BROAD-BAND COUPLING OF THZ RADIATION TO AN YBA2CU3O7-DELTA HOT-ELECTRON BOLOMETER MIXER

We report on broad-band coupling of THz radiation to an YBa2Cu3O7-delt a superconducting hot-electron bolometer. The bolometer-a micro-bridge of an YBa2Cu3O7-delta film with a PrBa2Cu3O7-delta, protection layer- was built in a hybrid quasioptical antenna system consisting of a plan ar self-complementary spiral antenna on a dielectric substrate and an extended hyperhemispherical lens. The planar antenna was integrated in to a coplanar transmission line for broad-band intermediate-frequency matching. Measurements performed in the direct detection regime betwee n 2.5 THz and 4.2 THz delivered a system coupling efficiency for the r adiation in the fundamental Gaussian mode of about 0.1 and a beam widt h of the radiation pattern of less than one degree. The pattern was co nsistent with the far-field radiation pattern that we calculated with a ray-tracing technique. At an intermediate frequency of 1.5 GHz we me asured an output noise temperature of approximate to 160 K for the bol ometer driven in the resistive state by both de and THz current and es timated, for the heterodyne regime, a system noise temperature of appr oximate to 3 x 10(5) K. We studied the effect of a protection layer on the rate of phonon escape from the micro-bridge and, thus, on the out put frequency bandwidth of the device. We discuss possibilities of a s ignificant improvement of the device performance.