This PDF file contains the front matter associated with SPIE Proceedings Volume 12324, including the Title Page, Copyright information, Table of Contents, and Conference Committee Page.

Terahertz (THz) wave (frequency range: 0.1-10 THz; 1 THz=1012 Hz) is in the electromagnetic spectrum region between microwave and infrared light, and has important application prospects in the fields of spectral detection, medical imaging, space communication, etc. It has become a hot spot of theoretical and experimental study. When driven by periodic periodic electric fields, electrons in semiconductor superlattices and multiple quantum well structures will exhibit different behaviors, which in turn affect their physical properties such as output current and power. We theoretically studied the transient output power of the optical-injected THz quantum cascade laser (QCL) driven by a time periodic current. It is found that the system displays transient instability. It is found that the transient output power of the QCL exhibits periodic, quasi-periodic, and chaotic oscillation states under the control of external field. The transient dynamic properties of THz QCLs under control of external field open a new way for designing novel THz imaging modality.

Realizing polarization manipulation of light at subwavelength scale has developed into an emerging field involving optical communication and quantum information processing. Metasurfaces, due to their extremely strong capabilities in polarization and wavefront manipulation, have been demonstrated to be useful for designing multifunctional and integrated polarization optics. In this work, a versatile terahertz metasurface platform relying on spatially interleaved nanoscale quarter-wave plate is established, to implement wavefront steering while realizing polarization conversion between linear polarization and circular polarization. It is verified by simulation that the constructed metasurface possesses the function of dual-channel polarization conversion, and the output waves generated under the incidence of linearly polarized or circularly polarized waves are focused vortex waves with different topological charges. The terahertz metasurface platform established in this study opens up new avenues for advanced research and application fields facilitating the development of miniaturized, integrated, and versatile optical devices.

The Leighton Chajnantor Telescope (LCT) project, sponsored by Shanghai Normal University in collaboration with Caltech and the University of Concepción, is seeking to relocate the Caltech Submillimeter Observatory (CSO)[1] from Mauna Kea, Hawaii to Llano de Chajnantor Observatory on the Chajnantor Plateau in Chile. The LCT will be equipped with a new 345-GHz band heterodyne array receiver of 3×3 beams and quantum-limited sensitivity. Based on superconducting Nb/Al-AlOx/Nb tunnel junction (SIS) mixers, we have developed a compact 1×3 array as one unit of the new heterodyne array receiver. Detailed design and measurement results will be presented.

A highly sensitive terahertz parametric up-conversion detector based on KTiOPO₄(KTP) crystal pumped by 1064nm laser was demonstrated in this paper. THz wave was generated in KTP crystal with a terahertz parametric oscillator (TPO), which can generate THz wave from 1.17-5.98 THz by varying the phase-matching angle between the pump and Stokes wave inside KTP crystal. THz wave and pump wave were mixed in KTP crystal to generate up-conversion signal based on stimulated parametric scattering. The up-conversion signal was amplified in another two KTP crystals based on non-collinear and collinear phase matching to improve detection sensitivity. Spectrometer and photodiode were used to measure the wavelength and pulse energy of up-conversion signal respectively. The detectable THz frequency range was 4.26-4.50 THz and 4.80-4.92 THz. The minimum detectable energy of 250 pJ was realized with dynamic range of 32 dB at 4.40 THz, and the minimum detectable energy at 4.85 THz was 9.4 pJ with dynamic range of 48 dB. All experiments were carried out under pump threshold conditions of spontaneous parametric noise generation. Compared with LiNbO₃ crystal, the parametric up-conversion detection based on KTP crystal can realize high frequency range (>3 THz) THz wave detection, filling in the gaps for high-frequency detection.

Superconductor–insulator–superconductor (SIS) mixers remain the choice of heterodyne mixers for single-dish telescopes and interferometers at millimeter and submillimeter wavelengths. Compared with conventional Nb/AlO_x/Nb superconducting tunnel junctions, Nb/AlN/NbN ones have larger gap voltage and may reach critical-current density beyond 10kA/cm², which are both of particularly interest in developing broadband SIS mixers. Here we report on the design and measurement of an SIS mixer based on Nb/AlN/NbN parallel connected twin junctions (PCTJ) incorporating NbTiN/SiO₂/Al microstrip circuit. The junctions have a gap voltage of 3.18mV and a critical-current density of 15kA/cm². The measured receiver noise temperature reach 5hν/k_B among 200-260GHz band, and the mixer’s fractional bandwidth is about 40% centered at 230GHz.

In drone detection and short-range automotive radar, the detection system should have the ability to monitor a large area and to detect the distances and directions of multiple targets. Suffered from electronic bottleneck, it’s difficult to achieve a high-performance detection via conventional radar system. Microwave photonics has the advantages of low loss, large bandwidth and so on, and greatly improves the performance of radar system. To realize simultaneous distance and direction detection, microwave photonic phased array radar steers the beam for inertialess and squint-free scanning to detect targets from different directions. However, limited by the narrow angle scanning range, its field of view is inadequate. Photonics-assisted AOA (angle-of-arrival) detection method can detect the direction of target in a large-angle range. Nevertheless, it can hardly obtain the distance information of targets. It is a key issue to perform wide-angle simultaneous detection of distances and directions for multiple targets. We introduce a microwave photonic radar for distance and direction measurement of multiple targets. At the transmitter, a linear frequency modulation signal is generated by photonic frequency doubling and transmitted for detection. At the receiver, echo signals are received by a uniform linear array with three antennas. The intermediate antenna supplies overlapped reference spectra by polarization-multiplexing modulation. The de-chirped spectra possess specific symmetry, thus de-chirped frequencies of each antenna are clearly distinguished and distances and directions of multiple targets can be obtained. Two-target proof-of-concept experiments have verified that the average measurement errors of distance and direction are 2.4 cm and 0.6°, respectively.

GaAs Schottky barrier diode (SBD) based terahertz mixer and frequency multiplier represent one of the most important method for terahertz signal emitting and receiving from 0.5THz to 5THz. Compared with original GaAs substrate, quartz using as GaAs SBD circuit base could suppress transmission loss and high order transmission mode on chip, benefits from low dielectric constant of quartz. In this paper, GaAs SBD was integrated on quartz substrate using transfer printing technique, which could achieve membrane device transfer and low cost high output of original GaAs wafer.

Optical regulation strategy with the aid of hybrid materials can significantly optimize the performance of terahertz devices so as to be used in the real world. Gold nanobipyramids (AuNBPs) with synthetical tunability to the nearinfrared band show strong local electric field enhancement, which improves the optical coupling at the interface and benefits the modulation performance of all-optical devices. Here we design AuNBPs-integrated terahertz modulators with multiple structured surfaces and indicate that introducing AuNBPs can effectively enhance their modulation depths. In particular, an ultrahigh modulation enhancement with one order of magnitude can be achieved in the AuNBPs hybrid metamaterials accompanied with the multifunctional modulation characteristics. Applying the coupled Lorentz oscillator model, the theoretical calculation suggests that the optical regulation with AuNBPs originates from increased damping rate and higher coupling coefficient under pump excitation. With the help of excellent modulation enhancement in the AuNBPs integrated metastructures, a prototype of novel spatial light modulator is constructed. As a novel terahertz photonic device with the low-power consumption and multifunctionality, this modulator is promising for the potential application in spatial and frequency selective imaging.

In this work, we use terahertz (THz) pulses and near-infrared laser pulses to excite molecular motion modes of liquid water in resonant and non-resonant states, and record the corresponding THz Kerr effect (TKE) and optical Kerr effect (OKE) curves at the sub-picosecond time scale. We analyze the frequency dependence and energy dependence of TKE and OKE responses in water. The characteristics and advantages of the two schemes in low-frequency molecular dynamics observation of liquid water are summarized by calculating the signal-to-noise ratio (SNR). Our work provides valuable insights into the microscopic origin of dielectric properties in water, and provides a new method for observing low-frequency molecular motion in various aqueous solutions and even in biological tissues.

Terahertz wave becomes key technology for 6G next generation communication and advanced driver assistance (ADAS) applications. US Federal Communications Commission opens experimental spectrum licenses for 6G. The spectrum falls in the 95 GHz to 3 THz range. The automotive ADAS employs sub-THz radar to plan routes and avoid obstacles. The sub-THz radars usually select the spectrum of 24GHz, 77GHz and 79GHz. Composite communication materials, such as quartz glass, bakelite and polycarbonate are the basic ingredients for 6G communication and ADAS application. In communication channel modeling, the microelectronics device packaging and environment materials’ dielectric parameters should be known in advance. In detail, the transmission characteristics (transmittance, reflectance) and complex permittivity of various materials need to be characterized. Vector Network Analyzer (VNA) is usually employed to measure specific band dielectric parameters. To get broadband characteristics, the VNA needs to be calibrated at each frequency band. To get materials broadband dielectric property, the VNA method is expensive and time-consuming. Terahertz time domain spectroscopy (THz-TDS) emitted a pulse THz signal. The frequency domain spectroscopic waveform was gotten by Fourier Transform, and dielectric parameters were calculated. The frequency range is broadband from 30 GHz to 2 THz. The frequency resolution is as high as 380MHz. The permittivity is consistent with the VNA measured result, but THz-TDS do not need band switching and calibration. It is a promising candidate for evaluating the dielectric characteristics of 6G communication and ADAS materials.

A novel method to modulate the phase-matching (PM) condition based on the linear electro-optic (EO) effect in cubic nonlinear crystals was proposed to enhance the efficiency and broaden the PM bandwidth of terahertz generation. Taking ZnTe and CdTe crystals as examples, monochromatic terahertz waves can be difference-frequency generated (DFG) and agilely tuned under perfect PM condition over a range of 2.25 THz and 1.84 THz, respectively, which also corresponds to large allowable wavelength and divergence angle of the pump beam. Simultaneous wideband terahertz generation via optical rectification (OR) modulated by the EO effect was also investigated. It introduces an extra degree of freedom to fulfill PM condition of different excitation wavelengths and polarization states by OR, where the polarization selectivity can be optimized by controlling the applied voltage.

We demonstrate here for the first time generation of tunable terahertz (THz) radiation based on photomixing continuous wave (CW) radiation of the DFB laser with fixed wavelength and self-sweeping Er-doped fiber one operating near 1560 nm. The latter sequentially generates longitudinal modes with duration of several milliseconds and regular hopping between them. In this case signal accumulation of THz radiation within generation of each mode can be performed to increase signal-to-noise ratio (SNR). We demonstrated the possibility of designing a spectrometer with spectral resolution down to 50 MHz in the tuning range of ~5 GHz near the central frequency of 100 GHz. The proposed approach can be useful in the development of spectroscopic systems, millimeter-wave lidars, and telecom devices.

It is known that such features of pulsed THz radiation as a small number of periods and an ultra-wide spectrum modify the appearance of linear effects like absorption, dispersion, and diffraction. In this work, we study the influence of the spatial localization of a THz pulse during its generation. It is shown that this leads to a spatial chirp of the pulse. Based on the results obtained, recommendations are formulated for working with such pulses, which make it possible to guarantee that spectral information will not be lost during the propagation of such pulses.

We proposed and demonstrated an ultra-high sensitive refractive index metamaterial sensor within terahertz region. The sensor consists of a gallium arsenide (GaAs) structure on the top, aluminum in the middle, and a polyimide as the bottom substrate. The simulation results show that such sensor has an absorption peak at 2.0 THz with the Q-factor up to 444. The sensor is very sensitive to the change of the refractive index (RI) of the surrounding medium. The metamaterial sensor with optimized structure has a sensitivity of 1.762 THz/RIU and a figure of merit (FoM) value of 392 RIU-1, respectively. Meanwhile, both the absorption peak position and intensity of such metamaterial sensor could be adjustable. The proposed terahertz metamaterial sensor with such high sensitivity and easily tunable properties has broad application prospects in research fields including biomedical sensing, disease diagnosis, and also trace detection of hazardous substances.

Theoretical models for the backscattering intensities of Lambertian tilted plates, spheres and cones in monostatic radar situation were proposed, and their one-dimensional (1D) range profiles were simulated in the terahertz range. Then the 1D range profiles of picosecond and nanosecond pulse incidence are compared, which reveal that more details of object shapes can be obtained with the ultrashort terahertz pulse. The influences of target size, posture, pulse width and waveform were also investigated, respectively.

Traditional Chinese medicine tablets have issues such as poor compressibility and delayed disintegration. We mainly get porous traditional Chinese medicine to solve the problem through pore-making technology. Terahertz technology provides high resolution, low energy loss, and is non-destructive and non-invasive. We have obtained the spectral data of Pueraria lobata tablets with terahertz time-domain spectroscopy. Porous and non-porous structures have effects on the efficacy of traditional Chinese medicine. The spectral data have been distinguished according to porous and non-porous structures. The time-domain and frequency-domain spectra were compared and analyzed respectively, and related parameters such as refractive index and absorption coefficient were studied simultaneously. By comparison, we have found that terahertz spectroscopy is sensitive to the substance structure of traditional Chinese medicine, and terahertz technology can be used to detect the porous and non-porous traditional Chinese medicine. The results contribute to the methodological advancement of process analytical technology (PAT) of traditional Chinese medicine.

Blast-induced TBI (bTBI) is a type of traumatic brain injury generated by a shock wave that causes the head to quickly accelerate or decelerate. It has huge challenges in the diagnosis and prognosis due to its absence of significant edema and traumatic areas. The major diagnosis methods for bTBI has some defects such as time-consuming, poor specificity, low sensitivity. Thus, it is highly desirable to establish a rapid and high-sensitivity method for the detection of bTBI. Raman spectroscopy technique has been used in biochemistry due to its label-free and non-destructive. It is expected to detect and monitor the progression and regression of bTBI from molecular perspective. In this paper, the Raman spectra of the hippocampus and hypothalamus tissue were measured at 3h, 6h, 24h, 48h and 72h after mild and moderate bTBI in 2800-3000cm^-1 range. The results showed both the two brain areas had a significant difference in intensity of Raman characteristic peaks at 2855, 2885 and 2934cm^-1 at different time points compared with the sham group. It demonstrates that the content of the lipids and proteins have been changed in the rat hippocampus and hypothalamus after bTBI, due to the Raman peaks at 2855, 2885 and 2934cm^-1 assigning to C-H stretching of lipids and protein. It infers that Raman spectroscopy technique has the potential to be a rapid and effective diagnosis and monitoring method in bTBI clinic.

Microwave photonic frequency conversion and transmission is highly needed in modern distributed communication systems. However, the periodically power fading and co-frequency interference limits the working frequency range, transmission distance, and the number of channels. To address the above questions, a novel dual-channel microwave photonic frequency conversion and transmission method is proposed. A DPol-DPMZM modulator is applied and the dual-channel intermediate frequency (IF) signals and the local oscillator (LO) signal are both applied to the DPol-DPMZM modulator to produce optical sidebands of IF and LO signals. Through the jointly manipulate phase of optical sidebands, the periodically power fading and co-frequency interference problems can be simultaneously addressed, which guarantees the broadband and multi-channel performance of microwave photonic frequency conversion and transmission system.

The jet high-altitude high-speed unmanned-aerial-vehicle(UAV) plume is the main source of infrared radiation and has special radiation characteristics compared with that of human aircraft，and is one of the anti-stealth technology points. In order to master the infrared radiation characteristics of high-altitude and high-speed UAV plume independently, and facilitate the design of subsequent algorithms and apply artificial intelligence technology to autonomous recognition and tracking, the authors make a systematic and comprehensive study on it. This paper is one of the important parts of the research. Geometric model of turbojet engine nozzle is established by Gambit software. The flow field of high-altitude high-speed UAV plume is numerically simulated by Fluent software. Considering airframe shading, infrared radiation intensity distribution of UAV plume is calculate by using spectral line Lorentz Broadening Effect and Doppler Broadening Effect in single-line spectral band theory(SLG). In this paper, the attenuation of infrared radiation in the atmosphere is calculated by the method of slope path transmission equivalent. The distribution of infrared radiation intensity of typical high-altitude high-speed UAV after atmospheric attenuation under different working conditions, different azimuth angle and observation elevation angle is calculated, and the influence of flight attitude of UAV on the distribution of infrared radiation intensity of its plume is analyzed.

This paper proposes a method of infrared moving target detection and its azimuth information display based on background difference. First of all, in view of the shortcomings of the current VIBE (Visual Background Extractor) algorithm, we propose a moving target detection algorithm based on the improved visual background extraction. This improved algorithm makes full use of the temporal and spatial domain information of the image, which can not only adapt to different detection environment, but also effectively eliminate ghosting to have better detection results for small targets. Then, we design an infrared video moving target detection system based on FPGA, including three modules, namely target detection module, GPS (Global Position System) information extraction module and character superimposing module. Furthermore, the system has the function of target extraction and azimuth information display. Compared to the traditional two-dimensional(2D) infrared moving target detection, this method superimposes GPS data onto 2D image information to facilitate real-time detection so that the moving state of the target can be understood more comprehensively, which has wide application value in the sea, land and air moving target detection and location.

Due to obtaining Unmanned aerial vehicle (UAV) thermal infrared remote sensing (TIRS) data being difficult, a few works are proposed based on TIRS images. In addition, these mosaic methods usually face dislocation and distortion problems when facing a large number of TIRS images. To overcome this limitation, this paper proposes a mosaic method for large-amount TIRS images with less distortion. By using the prior sequence and coordinates information recorded during UAV flight, we can obtain a reliable panoramic image. We build a UAV TIRS image dataset containing a total of 500 images, and we conduct experiments on this dataset. Our method achieves low RMSE value and satisfying mosaicked results, which demonstrates the effectiveness of our method.

Photodetectors are the fundamental element for realizing scientific and commercial applications. Perovskites have attracted much attention as alternative materials for next generation photodetectors, owing to the high absorption coefficient of visible light, high carrier mobility, and unusually high defect tolerance . Previous studies show the working waveband of perovskite photodetector usually be confined to the visible light range. However, the weak absorption in the infrared and the low carrier mobility of perovskites limit the fabrication of high-performance photodetectors. Introducing graphene and its derivatives into perovskites is a proven and effective method to improve the optoelectronic properties. In this work, sulfonated graphene oxide (s-GO) was blended into the organo-inorganic methylammonium lead halide perovskites to fabricate the high-performance near infrared (NIR) photodetector. s-GO is an efficient carrier transport, thereby carrier mobility of doped perovskites can be effectively increased. After blending, the photodetector is twice the responsivity at 1064nm laser of the photodetector with no additives. Meanwhile, the fast response (less than 17 ms) and the impressive low noise equivalent power are achieved.

Coding metasurface has attracted much attention due to its flexible design of coding sequences and powerful control ability of light beams. However, the traditional coding metasurfaces with pin-diode switches between two metallic patches are usually used in the microwave band. Few studies have been carried out in the terahertz (THz) region with tunable metastructures. In order to realize the dynamic modulation of terahertz metasurface, in this paper we use the phase change material vanadium dioxide (VO2) to activate modulation coding metasurface in the terahertz band. We designed a VO2 embedded hybrid structure with metallic patches as the metasurface unit, which can produce a 180- degree phase change near 0.69 THz during the phase transition of VO2 from an insulating state to the metallic state. Meanwhile, we have constructed a metasurface array with the above designed tunable VO2 components and non-tunable metallic units to realize the dynamic switching of the far-field beam at that frequency. Our simulated results indicate that when the VO2 conductivity increases from 200 to 200000 S/m, the far-field reflected beams of the metasurface array can change from the separation of about 41° apart to close together. Notably, this coding metasurface will remain the reflectivity higher than 0.76 at the working frequency and exhibit polarization insensitive feature to the incident light. The active coding metasurface we designed provides a new idea for flexible beam control and has broad application prospects in terahertz functional devices.

Terahertz metamaterials with electromagnetically induced transparency (EIT) have attracted extensive attention recently due to the broad application prospects in communication, optical storage, slow light effect, and biosensing. Here, we have studied the EIT effect caused by the interlayer coupling of two asymmetric split ring resonators with four gaps. The upper and the lower layers spaced by the intermediate Si have the same metastructures with the rotated angle of 90° to each other. By varying the length of the metallic arm, we find that the EIT effect becomes increasingly apparent as the asymmetry coefficient decrease. The simulation results indicate that with the increase of the thickness of Si layer, the EIT phenomenon will first emerge, gradually become the strongest with the thickness of 5μm, and finally tend to be weakened after further increasing the Si thickness. Meanwhile, the frequency of the transparency peak exhibits redshift with the Si thickness. It is also found that the EIT effect can be further optimized by adjusting the microstructure width of the split ring resonators. When the asymmetry coefficient and the thickness of the intermediate layer is determined, the EIT effect becomes most obvious with the width of 3 μm, and will gradually weaken with the increase of metallic width. The transparency peak frequency presents blue shift simultaneously. Our designed metastructure could provide the optional approach to modify the EIT behaviors and play an important role in the sensors and modulators.

We demonstrate terahertz (THz) coherent detection using ethanol and ethanol-water mixtures. Combined with the THz-induced second harmonic (TISH) radiated from the liquid plasma and the control second harmonic (CSH) generated by BBO crystal, when the polarization directions of the CSH and TISH beams are parallel, a measured time-resolved waveform of the THz field is obtained due to the four-wave mixing mechanism. Since the third-order nonlinear coefficient of ethanol is larger than that of pure water, our scheme further enhances the sensitivity and signal-to-noise ratio of coherent detection. Meanwhile, the amplitude of THz wave detected by ethanol-water mixtures were increased with the increase of the ethanol concentration. This work provides a new perspective for exploring the solute-solvent molecular interaction and lays an experimental foundation for the theoretical analysis of THz wave coherent detection of classical liquid containing hydrogen bonds.

Electromagnetically induced transparency (EIT) can be analogically achieved by terahertz (THz) metamaterial, which has extensive applications in sensing, filtering, and slow light devices. Here, we firstly construct a metastructure that can modulate THz transmission, consisting of an outer symmetrical split ring resonator (SRR) embedded with two inner closed ring resonators. The simulated THz transmission spectrum presents a simple lineshape superposition of two resonances, corresponding to the low frequency dipole mode at 1.184 THz from the external SRR and the high frequency dipole mode at 1.757 THz from the closed ring resonators, respectively. However, the EIT phenomenon can be observed by replacing the inner part with two asymmetric split ring resonators. We have attributed this to that the inner metastructure can induce an extremely weak LC resonance at 1.074 THz due to the breaking of structure asymmetry. This mode will couple with the above dipole resonance of the outer SRR to accomplish the EIT effect through the near-field coupling of the weakly accessible bright-mode and the strongly excited bright-mode in this system. By varying different parameters, we have found that the impact of the rings distance on the EIT effect is more obvious. To further modulate the EIT window, the semiconductor silicon was placed at the opening gaps of the two inner asymmetric split ring resonators. Our simulated results indicate that with the increasing of the silicon conductivity from 0 to 9000 S/m, the EIT peak will gradually weaken and finally vanish, which is consistent with the results of closed ring resonators and shows the switch on/off of EIT phenomenon. Our work provides a design approach to control the electromagnetic transparent peak and manipulate EIT effect, for the potential applications in versatile THz devices.

We demonstrate a low thermal mass long wave infrared broadband metasurface absorber employed for a double-layer umbrella 8μm small pixel pitch microbolometer, which is achieved by depositing an array of metal patches on the thin dielectric slab and forming an optical resonator with a metal back-reflector. The absorber achieves near-unity optical absorptivity within the long wave infrared regime (8-14μm) due to its optical impedance exhibits a good matching characteristic with the free space impedance of 377Ω, while reducing significantly the thermal mass and obtaining the time constant of 10.96 ms. Consequently, the thermal response speed of the microbolometer is increased by nearly 50% compared with the multilayer absorbing film. Furthermore, maximum thermal mass reduction of the device is performed, finding the optimal perforated diameter 0.75μm with the thickness of SiNx film 65nm, which reduces the absorbing area by 28.8%, lower the time constant to 9.7ms and further improve the response speed of the detector, while maintaining the average LWIR absorption as high as 96%.

Differences in hydroxypropyl cellulose (HPC) include the degree of hydroxypropyl substitution of cellulose or the molecular weight of hydroxypropyl cellulose. The differences have an impact not only on its physicochemical properties but also on the drug delivery process. It is sensitive to a wide range of intermolecular vibrations as well as lattice phonons using Terahertz time-domain spectroscopy (THz-TDS). We have obtained the spectral parameters of various kinds of hydroxypropyl celluloses by THz-TDS measurement. The experimental results show that the spectral characteristics of various kinds of hydroxypropyl celluloses are distinguishable. Using THz-TDS, we can quickly and accurately distinguish different hydroxypropyl celluloses and analyze the physicochemical properties of different hydroxypropyl celluloses.

The characteristic absorption peaks of the infrared spectrum correspond to different functional groups and chemical bonds. The infrared qualitative analysis of the functional groups can be carried out by analyzing the position of the characteristic peaks of the spectrum, and the quantitative analysis of the functional groups can be carried out by analyzing the peaks. In order to study the aging performance of hydroxyl-terminated polybutadiene (HTPB) propellants from the perspective of infrared spectrum, HTPB propellants under accelerated aging at 70°C were tested for both their dynamic mechanical properties and Fourier transform infrared spectrum. The loss factor curves, tanδ, obtained from dynamic thermomechanical analysis (DMA) were used for analyzing the aging rule of dynamic mechanical properties. For infrared spectrum, spectral subtraction was used to get the characteristics peak during aging process, by which the corresponding functional groups can be found to analyze the aging mechanism. Further, in order to quantitatively characterize the aging of HTPB propellant by using infrared spectra, second derivative data conversion was used to eliminate the interfering factors on spectrum such as ambient temperature and instrument. The correlation between peak value of peak α in loss factor curve and infrared spectra was analyzed, then. The results show that peaks at wavenumber 3600cm^-1-3100cm^-1(-OH), 1186cm^-1(C-O-C), 885cm^-1(epoxy structures) and 1650cm^-1(C=O) of FTIR spectrum second-order derivative have high linear correlation to peak value of peak α. The conclusion obtained from the correlation analysis is consistent with the oxidative crosslinking mechanism in the aging process of HTPB, which confirms the feasibility of quantitative characterization of propellant aging by infrared spectrum.

Electromagnetic induced transparency (EIT) in terahertz band can achieve group delay and transparent window, which is attractive in biosensing field. In this paper, based on the phase transition properties of vanadium dioxide (VO2), an EIT with metasurface is designed to adjust the frequency position of transparent window. The unit cell of the metasurface consists of a cut wire (CW) resonator embedded with VO2 and two C-shaped ring resonators. The simulations show that when VO2 is in the insulating phase, the EIT window appears at 0.58-0.74 THz. While VO2 is in the metallic phase, the EIT window is located at 0.37-0.50 THz. The proposed structure has the active regulation of the EIT frequency band, which provides great potential for the sensitivity detection in THz.

In this work, we report a separate absorption and multiplication avalanche photodiode (SAM-APD) with 100% cut-off wavelength of ~2.1 μm at 300 K grown by molecular beam epitaxy. The electron-dominated avalanche mechanism multiplication region was designed as a multi-quantum well structure consisting of AlAsSb/GaSb H-structure superlattice and Al_0.3In_0.7AsSb digital alloy. At room temperature, the device exhibits a maximum multiplication gain of 79 under -13.3 bias voltage.

In order to realize multi-function, the radar has wide instantaneous bandwidth and working bandwidth. Unfortunately, the traditional phased array radar antenna by microwave phase shifter in the way of phasing. With the increasing demands of radar network performance in terms of bandwidth, distance and synchronization accuracy of information transmission links, microwave photonic radar network is gaining great attention from researchers. This paper analyzes key technologies in microwave photonic optical beamforming networks, reviews the recent advancement and discusses its possible research directions in the future.

6G wireless communication technology is expected to provide a higher peak data rate, lower latency, high mobile speed, high spectral efficiency, and high network energy efficiency in the future. It has the advantages of wide coverage, high security, and low-cost efficiency. As one of the candidate frequency bands of 6G technology, THz wave (0.1-10 THz) bridges the infrared band and the microwave band and has a very important application prospect in the communication process. Due to the terahertz source power and the absorption of various particles in the air, indoor short-range terahertz wireless communication has practical research value. In this paper, for the three-dimensional scene of common indoor conference rooms, the ray tracing method is used to model the terahertz channel of the line-of-sight (LOS) path, the primary reflection path, and the secondary reflection path. The carrier frequency range used for the simulation is 220-330 GHz. By calculating the power loss and required time of each path from the transmitter to the receiver, parameters such as the power delay profile are obtained. Then, the related terahertz channel parameters such as Rician K-factor and root mean square (RMS) delay spread are analyzed.

The oscillation frequency of terahertz (THz) waves is between 0.1 THz and 10 THz. THz time-domain spectroscopy (THz-TDS) systems usually measure samples at the focus of THz beams. The Gouy phase shift has attracted widespread attention since it was discovered and has an important connection between the beam propagation dynamics and the dispersion of light waves. In this case, the Gouy phase shift affects the inversion dielectric spectrum of the sample. In order to more conveniently detect the phenomenon of Gouy phase shift, we propose a method of using a water column as a probe to detect the distribution of THz beams when they converge and directly observe the Gouy phase shift, owing to the sensitivity of this detection method to THz amplitude and phase. The Gouy phase shift in the THz band can be observed with simple processing by detecting the THz time domain signal under different water column position using THz-TDS. This method is of great significance for better selection of THz sources and coherence tomography of samples in the future. At the same time, it provides more help to control the Gouy phase shift to develop phase-shift interference technology and enhance the depth resolution of THz imaging.

The dependences of the refractive index of a congruent LiNbO3 crystal cut perpendicular to the x and z axes on the radiation frequency in the range of 0.25–1.25 THz are presented. These dependences are presented for different values of the crystal thickness - 0.52 mm, 1 mm and 2.21 mm. A comparative analysis of the obtained dispersion curves with the results from other works is presented. The comparison was carried out by estimating the dispersion broadening of a THz pulse with time in the process of simulating its propagation in a medium with a given dispersion. It is shown that a 1.5-cycle THz pulse is broadened in media with dispersions found in other works, which does not correspond to experimental data. In accordance with this, it was concluded that the dispersion curves for congruent LiNbO3 from the considered works do not agree with the real values of the refractive index in the THz frequency range.

Rapid progress in big data technologies results in the need for a drastic increase in wireless data transfer rates. To meet the demand, it is necessary to develop communication systems operating at carrier frequencies in the range 0.1–10 THz (so-called THz band). They can provide data transfer rates of more than 1 Tbps in in-door deployments. However, existing hardware solutions for THz transmitters/receivers usually rely on the use of hollow metallic waveguides, which already suffer from noticeable insertion losses of 0.24-0.31 dB/cm at 110 GHz. And these losses increase even more with further increase of operating frequency. Thus, use of metallic waveguides potentially compromises signal strength and sensitivity, increases design complexity and fabrication costs of a transceiver. One of the potential solutions is to use low-loss fully dielectric wave-guiding structures. In this work, we report on the development of a THz dielectric waveguide made from a high-resistivity Si substrate. A square lattice of openings is fabricated in the substrate via the Bosch process. Transmission and reflection spectra of the fabricated waveguide samples are measured over the frequency range 135–160 GHz. Similar to the simulation forecast, we measure insertion and return losses of 0.04 dB/cm and 20 dB at 150 GHz, respectively, which meet requirements of modern practical applications.

The growing interest in the low-dimensional structures research has necessitated the development of non-contact optical methods for their thermal properties measurement. Thermoreflectance method allows thermal conductivity evaluation based on the analysis of material reflection coefficient change due to the laser radiation absorption and subsequent heating of the sample. Transient thermoreflectance in the frequency-domain (FDTR) and in the time-domain (TDTR) are already widely used for such measurements. More recent steady-state thermoreflectance method (SSTR) has the advantage above TDTR and FDTR which lies in the possibility to perform direct measurements resulting in higher accuracy and ease of measurement procedure. The current work is dedicated to the development of the approach for SSTR technique simulation. It includes the description of experimental procedure and the data collection, analytical and numerical data processing, finite elements simulation and the verification of the results. The proposed methodology has been performed on the example of 3 samples: Ge, Si, GaAs. The experiment included the measurement of the radiation power absorbed and reflected by the samples. Then, analytical and numerical models have been derived and used to calculate absorption and reflection coefficients taking in account Fabry-Pérot effect. The finite elements model has been carried out to simulate electromagnetic heating of the studied samples and to evaluate their temperature. The model took into the consideration a normal distribution of a laser beam its diameter. For the model validation the temperature maps captured by a thermal imager have been compared with the numerically simulated ones. The discrepancy did not exceed 9%. The performed approach can be used for SSTR setup calibration and analysis of thermal processes in the samples under study.

The terahertz (THz) frequency range is electromagnetic radiation in the range from 0.1 to 10 THz, which has high potential for use in non-destructive testing and control of the moisture content of objects. For a long time there was no elemental base of elements (receivers, transmitters, modulators, etc.) to work in this range. At the moment, existing detectors are made according to difficult and inaccessible technologies (like as molecular-beam epitaxy). In this paper, terahertz detector based on a thin-film structured thermoelectric material (solid solution of bismuth-antimony with a concentration of antimony of 12%) and copper contacts on a mica substrate was numerically demonstrated.