Heterodyne receiver for 3-5 THz with hot electron bolometer mixer

Abstract

Heterodyne receivers for applications in astronomy need quantum limited sensitivity. In instruments which are currently build for SOFIA or Herschel superconducting hot electron bolometers (HEB) will be used to achieve this goal at frequencies above 1.4 THz. We present results of the development of a phonon-cooled NbN HEB mixer for the high frequency channel of GREAT, the German Receiver for Astronomy at Terahertz Frequencies, which will be flown aboard SOFIA. The high frequency channel is centered at 4.7 THz in order to detect the fine structure line of atomic oxygen (OI). We will present the design of the high frequency channel including the local oscillator, an optically pumped THz gas laser, and the quasi-optical phonon cooled HEB mixer. In order to optimize the performance mixers with different logarithmic spiral and double slot feed antennas have been investigated. Their noise temperatures and beam patterns were measured at several frequencies between 0.7 THz and 5.2 THz. Above 3 THz the best performance was achieved with a superconducting bridge of 0.2 x 2.0 µm2 incorporated in a logarithmic spiral antenna. The DSB noise temperatures were 2900 K, 4000 K and 6000 K at 3.1 THz, 4.3 THz and 5.2 THz, respectively. The results demonstrate that the NbN HEB is very well suited as a mixer for THz heterodyne receivers up to at least 5 THz.