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In this letter, a broadband bias-tunable terahertz (THz) quantum well infrared photodetector (QWIP) with a GaAs/AlGaAs asymmetric step multi-quantum well using the intersubband transition was proposed, designed, fabricated, and characterized. It is found that the THz QWIP has two photon response peaks at 4.5 THz and 6.5 THz and the bias-tunable photon response in the 4.0–6.5 THz frequency range of the device is attributed to the field-assisted tunneling of electrons in the second excitation subbands to continuum states. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Strong and sharp photocurrent peak at longitudinal optical (LO) phonon frequency (8.87 THz) is found in GaAs/(Al,Ga)As terahertz quantum-well photodetectors (QWPs). Two mesa-structure terahertz QWPs with and without one-dimensional metal grating are fabricated to investigate the behavior of such photoresponse peak. The experimental and simulation results indicate that the photocurrent peak originates from a two-step process. First, at the LO phonon frequency, a large number of non-equilibrium LO phonons are excited by the incident electromagnetic field, and the electromagnetic energy is localized and enhanced in the thin multi-quantum-well layer. Second, through the Frohlich interaction, the localized electrons are excited to continuum states by absorbing the non-equilibrium LO phonons, which leads to the strong photoresponse peak. This finding is useful for exploring strong light-matter interaction and realizing high sensitive terahertz photodetectors. [ABSTRACT FROM AUTHOR]

For the terahertz imaging systems, the fast detection of terahertz light is deemed as a key technology. We present a fast terahertz reflective confocal scanning imaging system employing a fast terahertz quantum-well photodetector (QWP) and a fast rotating translational platform. The excellent performance of the terahertz QWP indicates that the device can rapidly detect the 4.3 THz radiation generated from a pulsed electrically pumped terahertz quantum cascade laser, which is used as the terahertz source of the imaging system. Two 200 μm pinholes are placed in the two focuses to filter out the scattered light and improve the resolution of the imaging system, we achieve a lateral resolution better than 110 μm and an axial resolution of about 320 μm. The terahertz two-dimension images of some objects are obtained within 5 s with a high contrast. The three-dimensional sections of the object demonstrate the great axial selectivity of the terahertz imaging system. [ABSTRACT FROM AUTHOR]

A reflection imaging system based on the terahertz quantum-well photodetector is presented. Reflected terahertz imaging of a logo on a universal serial bus (USB) flash disk is demonstrated. A spatial resolution better than 0.4 mm is achieved. This experiment demonstrates that the quantum-well photodetector is potentially useful for reflection imaging applications. [ABSTRACT FROM AUTHOR]