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.

Ultrafast terahertz detectors

Terahertz (THz) technologies have broad application prospects in ultrafast space communication, heterodyne detection, biological detection, non-destructive testing and national security. Ultrafast THz detectors, which can respond to the THz light with modulation rate larger than 1 GHz, are the key component of fast imaging, space communication, ultrafast spectroscopy and THz heterodyne applications. Theoretically, the traditional THz detectors based on heat effects are difficult to meet the requirements for fast detections, while the semiconductor based THz detectors can work under the condition of ultrafast detection. Photoconductive antennas with ultrafast response time are suitable for room-temperature broad-spectrum THz detections. Schottky barrier diodes, superconductor-insulator-superconductor mixers and hot electron bolometers are promising candidates for high-speed THz spatial heterodyne and direct detections attributable to their high conversion efficiency and low noise. High-mobility field effect transistors based on two-dimensional graphene material have the advantages of high sensitivity and low impedance, which make this kind of device have great potential applications in room-temperature high-speed detections. THz quantum well detectors (THz QWPs) based on inter-subband transitions are very suitable for the applications in high-frequency and high-speed detections because of the advantages of high responsivity, small value and integrated packaging. Recently, we have demonstrated 6.2 GHz bandwidth modulation by using THz QWPs, the fast THz receiving device. On the other hand, low working temperature and low coupling efficiency are the main factors that restrict the applications of THz QWPs. From the Brewster angle, 45° polished facet coupling structure, to one-or two-dimensional metal grating and surface Plasmon polariton coupling configuration, researchers often explore the appropriate coupling mechanism which can not only couple the normal incidence THz light, but also improve the coupling efficiency substantially. The sub-wavelength double-metal micro-cavity array coupling structure has two advantages which make THz QWPs a key candidate for fast imaging and detection in THz band:firstly, the patch antennas on the device surface can effectively increase the light absorption region, and the periodic structure can make the normal incidence THz light fulfill the rule of intersubband transition. Secondly, the sub-wavelength size double metal structure can restrict the light within a very small volume, and the electric current will be enhanced by the resonance effect when the cavity mode is equal to the peak response frequency, which can suppress the dark current and improve the optical coupling efficiency of the device. In this paper, several ultrafast THz detectors are reviewed and the advantages and disadvantages of various detectors are also analyzed.