Receiver Measurements at 700 GHz with a Niobium Diffusion-Cooled Hot-Electron Bolometer Mixer

We report on heterodyne measurements with a Nb diffusion-cooled hot-electron bolometer in a fixed tuned waveguide mixer at 700 GHz. The device is a 300 nm long and 12 nm thick Nb microbridge, with a normal state resistance of 31 Si. We have measured the DSB receiver noise temperature using a standard Y-factor method with a 300 K and 77 K blackbody load. At 700 GHz, a bath temperature of 4.3 K, and an intermediate frequency of 1.25 GHz, the receiver noise temperature Tre, ,DsB is 1690 K after correction for the losses in the beamsplitter. Reducing the bath temperature to 3.3 K decreases Trec.Dsg to 1100 K. Careful inspection of the pumped differential I(V) characteristics does not reveal any influence due to direct detection. From the data we extract a maximum value of —17.8 dB for the conversion gain of the mixer at 4.3 K. The minimum mixer noise is determined to be 1170 K by applying a noise breakdown calculation. We expect significant improvement of the receiver noise performance upon further reduction of the bath temperature.