Highly sensitive and moderately cooled THz sensing: a challenge for Y-Ba-Cu-O superconducting thin films

Abstract

Hot electron bolometers (HEB) exploiting the properties of the high-TC superconductor (HTS) Y-Ba-Cu-O, are offering a competitive alternative to THz Schottky mixers, which require moderate cooling (e.g., 60 to 80 K). This arises from the HTS HEBs expected wide bandwidth (tens of GHz), and low local oscillator (LO) power requirements: tens of microwatts, whereas several milliwatts are necessary to pump Schottky diode LOs efficiently. In fact, the large instantaneous bandwidth is related to the extremely short electron to phonon relaxation time (1 to 2 ps, typically) in YBa- Cu-O. It is much longer in low-TC superconductors (LTS), e.g., about 20 ns in NbN. Besides, as required for LTS materials, it is mandatory to fabricate high quality and ultra thin HTS epitaxial films, so to process nano-bolometers exhibiting good mixing performances (i.e., double sideband noise temperature Tn and conversion gain G). Most of all, the challenge for Y-Ba-Cu-O remains the chemical reactivity and the aging effects, as reported previously. The main objectives of this paper are: i) to predict Y-Ba-Cu-O HEB heterodyne mixer performances, and ii) to exploit those predictions to simulate a stand-off passive detection system for, e.g., screening or security applications.