2DEG GaN hot electron microbolometers and quantum cascade lasers for THz heterodyne sensing

We present results on design, fabrication, and characterization of hot-electron bolometers based on low-mobility two-dimensional electron gas (2DEG) in AlInN/GaN and AlGaN/GaN heterostructures. Electrical and optical characterization of our Hot Electron Bolometers (HEBs) show that these sensors combine (i) high coupling to incident THz radiation due to Drude absorption, (ii) significant electron heating by the THz radiation due to small value of the electron heat capacity, (iii) substantial sensitivity of the device resistance to the heating effect. A low contact resistance (below 0.5 Ω·mm) achieved in our devices ensures that the THz voltage primarily drops across the active region. Due to a small electron momentum relaxation time, the inductive part of the impedance in our devices is large, so these sensors can be combined with standard antennas or waveguides. In the capacity of the THz local oscillator (LO) for heterodyne THz sensing, we fabricated AlGaAs/GaAs quantum cascade lasers (QCLs) with a stable continuous-wave single-mode operation in the range of 2.5-3 THz. Spectral properties of the QCLs have been studied by means of Fourier transform spectroscopy. It has been demonstrated that the spectral purity of the QCL emission line doesn't exceed the spectrometer resolution limit at the level of 0.1 cm^-1 (3 GHz). Discrete spectral tuning can be achieved using selective devices; fine tuning can be done by thermally changing the refractive index of the material and by applied voltage. Compatibility of the low-mobility 2DEG microbolometers with QCLs in terms of LO power requirements, spectral coverage, and cooling requirements makes this technology especially attractive for THz heterodyne sensing.