Aluminum hot-electron bolometer mixers at submillimeter wavelengths

Diffusion-cooled aluminum hot-electron bolometer (HEB) mixers are of intere st for low-noise high resolution THz-frequency spectroscopy within astrophy sics, Al HEB mixers offer operation with an order of magnitude less local o scillator power, higher intermediate frequency bandwidth and potentially lo wer noise than competing devices made from other materials, We report on mi xer experiments at 618 GHz with devices fabricated from films with sheet re sistances in the range from about 55 Omega down to about 9 Omega per square . Intermediate frequency bandwidths of up to 3 GHz were measured (1 mum lon g device), with absorbed local oscillator power levels of 0.5 to 6 nW and m ixer conversion up to -21.5 dB, High input coupling efficiency implies that the electrons in the device are able to thermalize before escaping from th e device. It was found that the long coherence length complicates mixer ope rations due to the proximity of the contact pads, Also, saturation at the I F frequency may be a concern for this type of device, and warrants further studies.