Broad-band coupling of THz radiation to an hot-electron bolometer mixer

Abstract

We report on broad-band coupling of THz radiation to an superconducting hot-electron bolometer. The bolometer - a micro-bridge of an film with a protection layer - was built in a hybrid quasioptical antenna system consisting of a planar self-complementary spiral antenna on a dielectric substrate and an extended hyperhemispherical lens. The planar antenna was integrated into a coplanar transmission line for broad-band intermediate-frequency matching. Measurements performed in the direct detection regime between 2.5 THz and 4.2 THz delivered a system coupling efficiency for the radiation in the fundamental Gaussian mode of about 0.1 and a beam width of the radiation pattern of less than one degree. The pattern was consistent with the far-field radiation pattern that we calculated with a ray-tracing technique. At an intermediate frequency of 1.5 GHz we measured an output noise temperature of for the bolometer driven in the resistive state by both dc and THz current and estimated, for the heterodyne regime, a system noise temperature of . We studied the effect of a protection layer on the rate of phonon escape from the micro-bridge and, thus, on the output frequency bandwidth of the device. We discuss possibilities of a significant improvement of the device performance.