PT Unknown AU Schubert, J Semenov, A Hübers, H Gol'tsman, G Schwaab, G Voronov, B Gershenzon, E TI Broad-band terahertz NbN hot-electron bolometric mixer SE Inst. Phys. Conf. PY 1999 BP 663 EP 666 VL 167 DE NbN HEB mixers ER