Abstract:The ensemble Monte Carlo method was used to calculate the time-variation of scattering mechanisms and the carrier nonlinear dynamics evolution of n-doped GaAs and InSb in the high terahertz (THz) field. The time information of the electrons scattering into the side valleys and that of the electrons relaxation back into the original energy valley was directly obtained. The carriers transient increase process was also traced. Meanwhile, it showed that the intervalley scattering is the main mechanism of GaAs, while the impact ionization is a key point for InSb in high THz field. Furthermore, the work discussed the influences of the two mechanisms on related physical quantities: average kinetic energy, average velocity, and material conductivity. It indicates that the two mechanisms lead to nonlinear effects and play inverse roles in the two materials. The response time of impact ionization in InSb is longer than that of intervalley scattering in GaAs. The results have some guiding values in THz modulation field.

Abstract:Terahertz beams are a kind of Gaussian beam with a big divergence angle and its wave front cannot be simplified to a plane or a spherical wave. All the theoretical derivations from the classical electromagnetic theory and ABCD laws support the following conclusions: firstly, a positive lens converges the terahertz beam in front of the focal plane instead of on the focal plane; secondly, a negative lens which matches the radius of the Gaussian beam’s wave front is more appropriate for terahertz beam collimation. Experiments show that a negative lens with f′=-188 mm at the matching position, z=100 mm, can improve the terahertz beam collimation from 6° to 0.1°. And 20-m terahertz space transmission was realized with a very simple optical scheme.

Abstract:118 GHz single pole double throw (SPDT) switches were designed and tested for radiometers in satellite applications. Filter synthesis method was proposed for the switches analysis. Based on equivalent circuit of the discrete AlGaAs/GaAs heterojunction PIN diodes, lumped element filter model of the switches was synthesized and its equivalent distributed circuit was developed. The size of the developed 118 GHz quasi-MMIC SPDT switch circuit is 6×2.5×0.1 mm3. For the packaged SPDT switch module, insertion loss (IL) introduced by mounting deviation of the waveguide microstrip transitions and the bonded ribbons is discussed. In the frequency range 110~120 GHz, the IL of the switch modules is less than 3.0 dB and typical value is 2.6 dB, and their isolation is higher than 22 dB. Response time, switch on time, switch off time and recovery time of the switches are lower than 18 ns, 20 ns, 10 ns and 18 ns, respectively. The 118 GHz switches can be widely used in transceiver systems.

Abstract:The LaAlO3/SrTiO3 heterointerface has been discovered to show two-dimensional interfacial conductivity and other intriguing emergent phenomena. Its interfacial conductivity was mainly characterized by electric methods previously. Here the authors present for the first time that scattering-type scanning near-field optical microscopy (s-SNOM) can be employed to spatially image the conductivity of the buried LaAlO3/SrTiO3 interface, providing a new means to study the physics of transition metal oxide heterointerface systems.

Abstract:The absorption spectra of four dinitrobenzoic acid isomers were acquired over the range of 0.3~2.2 THz using a terahertz time-domain spectroscopy (THz-TDS) system and over the range of 1 400~1 800 cm-1 using a Fourier transform infrared spectrometer (FTIR). These isomers generate very similar IR spectra but there are substantial differences in the THz region. Vibrational spectra were simulated using density functional theory and the results of these calculations explain the differences and similarities in both the THz and FTIR spectra. THz-TDS technique appears to be a powerful means of identifying isomers in the pharmaceutical and chemical industries.

Abstract:Back-gated (BG) Multi-layer MoS2 field effect transistors (FETs) have been fabricated on SiO2/Si(P++) substrate and electrically characterized. By optimizing the fabrication process and scaling down the SiO2 thickness to 10 nm, the device exhibit excellent switching performance with a subthreshold swing of 86 mV/dec and an Ion/Ioff ratio ~107. The little hysteresis and small SS jointly suggest tiny magnitude of interface traps or attached oxidants. The noise current induced by gate leakage can affect the measured switch ratio by overwhelming the effective Ioff current defined by VDS. According to the behaviors of MoS2 FETs expressed by this work and others’, BG devices with SiO2 insulator present good performance and valuable potentials underutilized for rich applications.

Abstract:An improved small-signal model for nanometer metal-oxide-semiconductor field-effect transistor (MOSFET) device is presented in this paper. The skin effect and multiple-cell effect are both taken into account. In the extracting procedure, the parameters of elementary cells are determined from the conventional model based on the scalable rules. This small-signal model was validated by the good agreement between measured and simulated S-parameters of 8×0.6×12 μm (number of gate fingers×unit gatewidth×cells) 90-nm gatelength MOSFET under three bias points up to 40 GHz.

Abstract:The position and orientation system (POS) information of a target can be obtained through airborne laser radar (LiDAR) technology combined with the Global Positioning System, an inertial navigation system, and a laser range finder. The camera is chosen by calculating the minimum field of view resolution, pixel number, focal length, and other parameters. The LiDAR multichannel spectrum image recognition system is composed of the POS information acquisition system and the multispectral camera. The multichannel matching fusion method can produce ultraviolet, infrared, and color pictures. The elliptical shape can be fitted and parsed using the Hough transform method, the immune genetic snake model algorithm, and the least squares method, which can solve anomaly recognition problem in the insulator. The average failure detection resolution of LiDAR multi-channel spectral image anomaly recognition system is 82.4%, and it is higher than the average for copter and manual detection of 2405%. The proposed system is a highly efficient smart grid patrol screening method.

Abstract:A beam-steering antenna array based on surface micro-machined process was proposed for 60GHz high data rate wireless communication. The air-filled coaxial structure based surface micro-machined process provides low loss rectangular micro-coaxial transmission line and feeding network. The insertion loss of designed transmission line is less than 0.18dB for both measured and simulated results. Simulated and measured results agree well. The size of beam-steering antenna array is 17.5×14.5×0.42 mm3 and bandwidth is around 10 GHz (55~65 GHz) under the condition of voltage standing wave ratio (VSWR) less than 2, which covers full frequency band standard. The steering beam directions are ±35° with 11.8dBi and ±11° with 12.1 dBi at 60 GHz, respectively.

Abstract:Immersion gratings are typically used in the infrared spectral band in which optical materials with high refractive index. Some issues inevitably arise due to their special operating mode. And they are critical to the application of immersion gratings. This study investigated the relations between the lengths of the long-and short-wave of normal gratings. For the features of the relevance of the refractive index with wavelength, the short-wave infrared spectral band (1.5~2.5 μm) was taken as an example to analyze the change characteristics of spectral line location and spectral resolution of immersion gratings. Results showed that when immersion gratings were adopted, significant differences were observed in the distribution of spectral lines as compared with normal gratings, and “trapezoidal” spectral lines tilted. For short-wave infrared spectral lines, the inclination of the “trapezoidal” spectral lines became more apparent due to larger changes in the refractive index of the medium. When Littrow conditions were registered with long-wave length (2.5 μm), the spectral line tilted toward the short-wave end. When registration was done with short-wave length (1.5 μm), the spectral line tilted toward the long-wave end, and the Littrow wavelength drifted away from the central wavelength. The refractive index had a few changes at the thermal infrared spectral band. The tilt of “trapezoidal” spectral lines was smaller and more similar to that of normal gratings. The spectral resolution of immersion gratings varied due to changes in the refractive index. At the same order, the wavelength increased, and resolution increased; between various orders, the order decreased, and the resolution decreased. Meanwhile, considering that the high-order resolution was larger than the low-order’s, the ratio of the lengths of spectral lines at various orders no longer meet the relation of normal gratings.

Abstract:The arsenic doping and activations on HgCdTe (mercury cadmium telluride) p+-on-n heterojunctions grown by MBE (molecular beam epitaxy) have got a lot of attention in fabricating high performance long-wavelength IRFPAs (infrared focal plane arrays). In this paper, the performances of HgCdTe diodes with different arsenic doping concentrations are presented. According to the I-V results and dark current mechanism, the effect of arsenic concentration on the trap-assisted tunneling current was calculated and analyzed. To achieve reproducible doping and activation process, the dependence between arsenic doping efficiency and Hg/Te condition was reported. As activation ratio could be higher than 60% under activated annealing at 300, 420 and 240 degree Celsius under an Hg saturated vapor pressure, which was demonstrated by means of Hall-effect measurements and SIMS (Secondary Ion Mass Spectrometry).

Abstract:Star surveys and model analyses show that many stars have absolute stable fluxes as good as 3% in 0.3~35μm wavebands and about 1% in the visible wavebands. The relative flux calibrations between stars are better than 0.2%. Some stars have extremely stable fluxes and can be used as long term flux calibration sources. Stellar brightness is several orders of magnitude lower than that of most ground objects. However, the stars do not usually appear in remote sensing cameras, which makes the stars inappropriate for being calibration sources. The calibration method using stars discussed in this paper is through a mini-camera attached to remote sensing satellite. The mini-camera works at similar wavebands as the remote sensing cameras and it can observe the stars and the ground objects alternatively. High signal-to-noise ratio is achieved for the relatively faint stars through longer exposure time. Simultaneous precise cross-calibration is obtained as the mini-camera and remote sensing cameras look at the ground objects at the same time. The fluxes from the stars used as calibration standards are transferred to the remote sensing cameras through this procedure. Analysis shows that a 2% accurate calibration is possible.

Abstract:This paper reports photoluminescence (PL) and magneto-PL study of CdZnTe single crystal grown by Bridgman method. Magneto-PL measurements on two CdZnTe samples in the sample crystal were realized at low temperature with sufficiently high spectral resolution and signal-to-noise ratio. PL spectra reveal that the Te inclusions near the CdZnTe surface affects obviously the PL processes energetically below 1.5 eV. Further analysis with curve-fitting process shows that (1) stress distribution exists inside the CdZnTe sample without Te inclusions, and the stress causes the splitting of the heavy-and light-hole subband. (2) The 1.57-eV PL feature originates from the shallow-donor to valence-band recombination.

Abstract:In this paper, the output current characteristics within a large range of incident light power of a double heterojunction uni-travelling-carrier phototransistor (UTC-DHPT) were analyzed in detail and compared with a single heterojunction phototransistor (SHPT) simultaneously. Firstly, the output current of UTC-DHPT is smaller than that of SHPT under small power incident light because the UTC-DHPT’s absorption section is only the base region, which is smaller than SHPT’s absorption section consisting of base and collector. Secondly, benefiting from the double hetero-junction, the photogenerated carriers are only generated in base in UTC-DHPT, and there are few photogenerated holes accumulated in the collector junction interface. The space charge effect, which is seriously in SHPT, is effectively alleviated. Furthermore, the saturation of the output current under small power incident light, which always occurs in SHPT, is avoided. Therefore, UTC-DHPT has a larger quasi linear range than SHPT. Lastly, the uni-travelling transportation in UTC-DHPT make photogenerated holes in the base region reach to the emitter junction interface by the rapid relaxation, and thereby effectively reduce the emitter junction barrier. This would increase the number of electrons transmitting from the emitter to the base in the unit-time, and the emitter junction injection efficiency is improved. Thus, UTC-DHPT can obtain higher output current than SHPT under the high power incident light.

Abstract:The diffraction efficiency of the holographic polymer-dispersed liquid crystal (H-PDLC) grating can be adjusted by the applied electrical field. Doping Ag nanoparticles (NPs) can effectively reduce the driving voltage of the grating. Due to the influence of polymerization kinetics, Ag NPs are distributed unevenly in the grating, namely, the distribution of Ag NPs in the polymer and LC area is not uniform and exhibit different electric field control characteristics. The relationship between the threshold driving voltage and the frequency of the applied field can be studied by equivalent circuit model. According to Wagner-Maxwell effect, in this paper, the equivalent circuit model of Ag NPs surrounded by LC and polymer, respectively, was established. Under the condition of different concentration of Ag NPs within the LC area, the relaxation time and relaxation frequency of Ag NPs doped H-PDLC grating was studied. The low driving voltage can be further obtained by adjusting the frequency of applied electric field. The distribution of Ag NPs in the grating can be determined by the optimum driving electric field frequency range, and it is confirmed that the Ag NPs are concentrated in the LC strip and distributed in a small amount in the polymer stripes.

Abstract:Zinc sulfide material is one of the important materials for infrared optical window in the long wavelength region. In this paper, the change of the spectral transmittance from room temperature to 600℃ was studied for the zinc sulfide materials prepared by chemical vapor deposition (CVD) and hot pressing (HP). Based on the two-dimensional correlation spectrum analysis technique, the interaction of the phonons in the long wave absorption region was obtained. In the long wave absorption band from 700 cm-1 to 1 250 cm-1, the phonon frequencies of CVD ZnS is the third harmonic frequencies in the transverse vibration mode (TO) and the third harmonic frequency in the longitudinal acoustic mode (LA). The phonon frequencies of HP ZnS are 3TO, 3LA and 4LA, and the response under temperature is 3LA, 3TO and 4LA in proper order. The vibration absorption of ZnS impurity prepared by the two techniques has an effect on the spectral transmittance in the range from 1250 cm-1 to 2000 cm-1 under the action of temperature, and the increase of the transmittance in different degrees occurs. Because of the difference of impurities, the transmittance of CVD ZnS is more significant than that of HP ZnS at high temperature environment.

Abstract:Optical signal splitter was optimized by adjusting the coupling length, coupling width, coupling rod radius between PCSR and waveguide in the two-dimensional photonic crystal structure. As the interactions between PCSRs were considered, electromagnetic wave coupling performances between waveguide and cavity in the structure were analyzed qualitatively by using coupled-mode theory. The transmission characteristics of the two output ports were studied by the FDTD method. It shows that the optical signal splitter based on PCSRs has the characteristics of high normalized transmission ratios, narrow bandwidth, stable signal transmission intensity, broadly tunable ranges of central wavelength in the ranges of design parameters. This structure can separate transmission signal power into two equal parts at the same central wavelength, and it can also separate signals of different central wavelengths. The micro optical signal splitter promises a potential building block of photonic circuits on chip. It is suitable for the designs of wavelength division demultiplexing optical communication systems and photonic integrated circuits etc.

Abstract:A physical model was established for studying near-space limb radiance contrast when the target is detected by space-borne infrared surveillance system. The definition of limb contrast and radiation issues of upper atmosphere of the model was described in detail. The target’s limb contrasts were simulated and analyzed to show upper atmospheric radiation properties’ effects to the target visibility based on non-local thermodynamics equilibrium atmospheric model and assumed detector’s noise. The results indicate that the detector’s noise must be considered for evaluating surveillance feasibility correctly. The contrast at the traditional waveband of the strong absorption by water vapor is better than the often-used ‘atmospheric windows’ for the upper atmospheric limb detection. The complicated contrast characteristics result from the relationship among target inherent radiation, atmospheric absorption and background radiation.

Abstract:Approximately more than 330 billion calibrated radiance measurements of the Moon had been acquired by Diviner over 7 years. Diviner data were routinely interrupted by pushbroom nadir mapping mode and had a small number of outliers during spacecraft or instrument anomalies. In one lunar day, the change of brightness temperature is relatively uniform at noon or throughout the night and abnormally severe in the morning or afternoon especially during sunrise and sunset. The Sinus Iridum bright temperature distributions of high spatial resolution and high coverage in six moments have been obtained by processing methods which include numerical simulation of bright temperature, elimination of singularities, piecewise fit and latitudinal direction correction. Remarkably, diurnal brightness temperatures are nearly equal to that calculated from the solar flux and vary depending on the distribution of topography range, material composition and channel photometric properties. The highest value appears in the direction of the slope towards the equator and the lowest value appears at the center of the crater where the elevation difference is the biggest. Nevertheless, nocturnal brightness temperatures mainly are sensitive to materials with differing thermophysical properties. Brightness temperature drop rate changes severly in the first half of the night and becomes gradually uniform in the latter half of the night. Results of the paper reveal the surface energy balance of the Moon, explain the complex and extreme nature of the lunar surface thermal environment. The results also provide a new comprehensive view of how regoliths on airless heavenly bodies store and exchange thermal energy with the space environment.

Abstract:The double cladding structure can reduce the loss of photonic crystal fiber. A highly nonlinear silica based photonic crystal fiber with double cladding was prepared. Silica based photonic crystal fiber made in our lab was pumped by the femtosecond Ti: sapphire laser in the abnormal region. The effects of the different pump powers and pump wavelengths on mid-infrared ultra short pulse soliton were investigated. Physical mechanisms of the generation of ultra short infrared ultrashort pulses solitons in highly nonlinear silica based photonic crystal fibers were also analysed. In this paper, the first mid-infrared soliton moves from 1933 nm to 2403 nm and the adjustable range lasts for up to 470 nm when the pump wavelength works at 827 nm with the pump power increasing from 0.1 W to 0.42 W. This provides a good condition for the broadband tunable ultrashort pulse source in the silica based photonic crystal fiber.