Study of terahertz interferometric imaging using optical techniques

<正>Principles of terahertz(THz) interferometric synthetic aperture imaging with heterodyne and optical techniques are presented.A THz interferometric experiment based on optical up-conversion is set up.The received THz signal is modulated into an optical carrier and transmitted in a fiber.To simulate phase differences between two THz receivers,the output of receiver is divided and a phase shifter is placed before electro-optical modulation(EOM).Interferometric spectra of these modulated optical signals are examined at different phase shifts.Otherwise,carrier suppression and phase error calibration are discussed for THz interferometric synthetic aperture imaging.     

Design and fabrication of 0.5 micron GaAs Schottky barrier diodes for low-noise terahertz receiver applications

Recent technological advances have made possible the development of heterodyne receivers with high sensitivity and high spectral resolution for frequencies in the range 1,000–3,000 GHz (1–3 THz). These receivers rely on GaAs Schottky barrier mixer diodes to translate the high-frequency signal to a lower frequency where amplification and signal processing are possible. At these frequencies, the diode quality is a major limitation to the performance of the receiver. The design, fabrication and DC evaluation of a diode for this frequency range is presented. A figure-of-merit cut-off frequency of over 10 THz is achieved with a record low zero biased capacitance of 0.5 fF. Results from RF tests are also given.This work has been supported in part by the National Science Foundation under contract ECS-8720850 and the US Army.     

Terahertz heterodyne receivers based on superconductive hot-electron bolometer mixers

We consider recent results in development of hot-electron bolometer mixers. Special attention is paid to optimization of the contacts between the antenna and the active area of a superconducting film. An important result in the study of the parasitic effect of direct detection is obtained during the measurement of the noise temperatures by the hot/cold load method. The latest results of studies of the waveguide hot-electron bolometer mixers and their successful practical applications are considered. Progress in development of high-frequency (over 1.3 THz) heterodyne receivers for several important international projects is discussed and new submillimeter radio astronomy projects ESPRIT and SAFIR are described.     

Recent developments in millimeter and submillimeter waves

Millimeter and submillimeter-wave observations provide important information for the studies of atmospheric chemistry and astrochemistry (molecular clouds, stars formation, galactic study, comets and cosmology). But, these observations depend strongly on instrumentation techniques and on the site quality. New techniques or higher detector performances result in unprecedented observations and, sometimes, the observational needs drive developments of new detector technologies, for example, superconducting junctions (SIS mixers) because of their high sensitivity in heterodyne detection in the millimeter and submillimeter wave range (100–700 GHz), HEB (Hot Electron Bolometer) mixers which are being developed by several groups for application in THz observations. For the sub-millimetre wavelengths heterodyne receivers, the local oscillator (LO) is still a critical element. So far, solid state fundamental sources are often not powerful enough for most of the applications at millimetre or submillimetre wavelengths: large efforts using new planar components (HBV) and integrated circuits, or new technics (laser mixing) are now in progress, in a few groups.The new large projects as SOFIA, FIRST, ALMA, … for astronomy; SMILES, EOS-MLS, … for aeronomy and other projects for the planetary science (ROSETTA, Mars Explorer, …), will benefit of the new developments.     

高速太赫兹探测器

太赫兹(terahertz,THz)技术在高速空间通信、外差探测、生物医学、无损检测和国家安全等领域具有广阔的应用前景.能响应1 GHz调制速率以上THz光的高速THz探测器是快速成像、THz高速空间通信、超快光谱学应用技术和THz外差探测等领域的核心器件.传统的THz热探测器难以实现高速工作,而基于半导体的THz探测器在理论上可实现高速工作.光导天线具有超快的响应速度,可实现常温和宽谱探测;肖特基势垒二极管混频器、超导-绝缘体-超导混频器和超导热电子混频器具有转换效率高、噪声低等优点,可用于高速THz空间外差和直接探测;基于高迁移率二维电子气的天线耦合场效应晶体管灵敏度高、阻抗低,可实现常温高速THz探测;THz量子阱探测器是一种基于子带间跃迁原理的单极器件,非常适合高频和高速探测应用,亚波长金属微腔耦合机理可显著提高器件的工作温度及光子吸收效率.本文对上述几种高速THz探测器进行了综述并分析了各种探测器的优缺点.     

Fabrication and analysis of GaAs Schottky barrier diodes fabricated on thin membranes for terahertz applications

The GaAs Schottky diode is predominantly used as the critical mixer element in heterodyne receivers in the frequency range from 300 GHz to several THz[1]. At operating frequencies above one THz the skin effect adds significant parasitic resistance to the diode which degrades the receiver sensitivity. A novel diode structure called the Schottky barrier membrane diode is proposed to decrease the skin effect resistance by reducing the current path between the Schottky and ohmic contacts. This is accomplished by fabricating the diode on a very thin membrane of GaAs (about 1 m thickness). A theoretical analysis has shown that this will reduce the substrate resistance by 60% at 3 THz. This reduction in resistance corresponds to a better frequency response which will improve the device's performance as a mixer element.This work has been supported by a grant from Texas Instruments and the National Science Foundation under contract ECS-8412477     

0.14THz高功率太赫兹脉冲的频率测量

提出了采用截止波导法与谐波混频法相结合的方式,进行0.14THz高功率短脉冲的频率测量.首先将两个截止频率分别为0.125和0.15THz的非标准矩形波导作为接收端,通过截止波导滤波法获得了太赫兹辐射源的频率范围.然后根据已知的频率范围,将本振频率选择为15—20GHz,则谐波混频的谐波次数确定为8.随后的Ka波段的脉冲测试和0.14THz连续波测试表明,该8次谐波混频器可用于0.14THz脉冲的混频测量.最后,0.14THz脉冲频率测量实验给出了太赫兹辐射源的准确频率为0.1465THz.该方法大大降低了对本振信号的频率要求,且结果准确可信,为长波段太赫兹脉冲的频率测量提供了一种新的思路.     

Some recent developments in the design of SIS mixers

SIS mixers in which superconducting tuning elements are integrated with the tunnel junctions have resulted in very low noise heterodyne receivers in the range 68–260 GHz. Above 120 GHz the need for extremely small reduced-height waveguides is avoided by mounting the SIS junctions in a suspended-stripline circuit coupled to a full-height waveguide by a broadband probe. The special characteristics of coplanar transmission line permit the design of SIS mixers with low parasitic reactances. Such a mixer operates over the full WR-10 band (75–110 GHz) without mechanical tuners.An earlier version of this paper was presented at the First International Symposium on Space THz Technology, March, 1990.The National Radio Astronomy Observatory is operated by Associated Universities, Inc., under cooperative agreement with the National Science Foundation.     

Terahertz Direct Detectors Based on Superconducting Hot Electron Bolometers with Microwave Biasing

Terahertz(THz) direct detectors based on superconducting niobium nitride(NbN) hot electron bolometers(HEBs) with microwave(MW) biasing are studied. The MW is used to bias the HEB to the optimum point and to readout the impedance changes caused by the incident THz signals. Compared with the thermal biasing method, this method would be more promising in large scale array with simple readout. The used NbN HEB has an excellent performance as heterodyne detector with the double sideband noise temperature(T_N) of 403 K working at 4.2 K and 0.65 THz. As a result, the noise equivalent power of 1.5 pW/Hz~(1/2) and the response time of 64 ps are obtained for the direct detectors based on the NbN HEBs and working at 4.2 K and 0.65 THz.     

Terahertz detectors and focal plane arrays

Terahertz (THz) technology is one of emerging technologies that will change our life. A lot of attractive applications in security, medicine, biology, astronomy, and non-destructive materials testing have been demonstrated already. However, the realization of 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. As a result, THz radiation is resistant to the techniques commonly employed in these well established neighbouring bands.     

THz radiation sensors

In the paper, issues associated with the development and exploitation of terahertz (THz) radiation detectors are discussed. The paper is written for those readers who desire an analysis of the latest developments in different type of THz radiation sensors (detectors), which play an increasing role in different areas of human activity (e.g., security, biological, drugs and explosions detection, imaging, astronomy applications, etc.). The basic physical phenomena and the recent progress in both direct and heterodyne detectors are discussed. More details concern Schottky barrier diodes, pair braking detectors, hot electron mixers, and field-effect transistor detectors. Also the operational conditions of THz detectors and their upper performance limits are discussed.     

Surface plasmon quantum cascade lasers as terahertz local oscillators

We characterize a heterodyne receiver based on a surface-plasmon waveguide quantum cascade laser (QCL) emitting at 2.84 THz as a local oscillator, and an NbN hot electron bolometer as a mixer. We find that the envelope of the far-field pattern of the QCL is diffraction-limited and superimposed onto interference fringes, which are similar to those found in narrow double-metal waveguide QCLs. Compared to the latter, a more directional beam allows for better coupling of the radiation power to the mixer. We obtain a receiver noise temperature of 1050 K when the mixer is at 2 K, which, to our knowledge, is the highest sensitivity reported at frequencies beyond 2.5 THz.     

Far infrared heterodyne detectors

The characteristics of far infrared detectors are reviewed. Three detectors, the InSb hot electron bolometer, the GaAs Schottky diode and the Josephson point contact junction, have been incorporated as mixers into sensitive heterodyne systems. The performances of existing heterodyne receivers/radiometers are described and compared. Other applications of submillimeter heterodyne techniques are discussed.Supported in part by the U. S. Army Research Office and the Department of the Air Force.     

New phase and polarization-insensitive receivers for coherent optical fibre communication systems

We propose new phase- and polarisation-insensitive receivers for coherent optical fibre communication systems which have the following characteristic features: (a) insensitivity to LO excess noise in addition to phase- and polarization-insensitivity; (b) absence of optical PLL and polarization control devices; (c) a smaller detector bandwidth requirement than for heterodyne systems; (d) the same source linewidth requirement as for heterodyne systems with non-coherent demodulation; (e) the possibility of a complete optoelectronic integrated circuit (OEIC) version in the future.     

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.     

超高灵敏度太赫兹超导探测器

太赫兹(THz)波段一般定义为0.1–10 THz的频率区间, 对应波长范围3 mm–30 μm, 覆盖短毫米波至亚毫米波段(远红外). 尽管人们早已认识到太赫兹波段具有非常重要的科学意义和广泛的应用前景, 但该波段仍然是一个有待全面研究和开发的电磁频率窗口. 因此, 太赫兹波段的天文观测在天体物理及宇宙学研究中具有不可替代的作用, 对于理解宇宙状态和演化具有非常重要的意义. 具有超高灵敏度的太赫兹超导探测器, 已经成为太赫兹波段观测的主要手段. 本文主要阐述了太赫兹超导探测器的基本类型和工作原理, 以及中国科学院紫金山天文台在该领域的主要研究成果和进展.     

Superconducting terahertz receivers for space-borne and balloon-borne radio telescopes

A superconducting heterodyne receiver based on superconductor–insulator–superconductor (SIS) junctions and a bolometric receiver based on superconductor–insulator–normal metal (SIN) tunnel junctions developed at the Kotelnikov Institute is described. The noise temperature and noise equivalent power of such a receiver fit the requirements for the TELIS, BOOMERANG, OLIMPO, and LSPE aerostatborne radio telescopes, and the space-based SPICA and MILLIMETRON projects, among others. Applications of such receivers for medical diagnostics and remote control are described.     

利用非锁定飞秒激光实现太赫兹频率的精密测量

频率是电磁波最重要的一个基本物理量,随着THz技术的发展,在光源研制、宽带通信、超精细光谱测量等领域都对THz频率的高精度侧量提出了要求.传统的Fabry-Perot干涉法与外差探测法难以实现THz频率的高精度测量,频率梳方法虽然测量精度很高,但测量系统复杂.本文提出一种利用重复频率自由漂移的飞秒激光器实现太赫兹频率精密测量的新方法.通过对非锁定的飞秒激光器的重复频率和THz拍频频率进行同时连续采集与计算,得到被测THz频率,测量精度可以达到10~(-10)量级无需对飞秒激光重复频率进行复杂的锁定控制,测量系统大大简化.