Superconducting Y-Ba-Cu-O hot electron bolometric nano-mixers for terahertz passive receivers

We report on the development of a terahertz (THz) wave mixer made from high critical temperature superconducting YBaCuO ultrathin films (10 to 50 nm). The work is part of the MASTHER ANR project aiming at a portable demonstrator for passive terahertz heterodyne detection, implementing simplified cryogenics (60 to 80 kelvin). The detection principle is that of the hot electron bolometer (HEB) so far mainly developed with low critical temperature superconductors. The HEB effect is implemented in an YBaCuO constriction (a few hundred nm in lateral dimensions). This structure can lead to a sensitive and fast THz detector (theoretical instantaneous bandwidth of 100 GHz). The THz radiation is coupled to the YBaCuO constriction by means of a wideband planar antenna. The new aspects first concern the modeling of heat exchange between electrons and phonons reservoirs (YBaCuO and its substrate). Our results establish the optimum operating conditions in terms of dimensions of the constriction and the local oscillator power required for high performance THz mixing (conversion gain and noise temperature). We are introducing in particular a new 'hot spot' modeling approach, which takes into account the influence of the terahertz frequency in the YBaCuO material and the impedance matching between the antenna and the constriction. Second, we have developed and optimized the HEB micro-fabrication process in clean room, especially the electronic and optical lithography steps, to obtain constrictions of 300 nm lateral size. Our first devices have been tested by direct detection in the infrared. The performance between YBaCuO ultrathin films prepared using various techniques are compared. (author)