Abstract:This paper introduces a low-thickness sandwich waveguide structure comprising silicon nitride-sapphire-silicon nitride layers. By exploiting its dispersion wave radiation effect and mid-infrared phase matching condition， combined with the waveguide pulse transmission model， this study examines the impact of different physical sizes of the sandwich waveguide on the phase matching point and spectral broadening. Through numerical simulation， a supercontinuum spectrum ranging from 0.5-4 μm is generated， producing a farther mid-infrared dispersion wave at a -40 dB level. Moreover， this model provides an in-depth mechanism for nonlinear waveguide pulse transmission. Theoretical analysis reveals that modifying the physical size of the silicon nitride and sapphire interlayer and altering the phase-matching conditions can regulate the position of the dispersion wave across a broader wavelength range.

Abstract:The cross-sectional scanning tunneling microscopy （XSTM） technique was used to study the cleaved surface of Hg_0.72Cd_0.28Te grown by molecular beam epitaxy. Measurements of scanning tunnel spectroscopy （STS） show that the width of zero current plateau （the apparent tunneling gap） of current-voltage （I/V） spectra is about 130% larger than the practical band gap of the material， implying the existence of obvious tip-induced band bending （TIBB） effect with the measurement. Based on the 3D TIBB model， the STS data can however be interpreted and the calculated I/V spectra are in good agreement with the measurement. Nevertheless， certain deviation appears for those I/V data which were acquired with a large imaging bias. This is because the current TIBB model does not take into account the transport mechanism of the material itself， for which the band-to-band tunneling， trap assisted tunneling etc. could be non-negligible factors for the tunneling.

Abstract:The influence of growth conditions of liquid phase epitaxy on the composition gradient of HgCdTe was studied， and the growth model of liquid phase epitaxy （LPE） of HgCdTe was established. HgCdTe with positive composition gradient was grown by slider liquid phase epitaxy by changing the mercury loss rate. The positive composition gradient structure of HgCdTe grown under the growth condition of specific mercury loss was confirmed by corrosion thinning spectrum and secondary ion mass spectrometry （SIMS）. The experimental results show that the HgCdTe with positive composition gradient had the similar surface morphology and infrared transmission spectrum curve to the traditional HgCdTe with negative composition gradient. It had high crystal quality， with a full width at half maximum （FWHM） of X-ray diffraction（XRD）double-crystal rocking curve of 28.8 arcsec.

Abstract:The photogating effect based on the vertical structure of a two-dimensional material allows high-sensitivity and broad-spectrum photodetector. A high-sensitivity photodetector based on the vertical heterostructure of indium selenide （InSe）/molybdenum ditelluride （MoTe₂） is reported， which exhibits excellent broad-spectrum detection capability from 365 to 965 nm. The top layer of InSe was used as the grating layer to regulate the channel current， and MoTe₂ was used as the transmission layer. By combining the advantages of the two materials， the photodetector has a fast response time of 21.6 ms and achieves a maximum detectivity of 1.05 × 10¹³ Jones under 365 nm laser irradiation. Under the illumination of 965 nm， the detectivity still achieves the order of 10⁹ Jones. In addition， the InSe/MoTe₂ heterostructure exhibits an external quantum efficiency of 1.03 × 10⁵ %， demonstrating strong photoelectric conversion capability.

Abstract:This study investigates an asymmetric tip design for a far-infrared metamaterial aimed at enhancing the Q factor and detection sensitivity. Employing the conventional double-split square ring resonator as a model， we conducted theoretical simulations to investigate the impact of different tip angles on the electric field distribution， resonance spectrum， and Q factor. The results show that the asymmetric tip increases the surface electric field of the resonator， decreases the full width at half maximum （FWHM） of the resonance peak， and increases the Q factor to over three times that of the conventional split ring. Our findings offer valuable insights for the development of highly sensitive far-infrared metamaterial sensors. Furthermore， we propose a straightforward and practical optimization approach to enhance the Q factor of conventional split ring metamaterials.

Abstract:In this letter， an In_0.53Ga_0.47As/In_0.52Al_0.48As InP-based HEMT with f_T > 400 GHz was designed and fabricated successfully. A narrow gate recess technology was used to optimize the parasitic resistances. The gate length is 54.4 nm， and the gate width is 2 × 50 μm. The maximum drain current I_DS.max is 957 mA/mm， and the maximum transconductance g_m.max is 1 265 mS/mm. The current gain cutoff frequency f_T is as high as 441 GHz and the maximum oscillation frequency f_max reaches 299 GHz， even at a relatively small value of V_DS = 0.7 V. The reported device can be applied to terahertz monolithic integrated amplifiers and other circuits.

Abstract:The homodyne mixing system is used to characterizing the performance of terahertz superconducting kinetic inductance detectors （KIDs）. However， homodyne mixing systems still have issues such as mixer imbalance， measurement system integration， and interference signals. The author designed a new single channel homodyne mixing hardware system and software algorithms to achieve integration of the measurement system， calibration of IQ-mixer imbalance， and performance characterization of KID； Furthermore， noise measurement of KIDs in VNA （vector network analyzer） CW mode is achieved； Finally， the method of hardware circuit design by dual channel homodyne mixing system based on autocorrelation algorithm effectively suppresses interference signals. It is worth noting that these research results are applied to characterize the performance of KIDs， which is important in the design of KIDs arrays.

Abstract:Generally， the porosity of traditional Chinese medicine is measured by the destructive method of density measurement， but there is a lack of non-destructive quantification methods. As one important non-destructive method， terahertz radiation has been used to non-destructively extract the drugs'' time-domain and frequency-domain optical information. For direct compression traditional Chinese medicine tablets， the effective refractive index of various tablets was obtained by different methods of terahertz time-domain signal processing and frequency-domain signal processing. It was found that the effective refractive indices with either signal processing method show a good linear relationship with the porosities of the tablets. The porosity of Puerariae Lobatae Radix tablets was extracted and modeled by linear regression based on four effective medium theory models. The porosity regression model based on the effective refractive index by time-domain signal processing shows better performance on the model interpretation and cross-validation accuracy， with the best model of the Bruggeman model （RPD=11.3325）. It provides support for the process optimization of porous powder preparation of traditional Chinese medicine.

Abstract:A homodyne detection system to acquire the thickness of silicon wafers is constructed and described. By harnessing the relationship between the transmission phase change of a 4.3-THz light beam and the incident angle controlled by a mechanical rotating stage， the thickness value of sample can be precisely deduced using the standard residual error method. The results indicate that the fitted thickness of the sample differs by only 2.5~3 μm from more accurate results measured by optical microscopes， achieving terahertz non-destructive thickness measurement with micron level accuracy. The experiment validates the effectiveness of terahertz quantum-cascade laser in non-contact and nondestructive measurement.

Abstract:The current main detection methods for packaging chip defects with small size， dense wiring， and high integration have drawbacks such as low accuracy and long cycle time. To compensate for the shortcomings of traditional detection methods， this study combines terahertz technology with time-domain reflection technology to explore the feasibility of detecting metal wire defects on chips. Firstly， different proportions of convex defects and concave defects were processed on metal microstrip lines of different widths to simulate incomplete opening/short circuits of metal wires in integrated chips. The time-domain reflection signals were collected using a terahertz time-domain reflectometer. Then， qualitative analysis was conducted on different defect degrees and types based on the corresponding time of time-domain reflection pulses， and the defect positions of the metal wires on the chip were accurately calculated. Finally， the finite element analysis method was used to simulate and analyze the metal wires with defects on the silicon substrate， which showed perfect consistency with the experimental results. This research shows that the combined terahertz technology with time-domain reflection technology can achieve the diagnosis and detection of metal wire defects on chips， providing an empirical reference for defect detection in integrated chips.

Abstract:Metasurface provide a new platform for studying planar and ultrathin optical components. The traditional geometric-phase-based metalens are limited to the polarization locking， which inevitably hinders its function in multiplexing of multiple functions. Herein， an approach based on the geometric phase is proposed to realize polarization decoupling of metalens. This method can be applied to design metalens that enables functionalities of polarization switchable multi-functions（focusing and vortex beam etc...） multiplexing in the longitudinal direction or both the longitudinal and transverse directions. The results provide a new avenue for multifunctional integrated planar components.

Abstract:As the connecting spectral band of visible-near infrared and thermal infrared， mid-infrared （MIR） combines the reflective characteristics of short-wave radiation and the emissive characteristics of long-wave radiation， which is of great importance for application scenarios such as temperature detection and target identification. At present， the radiative transfer （RT） calculation of the MIR spectrum still faces the problem of many input background parameters which are not easy to obtain. Based on this， a simplified parameterization scheme of MIR RT for satellite remote sensing is proposed. Using the MODTRAN model （MM）， the background surface and atmospheric parameters involved in the MIR daytime RT process were quantitatively simulated and analyzed to obtain the key parameters affecting RT， from which an MM-based parameter simplified scheme （SS） was developed. The total radiance calculated by SS and MM is compared and validated at the center wavelength of 6 MIR channels of MODIS， and the RMSE is less than 0.004399 with high accuracy. The SS only relies on surface temperature， surface emissivity， atmospheric profile type， water vapor and cloud optical thickness， and does not require input of aerosol， CO₂ and O₃ data. Compared with the MM， the SS reduces the input parameters from 8 categories to 5 categories， and the calculation time efficiency increases by 9.02%. With limited computational resources， the SS proposed in this paper can provide support for the application areas such as fast processing of MIR images and remote sensing simulation of large scenes.

Abstract:With the advantage of high sensitivity and high repetition rate， spaceborne photon counting Lidar has shown great application in ocean areas. The photon counting detector can not only respond to the weak echo signal， but is also susceptible to solar radiation. Due to the great impact of background noise on the performance of Lidar systems， as well as the impact on the data volume， accurate estimation of noise level is crucial in the design of satellite Lidar systems. A noise model of oceanic spaceborne photon counting Lidar was proposed that considers the contribution of the atmosphere， water surface， and water column. By inputting the system parameters of the new generation photon counting Lidar ATLAS and the environmental parameters， the MAPEs （mean absolute percentage errors） between estimated noise and the ATLAS measured noise are within 15%， which confirms the effectiveness of the noise model.

Abstract:The spaceborne light detection and ranging （LiDAR）， as a novel active remote sensing technology， offers possibilities for global diurnal research. In this study， global sea surface chlorophyll-a （Chla） concentrations were inverted using satellite data from Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations （CALIPSO）. A feedforward neural network model based on LiDAR data （FNN-LID） was developed to reconstruct a long-term diurnal dataset of sea surface pCO₂ in the Arctic Ocean. Subsequently， verification and analysis were conducted on the polar sea surface Chla concentrations and sea surface pCO₂ based on active remote sensing. The results demonstrated that the inversion products generated by this algorithm exhibit high data quality and exhibit favorable consistency with both other passive remote sensing products and buoy observations. Moreover， these products effectively fill data gaps during polar winters. Along the Arctic Ocean， margin seas significantly influenced by terrestrial sources consistently display high sea surface Chla concentrations. The spatial distribution of sea surface pCO₂ in the Arctic Ocean manifests meridional variations， with marked seasonal fluctuations， even higher than 80 μatm. Over the past two decades， the Arctic Ocean has consistently acted as a carbon dioxide sink， while areas with substantial sea ice decline such as the East Siberian Sea and Kara Sea exhibit pronounced increases in sea surface pCO₂.

Abstract:This paper describes the design and fabrication of one kind of dichroic beam-splitter that operates in an ultra-broadband spectral range from visible to longwave infrared regions simultaneously. The use of metal-dielectric coatings makes it implement structures that transmit the visible/near infrared radiation from 0.4 to 1.05 μm and reflect infrared radiation from 1.36 to 13 μm. At the same time， the structures are designed to obtain a low linear polarization sensitivity （LPS） in the visible/near infrared region. The transmission of visible/near infrared region is more than 85% and the average reflection of infrared region is more than 90%. The LPS of the region 0.4-1.05 μm is less than 4%.

Abstract:A dual-channel macroscopic imaging system has been developed based on optical methods， which can simultaneously capture the visible light and near-infrared second window （NIR-II） fluorescence. It could provide high-quality bright-field real-time images with the NIR-II fluorescence information， addressing the significant disparity issue between fluorescence and bright-field images in conventional NIR-II macroscopic imaging systems. In the experiment， the anti-scattering capability and imaging performance of NIR-II fluorescence signals of indocyanine green （ICG） were tested using different thicknesses of adipose tissues in the band of 1 100-1 700 nm and 1 300-1 700 nm respectively. Subsequently， the dual-channel macroscopic imaging system was used to obtain lymph node images of mouse and rat models， simulating the lymph node resection surgery and mimicking the process of abdominal lymph node clearance. Finally， different thicknesses of biological adipose tissues were added to the rat model to simulate the presence of adipose tissues covering the lymph nodes during actual surgery， and the penetration capability of the dual-channel system was observed. The visible and near-infrared second window dual-channel fluorescence imaging system provided the intuitive visual information to the operator， reducing surgery time and improving the patient prognosis， and held great potential for application in clinical surgical navigation.

Abstract:Image impulse noise removal is essential for obtaining high-quality images. A novel pixel gradients-based adaptive iterative median filter is proposed to remove image impulse noise by utilizing the principles of thermal infrared camera imaging. Firstly， the maximum pixel gradient of the original image is computed based on the camera''s modulation transfer function （MTF）， and a corresponding set of pixel gradients is established. Subsequently， the gradient weight root-mean-square error （GWRMSE） set of the original image and the corresponding pixel gradient filtered image is computed， and the optimal pixel gradient is determined as the one corresponding to the maximum value of Gaussian distribution of the GWRMSE set. Finally， the adaptive window size and number of iterations for the proposed filter are determined according to the density and complexity of the impulse noise in the image. Extensive experimental results demonstrate that the proposed filter exhibits excellent robustness in removing 8-bit and 16-bit single-channel impulse noise images. In comparison with other state-of-the-art methods， the proposed method can remove low-density random-valued impulse noise （RVIN） and salt-and-pepper noise （SAPN） in real thermal infrared camera-acquired images in real-time while preserving more than 99.5% of original pixels during the noise removal process. Additionally， for high-density SAPN removal， the proposed method achieves competitive results， demonstrating better peak signal-to-noise ratio （PSNR） and structural similarity index （SSIM） in comparison with filtering methods of faster running time and faster execution time in comparison with denoising methods of superior PSNR and SSIM. Moreover， it can recover meaningful image details even for images severely damaged by extreme SAPN （99%）.