Terahertz imaging using photomixers based on quantum well photodetectors

Abstract

Due to the fast intersubband transitions, the terahertz (THz) quantum well photodetector (QWP) is supposed to work fast. Recently it has been demonstrated that the THz QWP can detect the THz light modulated at 6.2 GHz and therefore it can be used as a photomixer [H. Li et al., Sci. Rep. 7, 3452 (2017)]. In this work, the authors report a novel active THz imaging using THz QWP photomixers. The THz radiation source used for this imaging application is a multi-mode THz quantum cascade laser (QCL) operating in continuous wave mode. When the fast THz QWP is illuminated by the multi-mode THz radiation, the intermediate frequency signal that is resulted from the frequency beating between the neighbouring THz modes of the QCL can be extracted from the QWP mesa for imaging applications. Employing the technique, the frequency can be down-converted from the THz range to the microwave regime. And therefore, the signal can then be amplified, filtered, and detected using the mature microwave technology.

4.3 THz quantum-well photodetectors with high detection sensitivity

“…[4][5][6][7][8] Additionally, due to the inherent short carrier lifetime, THz QWPs are very attractive in many high-speed applications. [9][10][11][12] The active region of QWPs, which consists of multiple-quantum-well (MQW) structures, can only respond to the light that satisfies the intersubband transition (ISBT) rules, [13][14][15] so the incident planes of the devices are usually polished to an angle of 45 • . [16][17][18] Generally, the ohmic contact resistance of the THz QWPs is relatively high (∼ 100 Ω) due to the low doping density of the top and bottom contact layers, [16,19] while if we raise the doping density of the contact layers, the dark current of the device will be relatively high.…”