The study of suspensions of red blood cells was carried out by THz spectroscopy and dielectrophoresis. An increase in the degree of liver fibrosis was associated with an increase in the number of deformed cells prone to aggregation and destruction, with a reduced surface charge, thickened membranes with high electrical conductivity, low deformability on the background of high summarized viscosity, and rigidity indicators (p  <  0.0001 to 0.05). Strong correlations of THz spectroscopy indices with electrical and viscoelastic parameters of red blood cells were obtained. The revealed possibilities of the study of blood and its cell components are very promising in the diagnosis and differentiation of the degrees of liver fibrosis.

The mechanism of formation of a terahertz jet by a dielectric cuboid and a sphere surrounded by ideally conducting screens is considered. The maximum screen influence is observed when the screen is located near the alight cuboid base. The screen influence eases when the screen is shifting along the dielectric object. Power flux density localization area is almost completely shifted inside the object when the screen is situated in the center. The minimum influence is observed when screen placed in the shadow plane of the cuboid base. This effect caused by the screen influence on electric field component tangential to the side edges and thus on the power flux directed to the central axis of the object. The screen influence on the terahertz jet in spherical object has been compared. Value of the power flux density after passing the object is higher in a cuboid, but the focusing characteristics are better (appearing on shorter distances) in a sphere.

A simple millimeter wave and terahertz (THz) receiver scheme that uses subwavelength focusing of electromagnetic beam on the point-contact detector area with waveguide dimensions is studied. A detection system with such an optical coupling scheme is implemented, where the signal to be detected is coupled to a detector through a mesoscale dielectric particle lens based on terajet effect. We have experimentally demonstrated an enhancement of the point-contact detector sensitivity higher than 6 dB and with 1.5 times decreasing of the noise equivalent power value. The results show that the proposed method could be applied to reduce the size and increase the sensitivity of various THz systems, including imaging, which would enable significant progress in different fields such as physics, medicine, biology, astronomy, etc.

Heterostructures with quantum wells (QWs) based on HgCdTe are promising systems for making compact semiconductor sources of the mid-infrared range. This range has paramount practical importance due to the presence of absorption lines of common pollutant gases. Stimulated emission (SE) from HgCdTe heterostructures has been observed previously at the wavelengths up to 3.7  μm at temperatures above 200 K, but the optimal design of the QW is still debated. We present the spectra of SE from a HgCdTe-based heterostructure obtained at room temperature (RT). We investigate the effect of optical excitation wavelength on the characteristics of SE and discuss possible routes toward improving the design of the active region of the sample. Obtained results demonstrate that HgCdTe heterostructures can be used for the creation of lasers operating at RT at the wavelengths in the vicinity of at least 2.5  μm.

Temperature-driven photoconductivity spectra are studied in HgCdTe thin films and quantum well (QW) heterostructures grown by molecular beam epitaxy (MBE). It is shown that the absorption edge steepness in narrow gap HgCdTe epilayers approaches the fundamental limit. The corresponding Urbach energy is 1.5 to 4 meV at 4.2 to 77 K, which is an order of magnitude lower than values reported previously, indicating a significant progress in the quality of structures grown by MBE. Auger-suppressed multi-QW heterostructures that can be used for development of long-wavelength lasers/detectors are shown to have the comparable steepness of the absorption edge. The corresponding “Urbach” energy is much less than the threshold energy of the Auger recombination, which means that furthering the operating wavelengths beyond 20  μm is feasible for optoelectronic devices based on HgCdTe structures.

The carrier transport mechanism of layered molybdenum disulfide (MoS₂) is studied by terahertz time-domain spectroscopy. The sheet conductivity and the change of sheet conductivity of layered MoS₂ under different pump excitations is studied. The carrier mobility is obtained by analyzing the optical conductivity with the Drude and Drude–Smith models. The carrier dynamics of layered MoS₂ is investigated by an optical pump-terahertz probe. It was found that multilayer MoS₂ exhibits a higher optical conductivity and mobility than monolayer MoS₂, especially under pump excitation.

The features of a liquid jet-based broadband terahertz (THz) generator that includes an optical part and system for a liquid jet formation are described. This system is based on the trapezium-shaped slit-type nozzle, the output of which is limited by two parallel blades. The applications of this generator for studying the optical-to-THz conversion efficiency for various liquids and THz energy temperature dependence are demonstrated. A trend for a decrease in the optical-to-THz conversion efficiency when using high-temperature water is revealed. This trend is interpreted through the increase in the THz wave absorption of the water jet. This system allows to achieve the optical-to-THz conversion efficiency up to 0.1% in the case of α-pinene double-pulse excitation.

Terahertz scanning-probe near-field optical microscope (THz-SNOM), operating in transmission mode and relying on a flexible sapphire fiber, was developed. For this aim, a 300-μm-diameter step-index sapphire optical fiber with flat ends, high surface, and volume quality was fabricated using the edge-defined film-fed growth technique. For the lowest-order guided mode of such fiber, numerical analysis revealed small-to-moderate THz-wave propagation loss, as well as strongly subwavelength guided mode confinement in a fiber core thanks to a high refractive index of sapphire in the THz range. Experimental setup realizing the THz-SNOM principles was assembled based on thus fabricated fiber, as a probe, a backward-wave oscillator, as a continuous-wave emitter with the output wavelength of λ  =  1.2  mm, and a Golay cell, as a THz beam power detector. By combining numerical analysis and experimental study, a subwavelength resolution of our THz-SNOM setup was demonstrated. The resolution depends on orientation of the guided mode polarization toward the image plane, but always remains strongly subwavelengths and overcomes the Abbe diffraction limit. For certain orientations, it can be as low as 0.23λ. In our opinion, thanks to the subwavelength resolution, high-energy efficiency, and flexibility of design, the developed sapphire-fiber-based THz-SNOM holds strong potential in different branches of THz science and technology.

The potential for post-acceleration of relativistic electrons in vacuum with the help of monocyclic THz electromagnetic pulses augmented with constant magnetic fields is explored. The directions of the electromagnetic wave propagation and electron injection are completely or closely aligned to enhance the time of energy transfer from the field to the electron. Monocyclic THz pulses efficiently capture and drive fast electrons until they are released due to field-induced transverse displacement or overtaken by the electromagnetic radiation. While the unidirectional motion of the electromagnetic wave and the relativistic electrons provides for longer overall capture of the particles by the field, protracted drift of an electron across the accelerating part of the monocyclic THz pulse may be achieved by calibrating the additional magnetic field. Simulations demonstrate that, under optimized parameters, the eventual electron energy gains may reach the magnitudes ranging from several to tens of electron rest energies depending on the injection energy. The suggested scheme of post-acceleration of relativistic electrons by combinations of comparatively moderate-intensity THz electromagnetic pulses and constant magnetic fields may thereby provide a viable alternative to the conventional laser accelerator designs employing extremely powerful laser systems that generate relativistically intense electromagnetic envelopes.

Complex conductivity of the topological insulator (TI) Bi_{2  −  x}Sb_xTe_{3  −  y}Se_y samples of various thicknesses and chemical compositions is studied by terahertz time-domain spectroscopy method in the range 0.5 to 2.5 THz. For the first time, a decrease in conductivity in the terahertz range has been observed as the chemical composition approaches the Ren’s curve. The generalized approximate expressions are obtained for complex conductivity with account of the lowest Eu1-phonon mode. Calculations of the Fermi energy and concentration of bulk carriers are performed. Based on the experimental data, an estimate of conductance of the topological states is obtained. The results can be useful in developing terahertz devices based on the specific surface transport in TIs.

Polarized THz radiation is increasingly being investigated and applied in the new interdisciplinary field “TeraNano.” We report the generation of terahertz (THz) radiation by a linearly polarized femtosecond optical pulse in black phosphorus (BP) crystallites grown by chemical vapor deposition. Characterization of THz radiation is performed by time-resolved THz spectroscopy. It was found that BP emits elliptically polarized THz radiation with an ellipticity of ε  =  0.77  ±  0.03. A saturation was found in the dependence of the THz intensity on the intensity of the incident optical pump for the latter higher than 6  mJ  /  cm². The results of our work can provide new fundamental knowledge needed to create detectors and generators based on BP in the THz range.

Our work studies the possibility of frequency doubling in InP:Fe pumped with subterahertz radiation. The attainable power of a second harmonic is calculated for an InP:Fe wafer placed in a parallel plate waveguide. It is shown that although phase matching may be compromised when the waveguide plates are introduced, there is an optimal configuration providing conversion efficiency up to 4.5% under a moderate power of 1 kW at the fundamental frequency. A zero-frequency part of nonlinear polarization is also considered in regard to diagnosis of the power at the fundamental frequency via “optical detection.”

The generation of terahertz (THz) radiation using a ZnGeP₂ (ZGP) large-aperture photoconductive antenna (PCA) was demonstrated. The semiconductors were excited above and below the bandgap (400 and 800 nm) by a femtosecond Ti:sapphire laser. The THz pulse waveform generated by the ZGP antenna was measured using a time-domain spectroscopy technique. The antenna’s THz pulse energy dependence on the optical pump energy was measured, and saturation fluence and carrier mobility were estimated. The ZGP and a chemical vapor deposited ZnSe-based PCA were compared.

We consider the possibility of using a combination of machine and deep learning in the spectral analysis for multicomponent gas mixtures. The experimental setup consists of a quantum cascade laser with a tuning range of 5.3 to 12.8  μm, a peak power up to 150 mW, and a Herriot astigmatic gas cell with an optical path of up to 76 m. We used acetone, ethanol, methanol, acetaldehyde, and ethylene as test substances for the described techniques. For detection and clustering biomarkers, we used machine learning methods such as t-distributed stochastic neighbor embedding, principal component analysis, and classification methods such as decision tree, k-nearest neighbors, logistic regression, and support vector machines. We used a shallow convolutional neural network (CNN) based on TensorFlow (Google) and Keras for spectral analysis of gas mixtures. We modeled IR spectra of pure substances using an NIST database as training and validation sets. Then we used experimental spectra as a test set. We showed that logistic regression gives us the best result for pure substances’ classification. Next, we modeled gas mixtures from synthetic IR spectra for CNN as training and validation sets. We showed that neural networks trained on synthetic spectra can recognize synthetic gas mixtures and experimental individual gaseous substances. We suggest using machine learning methods for pure substances clustering and classification and CNN for gas mixture components identification. We experimentally obtained minimum detectable concentration at ppm level for pure substances. Finally, we estimated that detection limits for the described experimental setup and numerical techniques occurs at levels around 10 to 50 ppb.

Our simulations using the validated multisegment simulation program with integrated circuit emphasis (SPICE) model show that the terahertz (THz) spectrometers using plasmonic field effect transistors could be used for the frequency to digital conversion. The THz SPICE unified charge control model that uses the distributed channel resistances, capacitances, and Drude inductances is validated by technology computer-aided design simulations. The THz spectrometers using Si plasmonic field effect transistors (or TeraFETs) determine the frequency of the impinging radiation by measuring the gate bias where the responsivity changes sign. This crossover frequency is sensitive to the channel length and nearly insensitive to the gate bias in the weak and moderate inversion regions. This unique feature allows the use of multiple THz spectrometers with different channel lengths to implement the frequency-to-digital converters in the sub-THz and THz frequency range. The simulations predict the operation frequency from 110 GHz up to 4 THz for the frequency-to-digital converters using Si TeraFETs with feature sizes from 20 to 130 nm.

We describe a method of parameters extraction for the lumped element network representing resonant tunneling diodes (RTDs). The method is based on onchip reflection coefficient measurements in a wide frequency range from 1 kHz up to 60 GHz in combination with differential resistance measurements. We have proposed and fabricated double-barrier GaAs/AlAs RTDs embedded into the 50-Ohm coplanar transmission line section, suitable for onchip RF-measurements using a probe station and a vector network analyzer. A good agreement between the experimental S11-parameter curves and the curves calculated from the equivalent lumped network is obtained for various RTD bias voltages. A possible operation of a distributed RTDs as an active microstrip transmission line (MTL) is also discussed. Experimentally extracted parameters of the lumped equivalent network are used to define amplification conditions in MTLs based on distributed RTDs.

Quantum cascade lasers (QCL) are widely adopted as prominent and easy-to-use solid-state sources of terahertz radiation. Yet some applications require generation and detection of very sharp and narrow terahertz-range pulses with a specific spectral composition. We have studied time-resolved light-current (L–I) characteristics of multimode THz QCL operated with a fast ramp of the injection current. Detection of THz pulses was carried out using an NbN superconducting hot-electron bolometer with the time constant of the order of 1 ns while the laser bias current was swept during a single driving pulse. A nonmonotonic behavior of the L–I characteristic with several visually separated subpeaks was found. This behavior is associated with the mode competition in THz QCL cavity, which we confirm by L–I measurements with use of an external Fabry–Perot interferometer for a discrete mode selection. We also have demonstrated the possibility to control the L–I shape with suppression of one of the subpeaks by simply adjusting the off-axis parabolic mirror for optimal optical alignment for one of the laser modes. The developed technique paves the way for rapid characterization of pulsed THz QCLs for further studies of the possibilities of using this approach in remote sensing.

Recently, we have proposed a voxel processing algorithm of optical coherence tomography angiography (OCTA) images to increase the accuracy of quantitative assessment of retinal and choroidal blood vessels volume. The purpose of this is to ascertain the effectiveness of the algorithm and to use it for determining the volumetric flow index ratio of the vascular network in the choroid (V_Chor) and the total vascular network (V_All) across the retinal and choroidal layers. Five patients (seven eyes) aged 25 to 68 years without ophthalmic pathologies and nine patients (12 eyes) aged 64 to 79 years with stage II of primary open angle glaucoma (POAG) were given diagnostic OCTA tests (Spectralis OCT2, Heidelberg Engineering, Germany), each consisting of 512 sagittal scans of the peripapillary area. Testing the feasibility of the method showed that the difference between the values of V_Chor  /  V_All ratio in two successive OCTA tests of the same patient’ eye does not exceed 5%, whereas the difference is significant between subjects without eye pathologies and patients with POAG (p  <  0.05 according to Mann Whitney U test). The voxel processing algorithm proved useful in quantitative assessment of the volumetric flow index of retinal and choroidal blood vessels and in the estimation of the V_Chor  /  V_All—the share of choroidal blood vessels volume in the total blood vessels volume within the peripapillary “retina-choroid” layers.

We present methods enabling rapid non-uniformity and range walk error correction of 3D flash LiDAR imagers that exhibit electronic crosstalk caused by simultaneously triggering too many detectors. This additional electronic crosstalk is referred to as simultaneous ranging crosstalk noise (SRCN). Using a method in which the 3D flash LiDAR imager views a checkerboard target downrange, the SRCN is largely mitigated. Additionally, processing techniques for computing the non-uniformity correction (NUC) and range walk error correction are described; these include an in-situ thermally compensated dark-frame non-uniformity correction, image processing and filtering techniques for the creation of a photo-response non-uniformity correction, and characterization and correction of the range walk error using data collected across the full focal plane array without the need for sampling or windowing. These methods result in the ability to correct noisy test validation data to a range precision of 8.04 cm and a range accuracy of 1.73 cm and to improve the signal-to-noise ratio of the intensity return by 15 to 49 dB. Visualization of a 3D scene corrected by this process is additionally presented.

In augmented reality scenarios, virtual images observed through optical see-through head-mounted displays (OST-HMDs) contain color distortion due to external light. This phenomenon is known as background color blending. Existing methods to overcome this issue based on background subtraction compensation produce negative values when the background is brighter than the virtual image. In such cases, virtual images lose their structural integrity and image quality deteriorates. To solve this problem, we propose a gamma-curve-based virtual image boosting method. We derive a measurement model to determine the appropriate gamma value for virtual image boosting using empirical data obtained from extensive real experiments featuring a variety of virtual images and backgrounds. These diverse experiments prove that the proposed boosting method can effectively improve background color blending. With diverse combinations of natural virtual images and bright backgrounds, including even colorful backgrounds featuring high color saturation, the proposed boosting method produces visually pleasing image quality.

As a common kind of cooperative target, checkerboard is widely used in camera calibration and position–pose measurement in which the corner detection plays an essential role. However, due to the influence of the complex external environment, the taken checkerboard image is often incomplete, and the pixel coordinates of the detected corners do not correspond one-to-one to their real-world coordinates. To overcome this problem, a robust corner detection method based on backward position matching for incomplete checkerboard is proposed. By employing a decision of the checkerboard edge after the initial corner detection, the prior corner defined by us is first identified, which provides the prior knowledge for the subsequent uncertain corner detection. Then the homography matrix is estimated using the pixel coordinates and the corresponding real-world coordinates of the prior corners. Finally, according to the homography backward mapping, the uncertain corners are successfully determined, and the one-to-one correspondence between the pixel coordinates and the real-world coordinates is further achieved. Experimental results demonstrate that our proposed method is able to identify accurately each detected corner of the incomplete checkerboard and achieves superior robustness in the corner detection under different interference conditions.

Real-time 3D reconstruction has always been a hot problem in mobile robotics. However, feature extraction algorithms used in traditional 3D reconstruction systems cannot work stably in challenging environments such as low-textured areas. The feature extraction method based on deep learning has higher accuracy and stability than traditional methods, but the complicated network structure leads to the lack of real-time performance. To overcome the limitations, a real-time 3D reconstruction system using multi-task feature extraction network and surfel is proposed. To enhance the stability and accuracy, we design a simplified convolutional neural network to extract feature. Moreover, surfel model is employed to implement the fusion and optimization of 3D point cloud. According to the experiments on the public dataset and real environments, the proposed system can run on Robot Operating System in real time, maintain high pose estimation accuracy in challenging scenes, and complete precise 3D reconstruction. The overall performance of the proposed system is better than that of the traditional 3D reconstruction system.

We present a method for measuring the van der Waals force between two microspheres based on photonic force microscopy. We trapped a microsphere as probe by optical tweezers. The restricted Brownian motion of Gaussian distribution could be found in this system. The vibration center of the probe was affected by the van der Waals force when a target microsphere was closer to the probe. We measured the vibration center of the probe at different separation between the pair of microspheres. Based on this, the measurement of the van der Waals force between the two microspheres was realized with a high precision. Our method can realize the direct measurement of van der Waals force without using the variation rules of it. This method results in a simple structure, would not damage the sample, and can be suitable for the surface of any shape. It is general and has a wide range of applications in other fields of micro-force measurement.

To study the application of laser shearing speckle interferometric vibration measurement technology in acoustic resonance landmine detection, we present an experimental and theoretical study of the method for laser shearing speckle interferometric landmine detection based on time-average. The relationship between the out-of-plane displacement gradient generated by the sound wave on the ground and the phase change of the shearing interference laser is analyzed. A high-power loudspeaker is used as the source of sound wave excitation, and the laser shearing speckle interferometric vibration measurement system is used in landmine detection experiments to investigate the changes of laser shearing speckle interference fringes under different experimental conditions such as sound wave intensity, detection target, buried depth, and soil environment. The study reveals that it is feasible and effective to use laser shearing speckle interferometry to detect landmines, and it provides a theoretical and methodological basis for realizing the rapid scanning of acoustic-optic landmine detection.

Advances in optical testing are as important as advances in fabrication because one can make only what one can measure. A high-precision metrology capability is utilized to close the gap between interferometric testing and lower precision metrology, laser radar, or contact-probe-based coordinate measuring machines (CMM) to accurately measure surfaces with large form error. This nearly universal optical testing method employs a precision CMM equipped with a non-contact, confocal probe. This technique was developed to characterize and align a broad spectrum of optical surfaces including ones with high slopes that are nearly impossible to measure using traditional interferometric testing without custom-made optics. Optical components covering a wide range of prescriptions, such as large convex conics, high-sloped aspherics, grazing-incidence x-ray optics, and highly deformed flats, were successfully measured. The resulting data were processed using custom-developed routines to determine the optic’s alignment, the departure from design surface, and the as-built prescription parameters. This information was used to verify and guide the development and fabrication of novel optics.

Imaging S-lidars have proven themselves in recent years as a new class of laser sensors for remote environmental monitoring and an alternative to traditional atmospheric lidars. Providing range-resolvable remote monitoring, these lidars use low-power CW lasers and advanced nanophotonics technologies to enable compact and cost-effective technological solutions. As a topical application, we have explored the potential of S-lidars to detect atmospheric pollution. We presented a generalized system structure adapted for such application field focusing on approaches to provide the necessary spatial selectivity. By adapting the universal lidar equation to S-lidar features, we have used a dimensionless parametric approach to provide a generalized description of this class of remote sensors. The possible wide variability of the ambient optical weather in the visible and near-infrared ranges was taken into account. It was shown how to apply the Q-criterion of spatial selectivity, we introduced for accounting the S-lidars specificity, to predict the borders of the operation range that can actually be covered by the sensor for reliable gaseous pollution detection. We have demonstrated how to estimate the possible narrowing of the range of concentration sensitivity with increasing requirements for spatial selectivity. The proposed methodology for analyzing the functional and diagnostic capabilities of S-lidars shows the presence of both undoubted advantages and some specific limitations of the achievable range of detectable gas concentrations. Following this methodology, it is possible to improve the validity of design solutions in a variety of applications of this promising class of lidars.

Actively tunable optical filters based on chalcogenide phase-change materials (PCMs) are an emerging technology with applications across chemical spectroscopy and thermal imaging. The refractive index of an embedded PCM thin film is modulated through an amorphous-to-crystalline phase transition induced through thermal stimulus. Performance metrics include transmittance, passband center wavelength (CWL), and bandwidth; ideally monitored during operation (in situ) or after a set number of tuning cycles to validate real-time operation. Measuring these aforementioned metrics in real-time is challenging. Fourier-transform infrared (IR) spectroscopy provides the gold-standard for performance characterization yet is expensive and inflexible—incorporating the PCM tuning mechanism is not straightforward, hence in situ electro-optical measurements are challenging. In this work, we implement an open-source MATLAB^®-controlled real-time performance characterization system consisting of an inexpensive linear variable filter (LVF) and mid-wave IR camera, capable of switching the PCM-based filters while simultaneously recording in situ filter performance metrics and spectral filtering profile. These metrics are calculated through pixel intensity measurements and displayed on a custom-developed graphical user interface in real-time. The CWL is determined through spatial position of intensity maxima along the LVF’s longitudinal axis. Furthermore, plans are detailed for a future experimental system that further reduces cost, is compact, and utilizes a near-IR camera.

Phase shifting profilometry (PSP) shifts the phase by projecting multiple fringe patterns with different initial phase values. Errors will be introduced when the dynamic object moves among the multiple fringe patterns. This paper presents a new 3D reconstruction method of moving object based on PSP by projecting single fringe pattern. The phase shift is caused by the object movement. Multiple images of the object with movement are captured. The object movement is tracked first; then, the phase variation caused by the movement in the single projected fringe pattern is analyzed and the reconstruction model describing the fringe pattern with movement is given; at last, the phase value is retrieved by utilizing the phase variation caused by the movement. The effectiveness of the proposed method is verified by the simulations and the experiment.

It is challenging to measure objects with a high dynamic range surface. We propose a three-dimensional measurement method of high dynamic surfaces based on multi-polarization fringe projection. The saturated pixels can be recognized to get the optimal exposure time by projecting a series of designed auxiliary patterns onto the surface of the object. It does not need to project multiple N-step phase-shifting fringe patterns for recognition, and it greatly improves the detection speed and efficiency. A multi-polarization camera captures full-resolution phase-shifting images with four polarization directions simultaneously. An appropriate polarization direction is selected by analyzing the per-pixel of auxiliary images captured by camera, and then optimal phase-shifting fringe images with high quality are obtained. The effectiveness and rapidness of this method are verified by the measurement experiments of different objects, and the method provides a new idea for rapid measurement.

Deep ultraviolet Raman spectroscopy measurements have been performed at the German Aerospace Center (DLR) with the aim of detecting traces (μg range) of explosive precursors. In this study, a backscattering Raman system was setup and optimized to detect urea, sodium perchlorate, ammonium nitrate, and sodium nitrate at a 60-cm short-range remote detection. The sample was tested at 264-nm ultraviolet laser excitation wavelength to experimentally observe any possible trace over textiles samples. For each colored sample textile, Raman spectra were acquired and no background fluorescence interference was observed at this laser excitation wavelength. Detection limits and system sensitivity with an acquisition time up to 3 s for microgram traces are presented.

Recent research in augmented reality (AR) eyewear has prompted interest in using volume holographic optical elements for this application. However, many sensing operations in AR systems require the use of wavelengths in the near-infrared (NIR) (750 to 900 nm). These wavelengths typically exceed the sensitivity range of available commercial holographic recording materials (450 to 650 nm), which complicates the design of optical elements with power since significant aberrations result when the reconstruction wavelength differs from the construction wavelength. Several methodologies for designing a waveguide hologram imaging system in NIR are reviewed and evaluated. The design approach presented in our work integrates the most effective practices such as fabrication point source location optimization and aberration analysis to realize effective holographic waveguide couplers formed with visible wavelength light and reconstructed in the NIR. The technique is demonstrated by designing and fabricating an input waveguide hologram in conjunction with a multiplexed output coupling hologram. The resulting input/output waveguide holograms can achieve an image resolution of (∼3  lp  /  mm) with a 0.6-mm-thick glass substrate that has a refractive index of 1.8.

A design of high-precision 4-in. transmission spheres that can work in wide-band interferometric measurement is proposed. A motorized precision rotation mount is used to drive one of the lenses in the design to make the transmission sphere work at any wavelength in the wavelength range of 532 to 1550 nm, where the F-number of the transmission sphere remains constant during the movement of the lens. OpticStudio software is utilized to verify the design under F-numbers of 1.0, 2.2, and 3.3, and the results show that a transmission sphere with an F-number of 1.0 to 3.3 can be designed based on our model. The tolerance analysis and the secondary adjustment results show that the peak-to-valley transmission wavefront error of the transmission sphere can be controlled within λ  /  20 in actual machining and assembly, which meets the requirement of high-precision interferometric measurement.

Based on temporal-multiplexing, super multi-view (SMV) technology gets implemented on a near-eye three-dimensional (3D) display, successfully overcoming the vergence accommodation conflict (VAC). The proposed system has two eyepieces for two eyes, respectively, each of which is constructed by an organic light-emitting diode (OLED) micro-display, a projecting lens, and a gating-aperture array. The gating-aperture arrays, consisting of gating apertures with an interval smaller than the diameter of the viewer’s pupil, are attached to the projecting lenses and gate different spatial segments of the projecting lens’s exit aperture in time sequence. Through refreshing the corresponding display contents synchronously, two or more light rays passing through a displayed point are guided by the gating-aperture array into each eye sequentially. Based on the persistence of vision, these light rays converge into a spatial light spot that the eye can focus on naturally, thus enabling a consistence between focusing distance and convergence distance. Further, gating-aperture array with polarization characteristics is designed to get a larger field of view (FOV), accompanied by a half-wave variable retarder being inserted between the OLED micro-display and the gating-aperture array in each eyepiece. Employing two OLED micro-displays of 120-Hz frame rates, a near-eye 3D display prototype has been demonstrated with an FOV 40 deg and a display frequency of 40 Hz, being free from VAC.

We have constructed photonic crystals (PC_s) of a standard honeycomb lattice composed of solid dielectric columns, hollow dielectric columns, and composite dielectric columns and studied the influence of their dielectric constant and filling ratio on the topological boundary states. The optical quantum spin Hall effect is realized by compressing and stretching the standard honeycomb lattice. In the waveguide structure with sharp bends, cavity, and disorder, the electromagnetic (EM) wave is transmitted unidirectionally and robustly, which verifies the transmission characteristics of the topological edge states. In addition, by replacing several solid dielectric columns near the junction of PC_s with different topological states with composite dielectric columns, disturbance is introduced to the edge states, and the influence of the relative effective permittivity of the composite dielectric columns on the edge states is studied. It is found that the EM wave in the gap between the upper boundary state and the lower boundary state cannot be transmitted, and with the increase of the relative effective permittivity of the composite dielectric columns, the gap becomes larger and larger, and the position of the gap gradually moves down. Based on this, a photonic crystal (PC) waveguide structure that has higher transmission efficiency, robust unidirectionality, and higher optical localization performance, is designed. It has more potential development in filters, optical switches, optical communication systems, and semiconductor technology.

The backlight foreign matters can undermine the display quality because they cause white spots on the thin-film transistor liquid crystal display screen. Research on the failure mechanism for such white spots shows that the issue is caused by the foreign matters between backlight films. Due to the variance in temperature and humidity or the influence of external force, such matters will expand or contract to the extent of off-specification, thus pressing or even scratching the film materials. As a result, the backlight module’s light path will be damaged, and the screen will present abnormal bright spots. To solve this problem, the key factors causing such matters in the process are screened out by fishbone diagram and cause-effect matrix. Then the countermeasures are developed by Six Sigma for the continuous improvement of standardization. Finally, the significance and effectiveness of such countermeasures are verified by double-ratio hypothesis testing and environmental testing. Experimental results show that the failure rate caused by backlight white spots decreases from 0.3% to 0.04% after the improvement. In addition, the fault feedback ratio can reach the standard continuously, indicating significant improvement.

Phase encoding and phase-shift profilometry are two commonly used 3D measurement techniques. However, the acquired phases in the techniques are subject to jump errors due to phase ambiguity and phase errors caused by multiple heterodyne. The phase-shifting profilometry also makes the selection of fringe period difficult. To overcome this problem and achieve high-precision measurement, a phase unwrapping method that combines dual-frequency heterodyne with double complementary phase encoding is proposed. First, two wrapped phases are obtained by two groups of sinusoidal fringes; the heterodyne phase is obtained after heterodyne processing, and the high-frequency phase is expanded by heterodyne phase. Second, the fringe levels are obtained using the complementary phase encoding fringes that are shifted by half an order, and then the absolute phase is obtained by selecting different phase coding levels according to different regions for the first phase unwrapping; Finally, the phase noise is removed by exploiting the difference between the phase slopes of adjacent pixels. Experimental results show that a system with the proposed method achieves an RMS error of 0.015 mm. In addition, the period of dual-frequency heterodyne synthesis does not need to cover the whole field of view, which breaks the limitation of frequency selection of the traditional dual-frequency heterodyne method and triple frequency heterodyne method, enabling high-precision measurement with higher frequency fringes. This method overcomes the limitations of the phase principal value error when using higher frequency fringes for high-precision measurement, improves the measurement effect of reflective objects, and effectively avoids the error caused by phase jump.

The square rooting is one of the most important basic operations in computers. Researchers have been studying its algorithms and obtained many algorithm implementations. But improving the accuracy and speed of the square rooting has always been a problem of great concern in electronic computers, embedded systems, and other new computing systems. Based on the parallel carry-free modified signed digit (MSD) adder, the bitwise square rooting of MSD numbers and the floating numbers are proposed in this paper. The parallel square rooting for single MSD integers and multiple MSD integers are presented on a ternary optical computer (TOC) processor. The clock cycles required for the square rooting of multiple groups of MSDs of equal length are the same with that of the single MSD data, which is   (  3  +  log  (    ⌈  n  /  2  ⌉    )    )    ⌈  n  /  2  ⌉   for n-bit MSD integer requires. It is proved that the square rooting for any MSD form and for the binary form of a decimal number may differ only at the lowest digit in their final binary square rooting representations. As an experiment, the square rooting is designed and implemented on a 192-bit prototype SD16 of TOC. Theoretical proof and optical experiments show that the bitwise MSD square rooting is feasible and can greatly improve the efficiency of square rooting for big data. It makes full use of the advantages of the ternary optical processor with a large number of digits, reconfigurable computing functions, and bitwise allocation.

Surface emission concentric shells that can emit both light rays and a large amount of heat from the whole outer surface are proposed here for a large globe light-emitting diode (LED) bulb. The concentric shells are composed of an outer total internal reflection (TIR) shell and an inner metallic shell adjacent to the outer shell. The TIR shell can guide light rays emitted from LEDs owing to the TIR and emits illuminating light from the outer surface. Assuming that the thickness of the TIR shell is sufficiently small, the TIR shell can also emit heat of the inner shell to the outside. It is theoretically predicted based on the heat transfer theory that the larger the concentric shells are, the larger the amount of heat that can be emitted to the outside. A large globe LED bulb prototype having the concentric shells with an outermost diameter of about 95 mm is therefore designed and fabricated, with which it is shown that an illuminating light flux of about 1064 lumens and heat of about 12.2 W can both be emitted to the outside. The prototype also shows to have a large hollow structure in which several kinds of devices can be accommodated.

A compact microring resonator (MRR) and its implementation for polarization rotation-based all-optical switching are proposed. The polarization rotation has been achieved by enhancing geometry-induced birefringence of the device and with the insertion of polarization rotator in the race-track-shaped MRR. The proposed device has been modeled using Jones matrix and the finite-difference time-domain simulation method. The insertion loss and the extinction ratio have been found to be −0.13 and 19.39 dB, respectively.

A low-cost and high-efficiency method for fabricating variable line-space gratings is reported. Specifically, oxygen plasma was used to process the stretched wedge polydimethylsiloxane substrate. After slowly releasing the stress, a gradient grating with a continuous change in line spacing was formed via self-assembly. The period of the self-assembly grating was 2.96 to 3.25  μm, from thick to thin, when the sample was stretched transversely. In longitudinal stretching, the period was 3.71 to 5.20  μm from thick to thin. The period variation range of the gratings obtained by the two types of stretching was 15% and 45%, respectively. Diffraction tests of the two samples showed that the diffraction spots were evident. The diffraction angles varied in the range of 11.31 deg to 12.41 deg and 7.12 deg to 9.93 deg. The gratings prepared using this method can compensate for the shortcomings of the traditional process and have a wide range of application prospects.

Modulation format identification (MFI) is widely used in elastic optical networks, such as optical performance monitoring and signal compensation. We propose an MFI scheme, through extraction of the amplitude probability distribution feature of the received signals and an analysis of the relative entropy, which is also known as the Kullback–Leibler divergence (KL). The modulation formats (MFs) can be distinguished via calculation of the KL value. Through this method, four commonly used transmission formats were investigated, namely, polarization-division multiplexing (PDM)-QPSK/16QAM/32QAM and 64QAM. A simulation transmission model was created, and the results were analyzed. The simulation results show that the MFs investigated can be distinguished when the optical signal-to-noise ratio (OSNR) is less than or equal to the corresponding theoretical 7% forward error correction (FEC) limit (bit error ratio  =  3.8  ×  10^−  3). In addition, two important factors, i.e., fiber nonlinearity and sample size, were investigated in relation to the proposed method. The results show that the method exhibits a certain tolerance to fiber nonlinearity and can be used to identify the MF at the FEC threshold even when the sample number is 1000. Furthermore, the experimental verifications were conducted to ensure the practicality of the proposed method.

A received signal strength-based visible light positioning system with multiple light sources is investigated. In this system, each light source is assigned a unique signature code, which is constructed based on the vertex coloring of graph theory. The signature code is separated into three parts for different purposes, i.e., discriminating the beginning of the received signals, distinguishing different light sources, and correcting errors when they occur. The length of the code is determined by the numbers of the light sources. A sequential decoding scheme is devised to identify different light sources stepwisely. The light source with the highest received light intensity is discerned first by utilizing the characteristic of the signature codes. Then it is reduced from the received signals, and the remaining light sources are discriminated one by one using this scheme. With the knowledge of the coordination of these light sources, the location of the receiver can be acquired based on trilateration positioning algorithm. The simulation and the experiment results verify that the proposed signature codes can eliminate the interference from multiple sources and achieve good positioning performance.

We demonstrate the theory and experiments of a power-ratio tunable dual-wavelength laser with coaxial diode end-pumping configuration by varying the pump wavelength, which is realized by controlling the working temperature of the pump laser diode (LD). Composite laser gain media containing an Nd  :  YVO₄ crystal and an Nd  :  GdVO₄ crystal was used for example. The dynamics of the dual-wavelength laser generation based on practical input operating parameters were simulated, and in the experiment, a total power of 3.72 W was obtained under the maximum LD pump power of 8 W, corresponding to the optical–optical conversion efficiency of 46.5%. The characteristics of the power-ratio tuning and output power agreed well with the theoretical predictions.

We present the amplifying features of the copper bromide brightness amplifier excited by a capacitively coupled longitudinal discharge. The dependences of the amplified spontaneous emission (ASE) power, gain, and the sensitivity of the brightness amplifier on the pulse repetition frequency (PRF) of the excitation have been obtained. The PRF of excitation varied from 12 to 24 kHz. At PRF 24 kHz, the output power was 2.1 W, pulse duration was 30 ns that is lower than for a copper bromide brightness amplifier with the typical excitation pulses. The small-signal gain reaches 0.25  cm^−  1, which is commensurate for a copper atom active media with the typical excitation charge. The sensitivity of the investigated amplifier is 0.01% of ASE power in the studied PRF range.

A terahertz (THz) hollow-core Bragg waveguide constructed by cascading waveguide units with supporting bridges (SPBs) on different air rings is proposed. The influence of the SPBs on the transmission loss of the waveguide is demonstrated numerically. Results show that the SPBs in the first air ring play the most important role in the transmission loss. By reasonably selecting the length ratio of the waveguide units, the waveguide absorption loss can be reduced to less than 0.4  dB  /  m in a wide bandwidth range of 0.53 to 0.73 THz, which is at least 2 orders lower than the material absorption loss.

We investigate the adverse impact of pointing errors (PEs) on the efficiency of free-space optical communication under varied weather conditions using hybrid subcarrier intensity modulation over a fading regime. The error rate is improved by implementing topological permutations at the trans-receiver to achieve a bit error rate of ^{10  −  10}. The derived combined probability density function expressions are defined as generalized infinite power series for precision, which supersedes Meijer-G. The outcomes are contrasted with BPSK-SIM and pulse position modulation, validating its agreement with favored aspects. The undesirable influence of PEs is balanced by widening the beam-waist at the expense of the received irradiance in a multiple-input-multiple-output, MIMO_{(2  ×  2)} system, until a normalized jitter cut-off of 3.35. Afterward, single-input-multiple-output, SIMO_{(1  ×  3)}, emerges to be more reliable, providing the flexibility to select the optimum order concerning the quality of service.

We demonstrate the performance of broadband, low loss, and compact three-mode converter and (de)multiplexer utilizing an asymmetric trijunction splitter, multimode interferometer, and a phase shifter; all are designed using on subwavelength grating technology (SWG) and built-in a silicon-on-insulator platform for the first time to the best of our knowledge. SWG enables the flexibility of refractive index engineering for more operating bandwidth and compactness compared with conventional counterparts. The designed device is capable of converting and (de)multiplexing three different input modes TE₀, TE₁, and TE₂ into a fundamental TE₀ output mode. The three-dimensional finite-difference time-domain simulation results confirm that the designed device has a low loss of 1 dB and the crosstalk reaches −65  dB over a broad wavelength of 160 nm (1470 to 1630 nm) in the C-band range with an overall size of the designed device is 95  ×  4  μm².

Laser ablation of zircon can be used to analyze its composition for geological history. However, the effect of laser properties on nanoparticle size has not been studied extensively. The effect of laser fluence and pulse duration on the diameter of zircon nanoparticles was analyzed using field-emission scanning electron microscopy and energy dispersive spectroscopy. The results showed that the diameters of the zircon nanoparticles induced by a femtosecond laser increased with increasing laser fluence, and that these particles were smaller than those induced by a nanosecond laser with the same laser fluence. Furthermore, the mechanism of zircon nanoparticle formation induced by laser ablation has been discussed. The explosion mechanism is the primary mechanism of nanoparticle generation. In particular, the zircon nanoparticles induced by the femtosecond laser were the result of Coulomb explosion, while phase explosion contributed to the zircon nanoparticles induced by the nanosecond laser. Therefore, the nature of zircon nanoparticles induced by laser ablation is mainly determined by the pulse duration.

We experimentally demonstrate C-band direct-detection (DD) power division non-orthogonal multiple access (PD-NOMA) aided wavelength-division multiplexed (WDM) data transmission over 50-km distance [standard single-mode fiber (SSMF) + non-zero dispersion-shifted fiber (NZDSF)] for far user at 10-Gbps data rate. For 5-Gbps data rate, a distance of 100 km is achieved for far user. The transmission system is made of mixed fiber types to include both linear and nonlinear fiber impairments. We have utilized successive interference cancellation technique assisted by K-means clustering algorithm for the detection of the multi-user 3  ×  20 Gbps on–off keying modulated WDM PD-NOMA signal. The obtained results reveal that the unsupervised K-means clustering-based DD receiver provides 7% hard decision- forward error correction BER limit of 3.8  ×  ^{10  −  3} for all three channels for both 100-/50-GHz channel spacing and 5-/10-Gbps data rates. For the case of a single-channel-based system, the BER well below the pre-FEC limit is achievable for both the data rates of 5 and 10 Gbps even after 50 km (for 10 G)/100 km (for 5 G) SSMF/SSMF + NZDSF for far user. Using the K-means algorithm, the maximum power penalty for the 10-Gbps single channel and WDM channels are 5 and 6.6 dB, respectively. The proposed detection technique is able to detect the data even for the far user at a distance of 100 km. Most significantly, no compensation techniques have been utilized for the fiber impairments in the proposed system.

We present the coupling characteristics of directional coupler comprising two identical single-mode triangular index fibers both in the presence and the absence of Kerr nonlinearity. Triangular index fibers of some typical V numbers are chosen for our investigation. Our analysis is based on the simple series expression for the fundamental modal field for triangular index fiber developed by Chebyshev technique. Analytical expression of the coupling length in the absence as well as in the presence of Kerr nonlinearity is prescribed. Using the said expression of coupling length, we apply a method of iteration to evaluate the said coupling parameter in the presence of Kerr nonlinearity. We also show that our results match excellently with the exact results obtainable by rigorous finite-element technique. The execution of our formalism requires little computation. Thus our simple but accurate formalism to study the triangular index directional coupler will prove a suitable alternative to the existing rigorous methods in the field of nonlinear optics. This user-friendly method will prove beneficial to the scientists and technologists working in the field of nonlinear optics.

A patterned laser lift-off technique is adopted for fabricating terahertz (THz) metamaterials with micrometer precision. A shaped UV laser pulse is used to remove low-viscosity silver nanopaste from a quartz substrate. Various types of metal THz metamaterials, including gradient metamaterials, can be obtained flexibly by movement of the sample and programming of the shaped beam. Two THz metamaterials, which are based on bridged double split ring resonator and its complementary resonator, respectively, are fabricated and tested. The results show that patterned laser lift-off based on a low-viscosity silver nanopaste film can supply a low-cost and easy-operation route in the fabrication of various THz metamaterials.

We propose an optical glucose sensor based on an enzymatic graphene oxide (GO)-functionalized π-phase-shifted long-period fiber grating (PS-LPFG) inscribed on high-birefringence fiber (HBF) by CO₂ laser irradiation. To fabricate a sensitive and selective sensor head, we coated the PS-LPFG inscribed on HBF (referred to as the HB-PS-LPFG) with GO and covalently immobilized glucose oxidase (GOD) on the coated GO film. When the enzymatic GO-functionalized HB-PS-LPFG (i.e., the sensor head) is immersed in glucose solution samples, the immobilized GOD catalyzes the oxidation of glucose to gluconic acid, resulting in refractive index (RI) changes in the local sensing region of the fiber. These RI changes lead to variations in the transmission spectrum of the HB-PS-LPFG, and the glucose concentration of the sample can be quantified by measuring spectral shifts. Owing to the birefringence of HBF, the fabricated HB-PS-LPFG had polarization-dependent attenuation dips, which exhibited variations in transmission level or wavelength for both glucose concentration and temperature changes (ΔC and ΔT, respectively). From these attenuation dips, we selected two dips (designated as AD 1 and AD 3) as sensor indicators because they showed linear and independent responses to ΔC in a glucose concentration range of 5 to 25 mM and ΔT in a temperature range of 25°C to 45°C, respectively. The glucose concentration and temperature-induced sensitivities of the two indicators were calculated as ∼0.0  pm  /  mM and ∼86.8  pm  /    °  C for AD 1 and ∼20.8  pm  /  mM and ∼149.2  pm  /    °  C for AD 3, respectively, which revealed the sensor capability of simultaneous measurement. With these unique ΔC and ΔT responses of the sensor head, our sensor can detect glucose concentration in a cost-effective way, minimizing the temperature-induced measurement uncertainty.

The nanoscale modulation of light can be achieved by integrating plasmonic nanostructures with phase change materials. A dynamic and tunable perfect absorber is designed by combining VO₂–Au hybrid nanodisc array with metal–insulator–metal structure. The finite-difference time-domain method is used to simulate the absorption curves under different structural parameters and temperatures. In the near-infrared band of 700 to 1800 nm, the average absorption rate and absorption bandwidth of the absorber in the insulating phase of VO₂ are 87.6% and 939 nm, respectively. And the maximum absorption modulation depth before and after phase transition of VO₂ reaches 63.8%. The absorber can tolerate a wide range of incident angles and exhibits polarization-independent features. We envisage that the overall advantages of this absorber will open new opportunities for optical modulation and optoelectronics.

An ultra-compact 1310  /  1550  nm wavelength demultiplexer based on multimode interference (MMI) coupler assisted by subwavelength gratings (SWGs) is proposed. Two parallel SWG-based slots are inserted into the MMI section symmetrically. Equivalent refractive index and width of the SWG are designed properly to reduce the device length while keeping a low insertion loss (IL) and high extinction ratio (ER). In this way, the device length shrinks to 34.48  μm. The performance when the device working as a multiplexer and as a demultiplexer are both investigated. From the transmission spectrum, ILs of <0.24  dB, ERs of larger than 15.2 dB and broad 1-dB bandwidths of larger than 90 nm are obtained for the two wavelengths.