Flat mirror for millimeter-wave and terahertz imaging systems using an inexpensive metasurface

Flat mirrors, also known as flat parabolic surfaces, for millimeter-wave and terahertz imaging systems are demonstrated. This flat mirror is based on the metasurface in which an inexpensive printed circuit board technology is used to realize copper patterns printed on an FR4 substrate. Compared to the conventional reflector antennas used today in diverse applications(for homeland security, medical systems, communication, etc.), the suggested mirror has major advantages in process simplicity, mechanical flexibility, frequency alignment, weight, and cost. The theoretical background, simulation results, experimental results, and proof of concept are given in this Letter.     

Optimisation of sprite detectors in anamorphic imaging systems

Previous analyses of the SPRITE detector signal-to-noise performance have ignored the effect of the increasing carrier density along the filament due to integration of the background flux and have consequently assumed the recombination probability of excess carriers, the electric field and the ambipolar velocity all to be constant. In this paper a more exact analysis is presented, and it is shown that with the lower carrier density and longer lifetime cadmium-mereury-telluride now becoming available, significant benefits in detectivity can be obtained by correct choice of the device geometry. Expressions are given from which the optimum length and taper functions can be determined for different system scan speeds and optical apertures. It is shown that anamorphic optics can increase the achievable detectivity in addition to improving the modulation transfer function.     

Planar sulfur-doped silicon detectors for high-speed infrared thermography

Planar extrinsic sulfur-doped silicon detectors for infrared (IR) semiconductor-discharge gap image converters intended for use in high-speed thermography of remote objects have been developed. The detectors were fabricated by high-temperature diffusion of sulfur into silicon wafers from the vapor phase. The dependence of doping efficiency on the sulfur vapor pressure in the course of diffusion was analyzed. The detector fabrication technology was optimized to meet the specific requirements for their operation in the microdischarge devices considered. The detectors were tested in a laboratory setup comprising a blackbody source of IR light, an image converter, and a pulsed CCD camera for recording the converted images. The converter equipped with the detector can provide imaging of objects heated to a temperature, T_min ≈ 200 °C, with a temporal resolution on the order of 10^?6 s and spatial resolution of about 5 lines/mm.     

Mesoscopic SNS junctions and their application to SQUIDs and millimeter-wave detectors

Mesoscopic SNS junctions have been studied both in the ballistic and diffusive regimes. SNS junctions in the ballistic regime behave as an ideal Fermion oscillator which is to be compared with the Boson oscillator or the Planck theory of blackbody radiation. The current of mesoscopic SNS junctions in the diffusive regime has the same phase dependence as that of dirty-limit short weak links derived by a transport equation. Recent theories of mesoscopic SNS junctions have successfully unified the theories of the tunnel Josephson junction, the clean-limit short weak link and the dirty-limit short weak link which look very different conceptionally. We can even observe transitions among the three types of junctions when we change the transmission coefficients of the barriers between the superconducting electrodes experimentally. We looked experimentally for the optimum transmission coefficient which gives the minimum low-frequency telegraph noise in order to make a low-noise SQUID magnetometor for brain science. We have observed signals of 5 fT from human brains with a good signal-to-noise ratio using the SQUID magnetometor of the SNS junctions. The 64-channel SQUID magnetometer of SNS junctions has confirmed that mesoscopic SNS junctions are important not only theoretically but also practically. These data could encourage people studying SNS junctions of high-T_c superconductors.     

Semiconductor detectors and focal plane arrays for far-infrared imaging

The detection of far-infrared (far-IR) and sub-mm-wave radiation is resistant to the commonly employed techniques in the neighbouring microwave and IR frequency bands. In this wavelength detection range the use of solid state detectors has been hampered for the reasons of transit time of charge carriers being larger than the time of one oscillation period of radiation. Also the energy of radiation quanta is substantially smaller than the thermal energy at room temperature and even liquid nitrogen temperature. The realization of terahertz (THz) emitters and receivers is a challenge because the frequencies are too high for conventional electronics and the photon energies are too small for classical optics. Development of semiconductor focal plane arrays started in seventies last century and has revolutionized imaging systems in the next decades. This paper presents progress in far-IR and sub-mm-wave semiconductor detector technology of focal plane arrays during the past twenty years. Special attention is given on recent progress in the detector technologies for real-time uncooled THz focal plane arrays such as Schottky barrier arrays, field-effect transistor detectors, and microbolometers. Also cryogenically cooled silicon and germanium extrinsic photoconductor arrays, and semiconductor bolometer arrays are considered.     

Planar laser-fluorescence imaging of combustion gases

An overview is provided of the planar laser-induced fluorescence (PLIF) method, which currently allows simultaneous combustion measurements at more than 10⁵ flowfield points. Important advantages of the method include its relatively high signal strength, ease of interpretation, and applicability for determining several flowfield variables (including concentration, temperature, velocity, pressure and density). Example results are shown for a turbulent non-premixed flame, a spray flame, a rod-stabilized premixed flame, and a diffusion flame from a fuel jet in cross-flow.     

The potentialities of hybrid model used for analysis of oscillatory systems for millimeter-wave magnetron

We consider the hybrid model from the viewpoint of its applicability to analyze equiresonator magnetron waveguide oscillatory systems used for millimeter band. The use of this model provides efficient analysis of the main characteristics including losses.     

Polarization radio imaging of various objects in the millimeter-wave range

We present methods and results of radio imaging of various technical objects at a frequency of 94 GHz for vertical and horizontal polarizations. We analyze the features of the formation of polarization-difference images in the millimeter-wave range and the typical properties and methods of processing of such images. The influence of experimental conditions (noise level, antenna focusing, and imperfect knowledge of the directional pattern) on the quality of image reconstruction is numerically simulated. To increase the spatial resolution of images, we propose filtering of the spatial spectrum of a reconstructed image, which allows for its a priori statistical characteristics. Possibilities of passive polarization stereo imaging and a method of its realization in the millimeter-wave range are considered.     

Manufacturability of type-II InAs/GaSb superlattice detectors for infrared imaging

Type-II InAs/GaSb superlattice detectors and focal plane arrays (FPAs) with cut-off wavelength at 5.1 μm have been studied. For single pixel devices, dark current densities of 1 × 10⁻⁶ A/cm² and quantum efficiencies of 53% were measured at 120 K. From statistics of manufactured FPAs, an average FPA operability of 99.87% was observed. Furthermore, average temporal and spatial noise equivalent temperature difference (NETD) values of 12 mK and 4 mK, respectively, were deduced. Excellent stability of FPAs after non-uniformity correction was observed with no deterioration of the ratio between spatial and temporal noise during a two hour long measurement. Also after several cooldowns the ratio between spatial and temporal NETD stayed below 0.6.     

Study of a millimeter-wave squint indirect holographic algorithm suitable for imaging with large field-of-view

In this paper a millimeter-wave （MMW） squint indirect holographic method is presented, which is suitable for imaging with a large field-of-view. The proposed system employs the squint operation mode to remove the background and twin- image interferences, which achieves a similar effect to off-axis holography but leaves out the large-aperture quasi-optical component. The translational scanning manner enables a large field of view and ensures the image uniformity, which is difficult to realize in off-axis holography. In addition, a corresponding imaging algorithm for the presented scheme is developed to reconstruct the image from the recorded hologram. Some imaging results on typical objects, obtained with electromagnetic simulation, demonstrate good performance of the imaging scheme and validate the effectiveness of the image reconstruction algorithm.     

Reflective ghost imaging free from vibrating detectors

The vibration is one of the important factors affecting imaging quality of conventional remote sensing imaging because the relative motion between the imaging system and the target can result in the degradation of imaging quality. The influence of the vibration of the detector in the test path on reflective ghost imaging(RGI) is investigated theoretically and experimentally. We analyze the effects of the vibrating amplitude and velocity. The results demonstrate that the microvibrations of the bucket detector have almost no impact on the imaging resolution and signal-to-noise ratio(SNR) of RGI, i.e., the degradation of imaging quality caused by the vibration of the detector can be overcome to some extent. Our results can be helpful for remote sensing imaging.     

更正北大核心CSCD

阵列设计作为多输入多输出(MIMO)雷达成像系统中的一项关键技术直接影响系统成像性能。通过分析MIMO雷达成像模型,探讨了基于等效阵列概念的MIMO阵列设计方法,并讨论了近场效应对成像结果的影响。基于所构建的MIMO雷达成像实验系统,进行了近场微小金属目标二维成像试验。实验结果表明:该阵列设计方法在120~150 GHz的适用性,验证了毫米波MIMO雷达近场成像能力。     

Novel detectors for fluorescence lifetime imaging on the picosecond time scale

Simultaneous acquisition of time and space information on the picosecond time scale became feasible with a recent advance in microchannel-plate photomultiplier-tube (MCP-PMT) technology: we present two novel MCP-PMT detectors for time- and space-correlated single-photon counting (TSCSPC), featuring a space-sensitive delay-line (DL) anode and quadrant anode (QA), respectively. The linear DL-MCP-PMT is characterized by a spatial instrument response function (IRF) of 100-Μm FWHM, resulting in 200 space channels, whereas the QA-MCP-PMT is a 2D imager with 400 x 400 pixels at 40-Μm resolution. The detectors have a temporal IRF of 75 ps (DL) and 80 ps (QA) FWHM, sufficient for 10 ps time resolution, at a dynamic range of 10⁵ of the uncooled detector. A throughput of 10⁵ cps is possible; in the imaging mode without timing, the QA-detector can achieve 10⁶ cps. We present time-resolved spectroscopy of DNA probes (DAPI, TOTO, C350) in solution, in micelles, complexed to DNA, protein, and fixed cells. Aging of DAPI stock solutions is reported. A polarity model for the photophysics of DAPI is proposed. First microscope lifetime images on the picosecond time scale show a clear potential for dynamic stray-light rejection and kinetic discrimination of probe-protein and probe-DNA complexes.     

Intensifying process of polarization effect within pixellated CdZnTe detectors for X-ray imaging

The intensifying process of polarization effect at room temperature in a pixellated Cadmium zinc telluride (CdZnTe) monolithic detector is studied. The process is attributed to the increase in build up space charges in the CdZnTe crystal, which causes an expansion of the space charge region under the irradiated area. The simulations of electric potential distributions indicate that the distorted electric potential due to the high X-ray flux is significantly changed and even deteriorated due to increasing space charges within the irradiated volume. An agreement between the space charge distribution and electric potential is discussed.     

Resonant-tunneling devices for millimeter-wave generation

Oscillations from resonant-tunneling diodes have been observed up to 200 GHz, and theoretical estimates predict that device performance should extend into the THz range. This paper addresses the issue of the ultimate frequency response and power generation capability of these devices. Techniques recently developed to solve the time-dependent Schrödinger equation are used to predict the rf power vs. frequency obtainable from resonant-tunneling diode oscillators, based on the calculated small-signal response. Factors limiting the rf power output from these devices are presented. Also, recently obtained dc experimental results for the In_.53Ga_.47As-In_xAl_1-xAs heterostructure material system grown on InP are presented. Using a quasi-static approximation, the rf power available from these devices under large-signal conditions is estimated.     

显示OH浓度分布图像的平面激光诱导荧光技术

用平面激光诱导荧光 (PLIF)技术测量平面火焰炉、狭缝火焰炉的单脉冲激光诱导OH荧光。由平面荧光图可得到氢氧基相对浓度分布和它的宽度。对于扩散火焰 ,高温区在OH带内侧 ;而对于预混火焰 ,二者基本一致。湍流火焰的PLIF图则清晰地显示出火焰面的不规则性。氢氧基的PLIF图像是研究火焰结构和流场的有力工具。