Abstract:The photovoltaic mid-wave infrared HgCdTe detector， which is operated at room temperature， is simulated to explore laser irradiation saturation characteristics. The results reveal that the heating effect on the HgCdTe material and the lowering of the zero-bias impedance due to irradiation， are significant factors affecting the quantum efficiency of the detector. The model of HgCdTe pn junction is established， and a one-dimensional numerical simulation method is adopted to compute the quantum efficiency and zero-bias impedance of device. The device is irradiated under steady-state. Moreover， the temperature field distribution is coupled in the simulation， the thickness of substrate affects the temperature of the detector， which significantly affects the saturation threshold of the device. Furthermore， the calculations show that the zero-bias impedance of device decreases when the light intensity is raised. The result is compared to the measured performance of device. Finally， the computational analysis serves as a foundation for the development of mid-wave IR HgCdTe detectors with high saturation irradiance threshold.

Abstract:The surface treatment is the beginning of the manufacturing process of HgCdTe infrared detector chip， and its quality will directly affect the yield of the chip. The mechanisms of four typical surface anomalies in the HgCdTe surface treatment process were explored using metallographic microscope， scanning electron microscope （SEM） and X-ray photoelectron spectroscopy （XPS） analysis methods， and the corresponding control measures are proposed. The water mark defect is triggered by oxygen absorption corrosion， and this defect can be controlled by rapidly drying the HgCdTe surface with a stable nitrogen gas flow. The staining is induced by the corrosive liquid unevenly diluted or contaminated by impurities such as water. To reduce the probability of staining， the contamination should be strictly avoided in the process， and the surface should be quickly rinsed after corrosion finish. The round spot originates from the adsorption of the cleaning solution at the material defect， which can be controlled via using isopropanol to soak the HgCdTe before drying. When toluene is in direct contact with HgCdTe， the surface roughness of HgCdTe will increase， thus this direct contact should be restricted.

Abstract:The longitudinal magneto-optical conductivity of monolayer molybdenum disulfide （ML-MoS₂） has been theoretically investigated under the quantizing Landau levels induced by applied magnetic field and proximity-induced exchange interaction via using the random-phase approximation （RPA） dielectric function approach. The effects of the proximity-induced exchange interaction and magnetic field on the longitudinal magneto-optical conductivity are examined. There are two magneto-optical absorption peaks induced by the transitions processes within the conduction band in Terahertz （THz） frequency range. The inter-band transitions between conduction and valence bands result in a series of magneto-optical absorption peaks in the visible frequency range. The results indicate that the magnetic field and proximity-induced exchange interaction could have important influence on the longitudinal magneto-optical conductivity of ML-MoS₂ and it could be applied in promising magneto-optics devices for spintronics and valleytronics working from visible to THz frequency range.

Abstract:The hetero-interface induced anomalous photoluminescence （PL） emissions in the vertical WS₂/Ga₂O₃ heterostructures was demonstrated. The WS₂/Ga₂O₃ hetero-interface varies type-II band structure and brings subsequent PL decline in the bottom WS₂ monolayer contacted with Ga₂O₃ layer. Such hetero-interlayer coupling interaction between oxides and 2D layered transition metal dichalcogenides （TMDs） in the stacked heterostructures impacts interlayer interaction between the bottom WS₂ monolayer and the upper WS₂ monolayer in a WS₂ bilayer， which leads to an anomalous PL enhancement in the bilayer WS₂. Stacked hetero-interface will benefit for controlling the optical or electronic behavior and modulating energy band structures by customizing transformative 2D heterostructures used in next-generation nanoscale optoelectronic detectors and photodetectors.

Abstract:Photodetectors play a key role in many applications， such as remote sensing， night vision， reconnaissance， medical imaging， thermal imaging， and chemical detection. With the increasing complexity of photoelectric detection tasks， photodetectors working in different bands are gradually integrated into broad spectral detection for the same scene. Limited by the volume and task module of the integrated system， conventional broad spectral detection tasks often require multiple detectors working in different bands to perform together， which greatly increases the complexity of detection system. Therefore， photodetector enabling to response ultra-broadband radiation （UV-vis-IR-THz） has gradually become a subject of great interest in recent years. However， there have been no reports on the review of ultra-broadband photodetectors so far. Hence， this review systematically summarizes the research progresses of ultra-broadband photodetectors in the past ten years. The factors affecting the response performance of photodetectors and the main types of common photodetectors are described first， and then the research progress， development status and challenges are reviewed and suggestions about the future research directions of ultra-broadband photodetectors are also provided.

Abstract:The high sensitivity terahertz detection module （HSTDM） is one of the scientific instruments of the China Sky Survey Telescope. HSTDM is a high-resolution spectrometer and the first space heterodyne receiver using niobium nitride （NbN）-based superconducting tunnel junction （Superconductor - Insulator - Superconductor （SIS）） mixer （the NbN SIS mixer）. The NbN SIS mixer must meet the specification requested for a space environment， such as high operation reliability， robustness to vibration， cosmic irradiation， and thermal variation. This paper presents the space qualification tests performed on the NbN SIS mixer， including sine and random vibration tests， single-particle irradiation test， total dose radiation test， and thermal cycling test. The mixer’s performance analysis confirms that it can meet the space application requirements of HSTDM.

Abstract:In recent years， terahertz （THz） imaging technology has received increased attention owing to the promising imaging tools now available for nondestructive testing. However， theoretical models of THz imaging systems are yet to be developed. In this study， we proposed a comprehensive mathematical modeling and simulation theory for the THz imaging system. Gaussian beam distribution was used in the mathematical modeling of the point spread function （PSF）. The target function was convolved using the PSF， and the analog transmission of THz images was realized. The PSF of the imaging system was calculated using the optical imaging method. Next， the image was deconvolution-enhanced using the PSF. The image restoration results revealed that the image resolution improved. The restored image contains more details.

Abstract:In this paper， a wide band cascode power amplifier working at 33~170 GHz is designed， based on the 500 nm InP dual-heterojunction bipolar transistor （DHBT） process. Two pairs of parallel input and output stub lines can effectively expand the working bandwidth. The output coupling line compensates the high frequency transmission. The measured results show that the maximum gain of the amplifier is 11.98 dB at 115 GHz， the relative bandwidth is 134.98 %， the gain flatness is ±2 dB， the gain is better than 10 dB and the output power is better than 1 dBm in the operating bandwidth.

Abstract:The interaction mechanism between terahertz waves and pore defect in glass fiber composites is explored， and the interaction relationship between porosity and terahertz characteristic parameters （refractive index， extinction coefficient and transmission coefficient） is analyzed at 0.075 and 0.713 THz frequency points. The experimental results show that the density and refractive index of glass fiber composites decrease with the increase in porosity. When the frequency is 0.075 THz， the extinction coefficient decreases and the transmission coefficient increases with the increase in porosity based on Rayleigh scattering theory. At 0.713 THz， based on Mie scattering theory， the extinction coefficient increases and the transmission coefficient decreases with the increase in porosity. In addition， the complex and changeable pore morphology results in a non-unique correspondence between porosity and terahertz characteristic parameters. When the different samples have the same porosity， the terahertz characteristic parameters are not the same.

Abstract:Based on the development of a magnetron injection electron gun（MIG） for gyrotron oscillator operating at 140 GHz and with about a megawatt output power， the temperature homogeneity of the cathode and thermal deformation of the MIG were analyzed with ANSYS code. Under an improved temperature homogeneity of the cathode， the geometrical and electrical parameters have been adjusted and optimized to eliminate the effect of thermal deformation on beam trajectory. The simulated temperature of the cathode will be compared with the tested one for evaluating the rationality of the simulation model， which may be helpful for the actual design.

Abstract:In this paper， a dual-tunable triple-band absorber based on bulk Dirac semimetal （BDS） and vanadium dioxide （VO₂） is proposed. The electromagnetic properties of the absorber are analyzed by the finite difference time domain method and equivalent circuit model （ECM）. When the VO₂ is in fully metallic state， the absorption spectrum of the dual-tunable absorber exhibits three obvious absorption peaks with the average absorptance being 98.64%. The resonant frequencies and absorptivity of the absorber can be dynamically controlled by adjusting the Fermi energies of BDS and the conductivities of VO₂. Finally， the relationships between the absorptivity of the dual-tunable absorber and the different thicknesses of the BDS， VO₂ and immediate dielectric layers are further discussed. This work provides potential applications in the designs of multi-banddual-tunable filters and absorbers.

Abstract:In order to enhance the multi-object detecting and tracking capability， a four-beam millimeter wave frequency scanning leaky wave antenna （LWA） based on spoof surface plasmon polaritons （SSPP） has been proposed. According to the theory of sinusoidally modulated reactance superposing surface （SMRSS）， Quad-beam LWA is realized by etching periodical slots on the top surface of substrate integrated waveguide （SIW）， and uniform slots etched on the bottom layer is used to eliminate the open-stopband effect. The measurement shows that the four beams can scan from -52° to 22° within the frequency band from 29 GHz to 30.2 GHz， achieving a total of 74° scanning range， and the beam scanning is up to 18°/%BW， which not only saves the spectrum source but improvs the multi-target detection efficiency.

Abstract:Based on the hybrid integration method， a 335 GHz unbalanced frequency tripler is designed with a symmetrical tapered gradient line matching structure. Under the condition of ensuring single-mode transmission， the matching structure can not only fix the diode position， but also increase the matching effect， and thus solve the problem of narrow 3dB bandwidth for high-frequency band multiplier. The measured results show that the output power of the frequency tripler is all greater than 5 mW in the frequency range of 330-356 GHz. The maximum output power even reaches 11.2 mW at a driving power of 220 mW. The solid-state terahertz local oscillator， as the core device， can drive the 670 GHz sub-harmonic mixer in the superheterodyne receiver.

Abstract:A high conversion efficiency quasi-optical mode converter prototype is designed for 140 GHz TE_22，6 applications. The Denisov launcher is designed based on the periodic perturbation concept， leading to primary radiation field with low edge diffraction. Full-vector physical optics integration solver is used to model and analyze the 3-mirror system. And the 3-mirror iterative phase correction is applied based on the co-polarization field component， so as to achieve high-quality mode conversion. Specifically， the correction of the 1^st mirror sufficiently refines the non-ideal radiated fields from the launcher. It is then validated by numerical investigations that， comparing to the original quaritic mirrors， the phase-shaped mirror system leads to excellent conversion performance. The Gaussian content （η_v） of the output fields rises from 92.7% to 99.6%， while the power transmission efficiency （η_p） reaches 98.8%.

Abstract:With the development of optoelectronic technology， low-light-level （LLL） imaging technology and its application have recently become a research focus. In LLL remote sensing images， it is difficult to completely separate forest fires from industrial combustion and urban heat sources based on radiance or temperature alone. Meanwhile， because of frequent data saturation in the low-light band， the existing fire detection products only provide detection information. To identify forest fires among various heterogeneous heat sources with high brightness and further improve the pixel-level characterization of fire detection products， a new spectral index， the enhanced noctilucent fire disturbance index （ENFDI），is proposed based on the principle that surface temperature decreases with the increasing vegetation density by Latent heat transfer. According to the results， ENFDI enhances the differences in spectral characteristics between forest fires and city lights and improves the ability of forest fire identification under low-light conditions. The forest fires’ ENFDI are significantly higher than those of urban heat sources.Moreover， ENFDI can also effectively relieve the impact caused by LLL band’s proneness to saturation. Not only can ENFDI clearly distinguish flame glow differences within potential saturation zones and enhance the distinguishability of forest fire’s pixels， but the correlation （R） between ENFDI and the mid-and far-infrared brightness temperature difference is as high as 0.94–0.97， which is considerably higher than that of NTL （0.82-0.83）.Furthermore， ENFDI is relatively stable —it is not affected by lunar phases in that forest fires at night are identified with or without moonlight. ENFDI recorded an 87.66% forest fire identification accuracy in this study， which is higher than the 83.91% accuracy of the conventional TMIR method. The forest fires identified using the ENFDI show a good overall correspondence with the NPP/VIIRS active fire product （VNP14IMG）， with a positional tolerance within 628 m. Therefore， ENFDI is sensitive， stable and accurate for identifying forest fires. Further， it may serve as a feasible reference for achieving further pixel-level characterization of fires.

Abstract:A large amount of noise will be generated while spaceborne photon counting LIDAR receive signals， and the signal-to-noise ratio is lower in complex mountainous land， which greatly affects the accurate extraction of vegetation point cloud signals. This paper proposes a density clustering algorithm based on the mountain slope to solve this problem. By analyzing the density of point cloud data and the terrain characteristics of forest targets， coarse noise removal is performed by using the maximum density center search method， and then the slope angle is calculated based on the point cloud data to optimize density clustering and complete the data fine noise removal. By classifying the extracted forest region signal， fitting the vegetation canopy profile and the surface profile， the results show that the proposed algorithm has high accuracy in the extraction of vegetation photon signal， and the RMSE of the ground and canopy profiles are 0.3588 m and 3.7449 m， respectively， which is more suitable for vegetation remote sensing point cloud data processing.

Abstract:This paper establishes the modulation transfer function （MTF） and signal-to-noise ratio （SNR） characterization models of the infrared array-aperture diffractive optical system based on the diffraction imaging mechanism. Subsequently， the imaging system diffraction efficiency is calculated based on the three-dimensional Finite Difference Time Domain （FDTD） method and the imaging characteristics are represented by combining the MTF and SNR. Finally， the effects of different working wavelengths， field of views and filling factors of the primary lens imaging characteristics are analyzed. The analysis results show that the diffraction efficiency， the MTF and SNR of infrared array-aperture diffractive optical system all have spectral and spatial variation characteristics， which reduce with the decrease of the primary lens filling factor. When the filling factor is 0.6， the integral area of MTF decreases by 45.42% and the SNR decreases by 4.92 dB compared with the ideal full aperture system. The established model can be used to characterize the imaging quality of infrared array-aperture diffractive optical system and provide reference to the imaging system design.

Abstract:A mode-locked fiber laser using Semiconductor Saturable Absorber Mirrors （SESAMs） is one of the popular candidate seed light sources for the construction of picosecond pulse fiber amplifier. In this paper， the influence of the fiber length of a gain fiber， the reflectance of the Fiber Bragg Grating （FBG）， the modulation depth， the unsaturated loss， and the saturation flux of SESAMs， the mode field radii of single-mode transmission fibers and a single-mode gain fiber， on the output pulse characteristics， have been theoretically analyzed using the nonlinear Schrodinger equations. The characteristics of the pulse and the spectrum of an outputted laser have also been investigated theoretically. According to the simulation results， we built an ytterbium-doped mode-locking fiber laser system based on the non-polarization-maintaining linear cavity and a SESAM. Without any compensation for intra-cavity dispersion and external polarization control， a stable mode-locked pulse laser output has been achieved with the center wavelength of 1.06 μm， the pulse width of less than 12.51 ps， the spectral width of 0.32 nm， the repetition rate of 37 MHz， and the output power of 2 mW， respectively. The spectral edges of laser pulses appear smooth in our experiment， and the spectral distribution is close to the Gaussian shape. Finally， the overall structure of the near-infrared mode-locked fiber laser has been optimized by the systematic simulation. The mode-locked fiber laser introduced in this paper has a compact non-polarization-maintaining fiber structure， simple intra-cavity configuration with fewer components， high-quality output pulse correlation characteristics， which might provide a practical seed light source with the excellent performance for the next-generation picosecond pulse fiber lasers.

Abstract:People easily get distracted or tired after long-duration actions such as driving and online classes， which can lead to accidents or poor efficiency. To detect such human behaviors， a head motion detection method based on low-resolution infrared array sensors is proposed with the protection of personal privacy. First， prominent areas of the human body are extracted based on image processing techniques. Then a 3D image fusion algorithm is developed to extract the change information in the spatiotemporal domain. Finally， an improved residual network is developed to achieve head motion classification. Ten head movements are designed for driving and online classroom scenarios. Experimental results show that in the detection range of 50 cm to 100 cm， our average recognition rate is 96.76%， and the processing speed is 9 frames per second， which is better than the existing state-of-the-art algorithms. The accuracy of the system is 93.7% when it is applied to the vehicle experiment.