A carbon nanotube (CNT) is a cylinder made of graphene. CNTs possess excellent properties of carbon materials, and the parallel-arranged nanotubes exhibit anisotropy additionally. The finite element method is adopted to simulate single-walled carbon nanotube (SWCNT) array in the terahertz spectrum. Under the standard size parameters we have set, Our simulation results show that between 1-3 THz, P-polarization transmittance, which is perpendicular to the SWCNTs, fluctuates around 0.5, while S-polarization transmittance, which is parallel to the SWCNTs, is around 4.8×10^-3. The reflectance parallel to the SWCNTs gradually increases to 0.407, and the reflectance perpendicular to the SWCNTs gradually decreases to nearly 1×10^-8 as the frequency increases. The simulation results provide theoretical support for further applications of SWCNTs in optical field.

In this paper, we used FDTD optical simulation software to simulate terahertz whispering gallery mode resonant cavities with cylindrical, circular ring, sphere, and hollow sphere structures. We used fiber coupling method and set the diameter of the fiber to 200um. Simulate resonant cavities with a radius of 5mm to 10mm in the range of 0.325-0.5THz. The resonant cavity uses quartz material and uses 2D simulation to save simulation time. We observed the distribution of mode spectral lines and electric field intensity, and characterized five indicator parameters of the terahertz whispering gallery mode resonant cavity after resonance phenomenon: resonance wavelength, free spectrum range, full width at half maximum, quality factor, and mode volume. The results indicate that terahertz resonant cavities exhibit good performance in specific frequency bands.

We study the process of THz soliton-like mode generation when an optical pulse with tilted wave front is launched into a crystal. Crucial role of the wave front tilt for the trapping of optical-terahertz solitons of a new type was proved theoretically and in experiments as well. These modes were developed as solutions of coupled Zakharov–Boussinesqtype equations. At that, diffraction was a contributing factor for soliton formation. Scenarios for soliton generation of terahertz radiation are quite possible to be observed in crystals with a characteristic size of a few millimeters. In the present work, using numerical simulation, we tackle the question of such soliton stability which still remains open.

Terahertz (THz)-based electron acceleration has potential as a technology for next-generation cost-efficient compact electron sources. Here we present a novel millimeter-scale multicell waveguide-based THz-driven photogun that exploits field enhancement to boost the electron energy, a movable cathode to achieve precise control over the accelerating phase as well as multiple cells for exquisite beam control. The short driving wavelength enables a peak acceleration gradient as high as ~3 GV m⁻¹. Using microjoule-level single-cycle THz pulses, we demonstrate electron beams with up to ~14 keV electron energy, 1% energy spread and ~0.015 mm mrad transverse emittance. With a highly integrated rebunching cell, the bunch is further compressed by about ten times to 167 fs with ~10 fC charge. High-quality diffraction patterns of single-crystal silicon and projection microscopy images of the copper mesh are achieved. We are able to reveal the transient radial electric field developed from the charged particles on a copper mesh after photoexcitation with high spatio-temporal resolution, providing a potential scheme for plasma-based beam manipulation. Overall, these results represent a new record in energy, field gradient, beam quality and control for a THz-driven electron gun, enabling real applications in electron projection microscopy and diffraction. This is therefore a critical step and milestone in the development of all-optical THz-driven electron devices, validating the maturity of the technology and its use in precision applications.

Terahertz Rotating Coherent Scattering (ROCS) method can realize the super-resolution imaging through the illumination of the evanescent wave. Under one illumination direction, it is treated as a coherent imaging process, and the local destructive interference may improve the imaging resolution. Then, the intensity images under different illumination directions are obtained by rotating the sample for several times, and they are added up incoherently to achieve the final result. However, in the experiment, the operation of rotating the sample is inconvenient and the rotational angle deviation, unstable light source output may exist to affect the results. In addition, simply summing limited intensity images may cause loss of sample information. To solve the above problems, we combine the deep learning method with the ROCS to realize the super-resolution imaging. Firstly, the reconstruction result is compared between the single-input and the multi-input network models, and the simulation results show that the latter one has the better quality. Then, with regard to that there is usually not enough experimental data, the transfer learning technique is introduced to the deep learning method. Use a large amount of source data and a small amount of target data to complete the training of the network. The simulation results show that the quality of reconstruction image is significantly improved. It implies that the ROCS imaging combining with deep learning method can further improve the resolution and the contrast of the reconstruction result.

Terahertz waves have shown great application potential in many fields, but they are limited by modulation devices. Currently, the dynamic modulator based on optical controlled devices show better performance, but is still far away from the actual application. In this work, a THz SLM system based on bare silicon wafers pumping with a continuous wave laser is built, the modulation performance on different thickness of the wafer and different pumping energy is analyzed. Over 80% modulation depth can be obtained at a pump power density of 2.5W/cm². The modulation of structured patterns can also be implemented in our system.

In this paper, a timing control circuit for infrared detectors is designed based on JESD204B high-speed serial interface technology, and a corresponding verification scheme is proposed. The timing control circuit of the infrared detector consists of a JESD204B high-speed data transmission module and a control clock generation module, used for data transmission and control timing generation in the infrared detector imaging circuit. The timing generation module can adjust the register status through SPI to meet the timing requirements of multiple detectors; The data transmission module outputs high-speed serial data through two JESD204B transmission interfaces for 8-channel LVDS digital data according to the protocol, reducing the data interface with the main control circuit. The high-speed data transmission function is verified using a timing control circuit as the transmitter and FPGA as the receiver. The FPGA received the data correctly and met a serial data transmission rate of 4.096Gbps; Verified the function of controlling timing generation, the timing control circuit is based on the main clock and can correctly generate control signals for various modules inside the circuit.

Infrared is an electromagnetic spectrum at a wavelength longer than the visible light. Infrared remote sensing systems can work both day and night, can cross thin cloud layer, and distinguish camouflage material, so they are more and more important and have applied in multiple fields. Point target detection infrared remote sensing technology has been widely used in astronomical observation, system defense, imaging guidance and etc. The point target is so far that the solid angle of the system is much smaller than the instantaneous field of view of the remote sensor itself, and it requires the infrared remote sensing system with high SNR. The design and construction of the SNR test system for point target imaging IR remote sensors was described in the article. The infrared target background simulator is the key component of the SNR test system. The simulator mainly consists of three parts: a cold plate, a vacuum cryogenic blackbody, and a 3-D electrical adjuster. The cold plate simulates the detection background, and micro holes are distributed on the cold plate to simulate the detection of point targets. By effectively controlling the thermal insulation and temperature control between the target simulator and the background simulator, as well as the thermal insulation between the background simulator and the tested remote sensor, stable testing can be achieved. In addition, by combining simulation optimization with practical experience, the influence of the simulator's cold plate thickness, the target phase, the collimator, etc. can be removed through simulation calculation, effectively improving the testing accuracy. By combining advanced thermal control, precise positioning, and simulation optimization, the simulator achieves high levels of accuracy and reliability in testing infrared remote sensing point target detection systems. The simulation analysis method and verification results presented in this article have important implications for the signal-to-noise ratio detection experiment of infrared remote sensing point target detection. By providing a detailed design and construction framework for a cryogenic vacuum infrared target background simulator, the article contributes to the advancement of remote sensing technology and enhances our ability to detect and analyze point targets in complex environments.

The InAs/InAsSb nBn structure detector without Ga (GA-free) has fewer internal defects, and the barrier blocks majority carrier while allowing the normal transport of photogenerated carriers. The unique structure can effectively suppress the generation-composite current generated by SRH, and achieve low dark current at high operating temperature. In this paper, a mid-infrared Ga-free nBn T2SL detector is investigated. The device exhibited 7.43x10^-6 A/cm² under 0.5 V bias at 127 K. At 120K, the detector achieves quantum efficiency values of 56%, exhibits excellent photoelectric performance.

In this work, a novel scheme to generate 108-tupling frequency millimeter-wave (mm-wave) based on the external modulation of dual-drive Mach-Zehnder Modulator (MZM) and highly nonlinear fiber (HNLF) is proposed, which is verified by simulation. In this system, the ±18 th order sidebands generated by six MZMs in parallel are used as the optical carriers and then injected to the HNLF. Due to the four-wave mixing (FWM) of HNLF, a new optical frequency component, which is up to 54-order can be obtained. The variation of Optical Side-band Suppression Ratio (OSSR) and the Radio Frequency Spurious Suppression Ratio (RFSSR) are analyzed by simulation. The simulation results show that our system achieves optimal performance at HNLF of 2km. At this time, a high-quality mm-wave can be acquired, with OSSR of 26.12dB and RFSSR of 44. 74dB. The analysis shows that this scheme has a high FMF, which can effectively reduce the frequency of local oscillator signal.

Terahertz wave has been widely used in medical imaging, sensing, and communication. The terahertz (THz) detector is an effective means to obtain information and images. Meanwhile, high-sensitivity THz detectors are necessary for detection and communication systems. However, most of the previously proposed terahertz detectors use butterfly or dipole antennas, which have low responsivity and narrow response bandwidth, limiting the detection of continuous and weak signals. In this work, a photoconductive detector based on metasurface is proposed. In order to improve the terahertz absorption effect of the detector, a metal-semiconductor-metal (MSM) unit array structure is employed. Gallium arsenide (GaAs) has the advantages of direct band gap (1.42 eV), high photo-electric conversion rate and high electron mobility, making it an ideal building block for the preparation of photodetectors. The influence of different structures on the light absorption intensity of the detector is simulated by the finite difference time domain (FDTD) method. The optimized structure achieves absorption of over 99.9% in the corresponding band. Therefore, theoretically determining the metasurface structure can enhance the light absorption intensity of the detector, and provide support for optimizing the structural parameters and realizing a broadband terahertz detector. The current and voltage response of the designed photoconductive detector under given conditions were modeled and analyzed by SILVACO. Among them, the responsivity of the device can reach 0.36 A/W at 0.3V bias voltage and 0.9mW incident light power. The designed metasurface and detector have simple structure and easy fabrication. This design provides a new idea and technical method for the design of terahertz detector.

Polarization filtering and shaping occupy crucial positions in optical systems, finding extensive applications across diverse domains such as optical imaging, detection, communication, and numerous other fields. Among them, metasurfaces, as a novel type of optical element, possess unique advantages in electromagnetic manipulation, enabling precise control over the phase, amplitude, polarization, and propagation direction of electromagnetic waves. Specifically in the terahertz band, the metasurfaces provide innovative solutions for polarization filtering and polarization conversion. To simultaneously regulate the transmission and reflection of circularly polarized light, this paper proposes and experimentally demonstrates a terahertz spin-selective metasurface, which can achieve circular polarization selection and wavefront manipulation. The metasurface is composed of split-ring resonators and realizes independent manipulation of these two types of circularly polarized light by combining transmission phase modulation and geometric phase modulation. When circularly polarized light is incident, the metasurface can achieve arbitrary wavefront modulation while simultaneously achieving the polarization selection, permitting only one type of circularly polarized light to transmit while reflecting the other type of polarized light. At the operating frequency, the cross-polarization efficiency exceeds 80%, exhibiting excellent performance. Through continuous optimization of the geometric parameters of each layer of the metasurface, we have achieved precise modulation of the phase and polarization state of the transmitted wave. Based on this metasurface, the designed deflectors and focusing lenses are demonstrated, and simulation results have verified the effectiveness and practicality of this approach. This metasurface holds tremendous potential for applications in optical information security, optical communications, and other fields.

Superconducting on-chip spectrometers have both imaging and spectroscopic capabilities. In general, the broadband signal coupled from an antenna goes through frequency dispersion via a series of filters that are connected with superconducting detectors like kinetic inductance detectors or bolometers. The filters have the same relative bandwidth, which determines the frequency resolution of the spectrometer. We here present the design and simulations on the twin-slot antenna, CPW-to-microstrip transition, and a ten-channel filter-bank of a verification-stage terahertz spectrometer chip at 350 GHz. The simulation results of the antenna and transition showed low return loss, and the simulation results of the ten-channel filter-bank show that each channel has good readout independence and coupling strength. These designs and simulations can provide assistance for the future development of terahertz on-chip spectrometer.

In this work, a 230 GHz all-NbN superconductor-insulator-superconductor (SIS) mixer was designed and analyzed. The NbN/AlN/NbN tunnel junctions (with an energy gap of 5.7 meV) and NbN/MgO/NbN tuning circuits were utilized. Full-height waveguides and bow-tie waveguide probes were adopted. The signal coupling circuit was designed and optimized to be transmissible to the given band. The quantum mixing characteristics of the all-NbN mixers were optimized with different-size SIS junctions. The mixer with a junction diameter of 1 μm achieves a conversion gain close to 0 dB and a noise temperature close to 40 K. The proposed design and analysis will provide technical support for the recovery and upgrade of the heterodyne receiver for the Leighton Chajnantor Telescope (LCT).

An all-dielectric terahertz metasurface biosensor consisting of two silicon cuboids deposited on a quartz substrate was proposed. By adjusting the length of the right cuboid and breaking the symmetry of the unit cell, a quasi-BIC driven ultra-high quality factor resonance is excited in x-polarization and y-polarization respectively at different resonance frequencies. For the C2v symmetry structure of the designed all-dielectric metasurface, two symmetry-protected BICs were aroused with complete destructive interference, since all of the coupling coefficients are symmetric. As a unique property of symmetry-protected quasi-BIC resonances, the quality factors can be manipulated as desired and even reach extremely high values by precisely changing the metasurface geometry. The sensitivity of the all-dielectric metasurface designed for biosensing purposes can reach up to 300 GHz/RIU, indicating superior sensing capabilities.

Chinese ancient murals are an important part of Chinese cultural heritages, with rich historical, artistic and scientific values. After decay of hundreds of years, there are generally a variety of serious diseases, such as hollow, crack, efflorescence in substrate layer and the peeling, fading of pigment layer. Although the diagnosis and identification of surface diseases in pigment layer are relatively easy, it is quite challenging to determine the crack, cavity, detachment, hollow, porosity and deep structural information in substrate layer. In this paper, the square heating thermography is used to conduct in-situ nondestructive testing and health evaluating of different murals. Due to the immovable of the mural and the limited space in its location, a portable device consisting of a long-wave infrared camera and two rotatable halogen lamps is used to facilitate detailed inspection of the murals. During the test, the heating time and imaging rate were determined, and a set of transient thermal images were taken in different regions one by one. A thermal tomography method was applied for quantitative 3D evaluation of murals, which can not only realize the detection and evaluation of cracks, hollow, cavities, porosity and supporting structure information in the deep structural murals, but also quantitative evaluate the disease distribution in the murals. Through the application of the infrared thermal imaging on the murals, it can deeply insight into the internal structure of the murals and provide an important basis for the assessment of the structural stability of the murals, which will benefit for the conservation and restoration of murals in the future.

Synthetic Aperture Radar (SAR) emits microwave electromagnetic pulses and detects remote targets through the backscattered echo signals. With the continuous advancement of technology, high-resolution SAR imaging can accurately observe various targets with small sizes, dense connections, and diverse shapes. The steep terrain or tall objects in SAR images display prominent shadows due the obstruction of ground-facing electromagnetic waves resulting in weak echo signals in the shadowed areas. Small targets can generate discrete shadows with contours similar to optical contours in high-resolution SAR images when the radar observing angle is appropriate. This characteristic, which shares similarities with the target contour, can be utilized to build the correlation between SAR images and other modal images such as optical images. This study uses aircraft as an example to validate the feasibility of this approach. It trains the YOLOv5 object detection network on the Remote Sensing Object Detection (RSOD) dataset of optical airport images, which is then utilized to detect the shadows of aircraft in high-resolution SAR images, indirectly achieving aircraft detection.

Firstly, the shadows in SAR images have an inverse relationship with the signal of the aircraft body in optical images in terms of grayscale. Therefore, it is possible to simply invert the grayscale of one of them. In this study, SAR images were chosen for grayscale inversion. After a simple grayscale inversion, the network detected a significant number of aircraft in the image at a confidence level of 0.2, while only part of aircraft were detected in the image without inversion. Besides, a series of adjustments were made to the brightness and contrast of the grayscale inverted image in order to find the optimal setting for aircraft detection. After scanning and adjusting the brightness and contrast, the network detects a certain number of additional aircraft in the grayscale inverted images at a confidence level of 0.2. The maximum number of aircraft detections was achieved at a specific filtering spatial frequency after applying filtering with different spatial frequencies. The overall detection result achieved an accuracy of over 80%.The maximum number of aircraft detections was achieved at a specific filtering spatial frequency after applying filtering with different spatial frequencies. The overall detection result achieved an accuracy of over 80%.

Wall penetrating porcelain sleeves are used for power distribution equipment and high-voltage electrical appliances in power stations and substations. They are used for wires to pass through grounding partitions, walls, or electrical equipment casings, supporting conductive parts to insulate them from ground or the casing. The sleeve is composed of porcelain parts, metal accessories, installation flanges, and conductive bars (rods), etc. Due to long-term use, porcelain components may experience cracking, aging, and other phenomena, posing a serious threat to the safe and stable operation of the power system. This article analyzes the defects, material parameters, and thickness of ceramic sleeves using a terahertz frequency modulated continuous wave system. We conducted tests on both standard samples and actual ceramic sleeves. The research results of this article provide a basis for the application of terahertz technology in nondestructive testing of power systems, which can achieve on-site testing of high-voltage ceramic bushings and ensure the safe operation of power systems.

Industrial hazardous gas leakage detection using infrared imaging technology has become an important area of research and application. However, existing single-band imaging methods face limitations in power consumption and cost. This paper proposes a novel design based on a micro-lens zonal filter array. This system can perform multi-spectral infrared imaging of gas clouds from various typical industrial gas leaks using an uncooled infrared detector. The key innovation is the use of a partitioned filter array to split the incident infrared radiation into different spectral bands and focus them onto distinct regions of the focal plane detector. By simultaneously imaging multiple infrared bands from 3 to 14 μm, the system can effectively detect and differentiate between various gas species in the gas cloud. Preliminary results demonstrate that this approach can improve the detection efficiency of hazardous gas leakage and enable the miniaturization and integration of the infrared imaging system.

A cost-effective, easy-to-produce calibration chessboard and a corner detection algorithm to enhance the calibration precision of an infrared (IR) imaging alarm system with a large field of view (FOV) are introduced. We present the manufacturing process for the calibration plate and conduct an experimental evaluation to verify its performance. The results confirm that our checkerboard calibration can achieve superior distortion correction accuracy, satisfying subsequent image processing requirements. This research provides practical insights and could serve as a theoretical reference for future IR camera calibration studies.

In this paper, two IBU-NIC cocrystal polymorphs of ibuprofen (IBU) and nicotinamide (NIC) were synthesized by melt recrystallization, solution evaporation and solvent milling, respectively. IBU, NIC, physical mixtures and their corresponding two cocrystals were characterized by terahertz (THz) spectroscopy. In addition, the theoretical structures of the two cocrystals of IBU-NIC were optimized and simulated by density functional theory (DFT) using Gaussian 16 software to obtain rich vibration mode information. By comparing the simulated results with the experimental results, the most reasonable molecular structures and vibrational modes for the preparation of IBU-NIC cocrystal polymorphs could be determined.

Tartaric acid is a common food additive with two mutually symmetrical chiral carbons, which is a very important class of four-carbon organic chiral sources. Centrosymmetric metamaterial sensors based on aluminum and polyimide were analyzed using CST microwave studio electromagnetic simulation software. The mechanism of the metamaterial resonance peak and the related electromagnetic field are analyzed. And the metamaterial was obtained by using coating and photolithography etching methods. The metamaterial has good detection performance and high sensitivity and can be used to identify tartaric acid chiral isomers (TACIs). The minimum detection concentrations of L-tartaric acid measured in the experiment were 0.001 g/ml. In addition, theoretical simulations of the crystalline cells of TACIs were also calculated using the density functional theory in quantum chemistry software. The absorption peaks of the TACIs simulation results are obtained, which are in general agreement with the experimentally measured results. Experimental results indicate that the metamaterial designed in this article can distinguish small differences TACIs, providing an effective method for both molecular identification and specific detection in biology and biomedical engineering.

Aerogel is known as super insulation materials and has already been applied to manufacture thermal protection parts of aerospace equipment. Terahertz (THz) wave has good penetration to non-metallic and non-polar materials. To explore its potential in aerogel defect detection, the absorption coefficient and refractive index of aerogel were studied. As a result, the absorption spectrum of aerogel has not obvious absorption peak, and THz wave has great penetration for aerogel. Additionally, two aerogel samples with different sizes of defects were imaged with two time domain parameters (pulse peak and pulse valley) and two frequency domain parameters (absorption coefficient and refractive index). In contrast to other three parameters, the refractive index presents the greatest performance in defect detection, defects in the aerogel can be observed more clearly through the refractive index. The study is of key importance for THz wave in nondestructive testing for aerogel, offering a rapid and non-destructive detection method for aerogel defects.

In this work, the generation of electromagnetic radiation of a wide spectrum, including microwave and terahertz ranges using three-dimensional ordered nanostructures such as photonic crystals were measured. Generation occurred when the exciting electron beam, created by linear accelerator LINAC-200, passed along the planes of orientation of the globules of the photonic crystal. By varying the orientation of the photonic crystal relative to the electron beam and the beam energy, a tunable narrowband microwave and terahertz source with a peak power at 10 W was created. Our experiments involved a set of photonic crystals with different globule diameters and elemental compositions and also included comparative studies using samples of dielectric and semiconductor monocrystals and powders with monodisperse globule sizes. We found that that electromagnetic radiation from single crystals has a similar frequency structure to that of a photonic crystal in the form of a set of narrow-band peaks with a width at half maximum of ~ several MHz appearing in the case, when the beam is passing along the crystallographic orientation axis of the single crystal.

Based on terahertz frequency modulated continuous wave nondestructive testing system, a ceramic pencil holder was studied. Different types of defects are designed on the outer wall of the ceramic pen holder. By using the high penetration and non-destructive characteristics of terahertz waves, the time domain, height and rotation angle information are obtained by scanning the ceramic pen holder in an all-round and multi-angle manner, and high-resolution three-dimensional terahertz images are constructed. At the same time, the processing technology of noise reduction is used to optimize the terahertz images. These images can accurately reveal the tiny defects on the ceramic surface, so as to improve the precision and efficiency of product quality control. Finally, the detection of ceramic pencil holder defects by terahertz frequency modulated continuous wave was realized.

Terahertz waves are capable of non-destructive detecting the structural information and evaluting structural integrity of wood. In this paper, leveraging the high penetration and nondestructive characteristics of terahertz waves, a 120GHz frequency-modulated continuous terahertz wave imaging system was employed to examine the wood samples. The results revealed that terahertz surface imaging can intuitively depict the light and dark textures on the surface of the wood. Furthermore, terahertz waves can penetrate wood, capturing a two-dimensional terahertz image of the interface between the wood and metal plate, and clearly identifying defects with distinct features within the figures.