Editor in chief：Jun-Hao CHU
International standard number：ISSN 1001-9014
Unified domestic issue：CN 31-1577
Domestic postal code：4-335
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Abstract:Continuouswave terahertz imaging was demonstrated based on an opticallypumped farinfrared gas laser and a Golay cell detector. The imaging system design and construction are discussed in the context of the choice and configuration of the involved terahertz optical components and devices. Performance characteristics of the system, including signaltonoise ratio, spatial resolution, detector response and imaging speed were measured and analyzed in detail. With this system, terahertz transmission images of various samples were obtained, confirming the quality of our setup and supporting the feasibility of terahertz imaging technique for applications in security screening and quality control. A data processing method for automatic calibration of the image intensities in the background region was also proposed to remove the influence of laser power drift on the image quality.
Abstract:CuIn1-xGaxSe2 (CIGS) is a promising direct bandgap semiconductor material for developing a new generation of highefficiency and lowcost thin film solar cells due to its variable bandgap structure and high absorption coefficient in visible range. In this paper, a series of CIGS thin films were fabricated by combination of DC sputtering and selenizing processes. The effects of the sputtering power for deposition of CuIn1-xGax (CIG) metal precursors and substrates on the microstructures and optical properties of the CIGS films were investigated. It was found that the film, deposited at 50W sputtering power onto Mocoated soda lime glass (SLG) substrate and then selenized at 550℃ for 40minutes, exhibited a single chalcopyrite phase, uniform and dense morphology, and columnar grains. It is also found that the optical band gaps of the films are in the range of 1.21~1.24eV.
Abstract:1kHz femtosecond laser was used to induce crystallization on amorphous Si film. Raman spectra show that the crystallization region depended critically on the laser fluence and profile. Furthermore, a textured surface with a mass of finegrained crystalline Si was observed through SEM. This structure might result from the explosive crystallization and epitaxial growth of Si nucleation on the interface of liquidsolid Si. Due to the simultaneous process of crystallization and surface texturing, this laser treated region enhanced its absorbance in the visible and infrared band.
Abstract:Transmission and reflection measurements have been carried out on AlInN thin films. With the aid of a detailed procedure developed for analyzing the spectra, we obtained the effects of temperature on the optical properties of AlInN, such as the absorption coefficient, bandgap, Urbach bandtail characteristics, and refractive index. The absorption coefficient of the complete Urbach and intrinsic absorption regions of InN thin films has been described by a series of empirical formulas. The refractive index dispersion of AlN thin films below the band gap can be described by the Sellmeier equation. A bandtail model based on the calculation of density of occupied states and the carrier–phonon interaction has been employed to analyze the bandtail characteristics of AlInN thin films. We have also investigated Raman spectroscopy of AlInN thin films, with the emphasis on temperature dependence of phonon frequencies, which can be described well by a model taking into account the contributions of the thermal expansion of the crystal lattice, the strain between AlInN thin films and sapphire substrates, as well as threeand fourphonon coupling. These optical properties provide an experimental basis for further theoretical investigation and the design of AlInNbased devices.
Abstract:The crystal structure, electronic and optical properties of Cr, O and CrO codoped GaN were calculated by firstprinciple calculation. The results show that codoping method brings in new intermediate band as well as lowering the formation energy in comparison with single oxygen atom doping. The formation of intermediate band enhances absorption of low energy photons, especially in infrared range of the sun spectrum. Our calculation predicts the feasibility of CrO codoped GaN used as a thirdgeneration solar cell semiconductor material.
Abstract:To further reduce the lasing threshold of organic semiconductor materials, the surface plasmon resonance effect of silver nanoparticles was investigated. The prepared silver nanoparticle solution was spincoated on glass substrate, and then PS:Alq3:DCJTB organic thin film was spincoated on the surface of Ag nanoparticles, and thus formed a planar waveguide. Pumped by YAG (λ=355nm) laser, the stimulated emission results show that compared with the case without Ag nanoparticles, the Agnanoparticleenhanced edge stimulated emission increases 10.38fold and its lasing threshold reduces to 0.5 mJ/cm2 from 2.0 mJ/cm2. The vertical surface stimulated emission increases 6.13fold and its lasing threshold reduces to about 1.5 mJ/cm2 from 2.5 mJ/cm2. This work may be of great significance in the realization of lowthreshold organic semiconductor lasers.
Abstract:The narrowband terahertz (THz) generation on lithium niobate (LN) crystals with nonperiodically poled domain structures was demonstrated. To realize THz pulse shaping, the optimal domain structures were designed by uing simulated annealing (SA) algorithm, for arbitrary preset frequency components and intensities of THz radiation generation. It was demonstrated that this method is feasible in THz pulse shaping.
Abstract:Zinc oxide thin films fabricated on si (111) and sapphire substrates by pulsed laser deposition (PLD) were annealed at different temperatures and in different ambient, respectively. The effects of annealing temperature and annealing ambient on structural and optical properties of ZnO thin films have been characterized by Xray diffraction (XRD) and photoluminescence (PL) spectra. The experimental results show that with the increase of annealing temperature the compressive stress of ZnO thin films is decreased and shifted towards strain stress. PL spectra of ZnO thin films annealed at different temperature indicate that the UV emission is increasing with increasing annealing temperature, and the visible emission is decreasing. The results annealed at various conditions show that the thin film annealed on sapphire substrate at 700℃ in oxygen ambient possesses a small strain stress and the highest ratio of UV emission to visible emission.
Abstract:Using the first principle method within the local spin density approximation, both the magnetism of defect induced in GaN and the effect of Si codoping on the magnetism in GaN with defect were investigated. It was found that defectinduced intrinsic magnetic moment of GaN is 3μB, while the magnetic moment is quenched to 2μB in Sicodoping GaN:Si. The magnetic moment decreases with the increase of the concentration of Si. The result is very helpful for experiments.
Abstract:A batch of BaF2:PbWO4 crystals are grown up by a multicrucible temperature gradient system. The transmittance and xray exciting luminescence (XEL) spectra of these crystals were studied. The results reveal that these scintillation properties are strikingly effected by the doped BaF2 content in the crystals. Proper BaF2 content in the crystal could improve the transmittance and the XEL peak intensity and make the XEL peaks shift to longer wavelength. For the crystal grown up from the melt with BaF2 1000molppm, this raising extent becomes evident for their transmittance and the XEL peak intensity. For the crystal grown up from the melt with BaF2 2500~3000molppm, their transmittance and XEL peak intensity raise greatly. For the crystal grown up from BaF2 3500molppm, there is no evidence change for the transmittance, but its blue XEL peak intensity becomes weak and the peak width enlarged. For the crystal grown up from the melt with BaF2 5000molppm, the blue peak intensity becomes low while the green peak intensity becomes strong. The enhancingeffect of V3 Pb and VO- and ［WO3F］- tetrahedron and the weakeningeffect of Oi ,Bai and ［WO2F2］ tetrahedron are related to the phenomena.
Abstract:SiO2 nanophotonic film is widely used in the areas of photovoltaic, nanophotonics and microelectronics. Plasmaassisted electron beam evaporation was used to produce the SiO2/Si nanofilm at low temperature. By using the ellipsometric method，we studied the optical properties of the nanofilms under three different process conditions, such as deposition rate, substrate temperature and radiofrequency power. The best condition to make the SiO2/Si nanofilm was obtained. The studies of the mechanical, chemical and optical properties of the SiO2/Si nanoflim show that the nanofilm prepared with plasma assisted electron beam evaporation under the best condition is superior to that produced by the conventional methods.
Abstract:Based on nonFourier equations of heat conduction, using the complex function and conformal mapping technique, multiple scattering of thermal waves by subsurface defects of arbitrary shape in the semiinfinite slab was investigated. A general solution of thermal wave scattering was given. The thermal waves are excited at surfaces of solid materials by modulated ultrashort laser pulse with a adiabatic boundary condition for defects. The effects of geometrical and physical parameters on the temperature distribution are analyzed. Meanwhile, the numerical results of temperature variation were demonstrated. The analytic method and numerical results can be used for analysis of heat conduction, thermal wave imaging, physical inverse problem and evaluation of internal defects in materials.
Abstract:The annealing temperature dependence of FWHM of Xray rocking curves of pGaN layers grown on sapphire substrate by metal organic chemical vapor deposition (MOCVD) were studied. The results show that the quality of pGaN became worse at annealing temperature higher than 1150℃. The implantation conditions were simulated by TRIM. The planar GaN pn detectors were fabricated by Si implantation into pGaN. The currentvoltage (IV) curve at room temperature shows that the dark current density is 4.7nA/cm2 at zero voltage bias. The peak responsivity is 0.065 A/W and 0.039A/W at 368 nm at room temperature and 80 K, respectively. It decreases obviously with the decrease of temperature as a result of the changes in bandgap, series resistance, and buildin potential with temperature.
Abstract:When the number of labeled samples is limited, Laplacian SVM needs as many as possible unlabeled samples to improve the performance of classification. However, when the number of unlabeled samples is large, the required time and space complexity would be unacceptable. In order to apply it to largescale classification problems like SAR image segmentation, a new method for SAR image segmentation named as improved Laplacian support vector machine algorithm (Improved Laplacian SVM) was proposed. Watershed algorithm was first used to decompose the original image into several small prototype blocks, and image features of each small prototype blocks were extracted as training samples. Then an improved Laplacian SVM algorithm was proposed to classify data sets. The proposed method was verified on three SAR images. The experiments show that the method not only improves the accuracy of segmentation but also greatly reduces the running time of Laplacian SVM algorithm for image segmentation.
Abstract:In order to improve frequency characteristics, AlGaN/GaN HEMTs were designed by reducing sourcedrain spacing, optimizing gatestructure and peripheral structure. The devices have been fabricated with domestic GaN epitaxial wafer and process. Measurements indicated that the AlGaN/GaN HEMTs can operate at Kaband. At VDS=30V, the HEMTs with 275μm gatewidth exhibited a current gain cutoff frequency (fT) of 32GHz and a maximum frequency of oscillation (fmax) of 150GHz; Under CW operating condition at 30GHz, the linear gain reaches 10.2dB. For the HEMTs with 675μm gatewidth, fT is 32GHz and fmax is 92GHz; Under CW operating condition at 30GHz, the linear gain reaches 8.5dB. The breakdown voltage is over 60V.
Abstract:The results of n onp HgCdTe midwavelength infrared detector with a passivation interface is presented. The interface passivation process was optimized using highdensity hydrogen plasma implantation. By implantation barrier layer deposition, ionimplantation window exposure, B implantation, passivation film deposition, plasma hydrogenimplantation, metallization and indiumbump arrays fabrication, n onp HgCdTe midwavelength infrared detector was obtained from a Hg1-xCdxTe film grown by MBE. Cutin voltages of HgCdTe midwavelength detector Photodiodes with interface optimized were 50mV approximately larger than those of the one without optimization. The dynamic resistances at zero bias and reverse bias region were improved 10 times and dynamic resistances at the larger forward region were decreased significantly. Thus, it is obvious that plasma hydrogenimplantation is beneficial to suppress the dark currents and improve the ohmic contact of HgCdTe midwavelength infrared detector photodiodes, and then to enhance the operating dynamic range and performance uniformity.
Abstract:Poor resolution and lower modulation thansfer function(MTF) value in infrared imaging system have been remaining for many years because of its large Airy disc of optical lens and the small fillfactor of focal plane array(FPA), respectively. This paper reported a popular way to improve the resolution and MTF of infrared images using microscanning technique. The principles were briefly described. The designing rotary IR microscanner was discussed. The rotary microscanner imaging system using a 240320 pixels αsiuncooled FPA, and got higher resolution and MTF value of IR videos obviously.
Abstract:A pumpprobe study of carrier dynamics in GaInNAs was carried out. Induced absorption by trapped electrons in deep level states becomes important as inferred from the sign of the transmittance change. A simplified differential equation model has been utilized to simulate photogenerated carrier dynamics in GaInNAs and to fit differential transmission results. The model can interpret the temporal evolution of the trapped carrier density and extract the time constants of the carrier relaxation processes.
Abstract:Based on the Poisson’s equation of charge distribution and the transport equation of carriers, the effect of doping concentration and disordered degree on the mobility of carriers in the Alq3 flexible organic lightemitting diodes has been studied selfconsistently. The density of states of the carriers was described with the double Gaussian distributions because of the energy disorders of organic molecules. It was found that the carriers' mobility is almost fixed with doping concentration smaller than 11020cm-3. The carriers' mobility increased nonlinearly when the doping concentration was larger than 11020cm-3, in well agreement with the experimental results. It was also showed that the increase of mobility as a function of doping concentration for smaller disordered degree is faster than that for larger disordered one. With low doping concentration, the disordered degree has little effect on the mobility. Finally，the doping concentration and the disorder degree dependence of the mobility is used to simulate the variations of the luminescence power density of the device with the voltage bias. It was indicated that the mobility results related to the doping concentration and the disorder is very important for developing organic LED with high performance.
Abstract:Based on the beam characteristics of semiconductor lasers, a new parameter for evaluating beam quality of high power semiconductor lasers was introduced. The shortcomings of M2 factor used in evaluating beam quality of semiconductor lasers were discussed and its limitations were pointed out. Moreover, some important aspects of the beam quality factor are discussed. The main factors which influence collimating the beam of semiconductor lasers are analyzed. The results suggested that the smaller the value of the new parameter is, the more easily the beam is collimated.
Abstract:It is proved that the radiometric accuracy of short wave infrared (SWIR) hyperspectral imager and its signaltonoise ratio could be affected by the background radiation. The quantified computing model of background radiation of dispersive SWIR hyperspectral imager is given. The relationship between the temperature and the radiometric accuracy is analyzed, which was demonstrated by experiments. Based on the analysis of the characteristics of background radiation, several methods of reducing the influence of background radiation were summarized, such as cold shield of small relative aperture, temperature controlling with high precision, cold optics, regular calibration and so on. The results in this paper is helpful to improve the quantification accuracy of hyperspectral imager.
Abstract:Infrared radiation measurement is one of the important ways for target signature acquirement and target recognition, wherein atmospheric correction is a requisite step for obtaining the real radiation of the target. In the conventional radiation measurement method, it is necessary to measure atmosphere parameters by using atmosphere observation devices, and calculate the atmospheric transmittance and the air path radiance from these parameters by using a atmospheric radiation transport calculation software. The uncertainty in the atmospheric transmittance obtained by the conventional method is about 10~20%, and the target radiation inversion precision is about 12~23%. To improve the radiation inversion precision, a novel radiation measurement method based on realtime correction was presented, which measured the atmospheric transmittance with high precision by using a reference blackbody near the target. Models for atmospheric transmittance calculation and target radiation inversion were proposed. Analysis showed that the uncertainty in the atmospheric transmittance obtained by the novel method is about 6~10.5%, and the radiation inversion precision obtained by the novel method is better than 3.5%. A radiation measurement was performed by using a MW infrared camera. Theresults showed that the radiation inversion precision of the conventional method is 7.5～24.7%, while that of the novel method is 0.1~3.4%. The novel method has an important significance for improving the target radiation inversion precision.
Editor in chief：Jun-Hao CHU
International standard number：ISSN 1001-9014
Unified domestic issue：CN 31-1577
Domestic postal code：4-335