Investigation of NbN phonon-cooled HEB mixers at 2.5 THz

The development of superconducting hot electron bolometric (HEB) mixers has been a big step forward into the direction of quantum noise limited mixer performance at THz frequencies. Such mixers are crucial for the upcoming ge neration of airborne and spaceborne TTHz heterodyne receivers. In this pape r we report on new results on a phonon-cooled NbN HEB mixer using e-beam li thography. The superconducting film is 3 nm thick. The mixer is 0.2 mu m lo ng and 1.5 mu m wide and it is integrated in a spiral antenna on a Si subst rate. The device is quasi-optically coupled through a Si lens and a dielect ric beam combiner to the radiation of an optically pumped FIR ring gas lase r cavity. The performance of the mixer at different THz frequencies from 0. 69 to 2.55 THz with an emphasis on 2.52 THz is demonstrated. At 2.52 THz mi nimum DSB noise temperatures of 4200 K have been achieved at an IF of 1.5 G Hz and a bandwidth of 40 MHz with the mixer mounted in a cryostat and a 0.8 m long signal path in air.