Lin Yen-Ju, Jarrahi Mona
University of California, Los Angeles, CA 90095, United States of America.
Rep Prog Phys. 2020 Jun;83(6):066101. doi: 10.1088/1361-6633/ab82f6. Epub 2020 Mar 24.
The high sensitivity detection of terahertz radiation is crucial for many chemical sensing, biomedical imaging, security screening, nondestructive quality control, high-data-rate communication, atmospheric, and astrophysics sensing applications. Among various terahertz detection techniques, heterodyne detection is of great interest for applications that require high spectral resolution. Heterodyne detection involves mixing the received terahertz radiation with a reference terahertz signal provided by a local oscillator and then down-converting it to an intermediate frequency for detection. The frequency of the intermediate frequency signal is usually chosen to be in the radio frequency regime, so that it can be accurately analyzed by well-developed radio frequency electronics, including amplifiers, filters, and spectrometers, for further processing. Heterodyne terahertz detection offers two major advantages over direct terahertz detection. First, the detected terahertz radiation is effectively enhanced by the reference local oscillator signal through the mixing process, thereby enabling the detection of very weak terahertz signals. Second, the detected noise power is effectively reduced by limiting the detected spectral bandwidth to the bandwidth of the intermediate frequency electronics. In this article, we present a broad overview of various types of heterodyne terahertz receivers, which utilize different electronic and optoelectronic techniques to down-convert the received terahertz signal to a radio frequency signal. We describe how the inherent nonlinearity of a Schottky diode, superconductor-insulator-superconductor junction, hot electron bolometer, and field-effect transistor can be utilized to mix the received terahertz radiation with a reference local oscillator signal from a gas laser, quantum cascade laser, photomixer, Gunn diode, IMPATT diode, and frequency multiplier and then down-convert it to a radio frequency signal. The down-converted radio frequency signal can be subsequently detected and analyzed by various backend spectrometers, including filter bank, acousto-optical, autocorrelator, fast Fourier transform, and chirp transform spectrometers. We also discuss how a photomixer pumped by a heterodyning optical beam can be used to down-convert the received terahertz radiation to a radio frequency signal with far fewer bandwidth constraints than conventional techniques. The advantages and disadvantages of different heterodyne receivers in terms of their noise performance, operation frequency, operation bandwidth, and operation temperature are discussed in detail.
太赫兹辐射的高灵敏度检测对于许多化学传感、生物医学成像、安全筛查、无损质量控制、高数据速率通信、大气和天体物理传感应用至关重要。在各种太赫兹检测技术中，外差检测对于需要高光谱分辨率的应用极具吸引力。外差检测包括将接收到的太赫兹辐射与本地振荡器提供的参考太赫兹信号进行混频，然后将其下变频到中频进行检测。中频信号的频率通常选择在射频范围内，以便可以通过包括放大器、滤波器和光谱仪在内的成熟射频电子设备进行精确分析，以进行进一步处理。与直接太赫兹检测相比，外差太赫兹检测具有两个主要优点。首先，通过混频过程，参考本地振荡器信号有效地增强了检测到的太赫兹辐射，从而能够检测非常微弱的太赫兹信号。其次，通过将检测到的光谱带宽限制在中频电子设备的带宽内，有效地降低了检测到的噪声功率。在本文中，我们对各种类型的外差太赫兹接收器进行了广泛概述，这些接收器利用不同的电子和光电子技术将接收到的太赫兹信号下变频为射频信号。我们描述了如何利用肖特基二极管、超导体 - 绝缘体 - 超导体结、热电子测辐射热计和场效应晶体管的固有非线性，将接收到的太赫兹辐射与来自气体激光器、量子级联激光器、光混频器、耿氏二极管、碰撞雪崩渡越时间二极管和倍频器的参考本地振荡器信号进行混频，然后将其下变频为射频信号。随后，可以通过各种后端光谱仪，包括滤波器组、声光、自相关器、快速傅里叶变换和线性调频变换光谱仪，对接收到的下变频射频信号进行检测和分析。我们还讨论了由外差光束泵浦的光混频器如何用于将接收到的太赫兹辐射下变频为射频信号，与传统技术相比，其带宽限制要少得多。详细讨论了不同外差接收器在噪声性能、工作频率、工作带宽和工作温度方面的优缺点。
