Recent progress of terahertz detectors based onphotothermoelectric effect

Abstract

<p indent=0mm>The unique physical properties of terahertz (THz) waves lead the THz technology to a broad application prospect in many fields, including security imaging, communication, nondestructive testing and biomedicine. However, as for now, the lack of high-performance, room-temperature THz detectors still hinders the practical applications of THz technology. According to the detection mechanisms, the THz detectors can be divided into two types, devices based on electronic methods and devices based on photothermal effects. The former is limited by the cut-off frequency and thus generally they can only work below <sc>1 THz,</sc> while the latter is usually limited by the slow response speed, so they cannot achieve high-speed THz detection. The photothermoelectric (PTE) detector, a new kind of THz detector, may have its own power to achieve sensitive THz detection. PTE detector is a new kind of THz photothermal detector appearing in the recent ten years. Compared with other kinds of photothermal detectors, it has many advantages, such as large bandwidth, energy conservation, high speed, and room temperature operation. In addition, thanks to the hot carrier effect, the response time of THz PTE detectors can reach several nanoseconds at room temperature, which completely makes up the shortcoming of other THz photothermal detectors. Therefore, since the researchers of the Tsinghua University first observed the THz PTE response in the carbon nanotube-metal contact in 2014, THz PTE detector has attracted much attention and become a hotspot rapidly. PTE effect, also regarded as the effect that carriers are driven by temperature gradient and thus flow from the hot region to the cold region along the channel, can result in a net current while external THz wave is illuminating on the detector. The most unique requirement of PTE detector is the asymmetric design, otherwise there will be no net current in the device. According to the design of asymmetry, there are two types of THz PTE detectors, devices with asymmetric temperature distribution and devices with asymmetric Seebeck coefficient distribution. The asymmetric temperature distribution is generally induced by the asymmetric illumination and asymmetric antenna structure. In both cases, a part of the detector is heated by THz wave while the other part remains cold. The carriers are driven by the temperature difference, leading to a photo response. The asymmetric Seebeck coefficient distribution is induced by the asymmetric electrode materials or p-n junctions. In both cases, there is a Fermi energy level difference in the detector. Under THz illumination, the carriers are driven by the temperature gradient and Fermi energy level difference, leading to a photo response. In order to evaluate different THz PTE detectors, several key figure-of-merits, including photoresponsivity, response time, NEP and detectivity are introduced. According to these factors, one can know clearly about the advantages and disadvantages of a specific detector. The recent progress of THz PTE detectors is comprehensively reviewed in this paper. The typical devices are analyzed in detail according to their working characteristics and performance. The future direction of THz PTE detectors is also prospected. PTE effect can be combined with many physical effects to further enhance the detector performance. For example, the phonon blocking effect and ballistic transport have been proved to improve thermoelectric performance. Besides, the development of new materials also brings new opportunities for THz PTE detectors. Intercalated two-dimensional materials with small organic molecules can effectively improve the thermoelectric properties and the two-dimensional-electron-gas system can also bring the large Seebeck coefficient and high carrier mobility, which are helpful to THz PTE detectors. All in all, although there is still a long way to go for practical application, THz PTE detector still has a bright prospect due to its unique advantages.