Nb-Au bilayer hot-electron bolometers for low-noise THz heterodyne detection

The sensitivity of present Nb diffusion-cooled hot-electron bolometer (HEB) mixers is not quantum limited, and can be improved by reducing the superconducting transition temperature T-C. Lowering T-C reduces thermal fluctuations, resulting in a decrease of the mixer noise temperature T-M. However, lower T-C mixers have reduced dynamic range and saturate more easily due to background noise. We present 30 GHz microwave measurements on a bilayer HEB system, Nb-Au, in which T-C can be tuned with Au layer thickness to obtain the maximum sensitivity for a given noise background. These measurements are intended as a guide for the optimization of THz mixers. Using a Nb-Au mixer with T-C=1.6 K, we obtain T-M=50 K with 2 nW of local oscillator (LO) power. Good mixer performance is observed over a wide range of LO power and bias voltage and such a device should not exhibit saturation in a THz receiver. (C) 2004 American Institute of Physics.