Detection of a 2.8 THz quantum cascade laser with a semiconductor nanowire FET

Since their first demonstration in 2002 [1], THz quantum cascade lasers (QCLs) had an impressive impact on the field of THz optics, due to the unique mix of properties they feature, namely mW output power and compactness. On the other hand, unlike low-power THz optoelectronic emitters, that ideally combine with low noise coherent detection schemes [2], THz QCLs still suffer from the lack of convenient detectors. THz power detectors, such as semiconductor bolometers, pyroelectric detectors, and golay-cells are either slow, or scarcely sensitive, or require cryogenic cooling. Room-temperature heterodyne detection schemes give access to fast response and high signal dynamics, but local oscillators are in general cumbersome or poorly tunable [3-5]. Fast electronic THz detectors, such as field effect transistors (FET), represent a novel promising detection approach, featuring fast response times and high sensitivity in the hundreds of GHz range [6]. The recent introduction of a high-electron mobility semiconductor nanowire as transistor channel has allowed extending the operating frequency range above 1 THz, with an impressive noise equivalent power, making them attractive devices for THz QCL detection [7,8].In this contribution we report a further frequency range extension of InAs nanowire FETs, by demonstrating the detection of a 2.8 THz bound to continuum QCL, based on a single-plasmon waveguide. The QCL output field was applied between the FET source and gate contacts (Fig.1) by means of a bow-tie dipole antenna, designed for frequencies higher than 2.5THz. The resulting experimental characteristics of detector were the following: responsivity -14V/W, response bandwidth -100kHz, and 75dB/Hz signal power dynamic range with 0.6mW of QCL output power. Thanks to such performances we were able to exploit the above source-detector pair to set up a simple raster scan transmission imaging system. With two pairs of f/1 off axis parabolic mirrors the QCL beam was firstly coll- cted and diffraction limited focused on the object plane, and then collimated and focused again on the detector. The imaged object was scanned in the former focal plane by means of two high resolution stepper motors, yielding highly resolved transmission images, as the example shown in Fig.2.