Development of 1.5 THz waveguide NbTiN superconducting hot electron bolometermixers

Abstract

We present a characterization of a 1.5 THz waveguide niobium titanium nitride (NbTiN)superconducting hot electron bolometer (HEB) mixer which can be pumped by acommercial solid state tunable local oscillator (LO) source. The NbTiN HEB mixer is madefrom a 12 nm thick NbTiN thin film deposited on a quartz substrate at room temperature.A gold electrode is formed in situ on the NbTiN thin film without breaking vacuum toensure good contact. The uncorrected DSB receiver noise temperature is measured to be1700 K at 1.5 THz, whereas the mixer noise temperature is derived to be 1000 K aftercorrections for losses of the input optics and the intermediate frequency (IF) amplifierchain. The required LO power absorbed in the HEB mixer is evaluated to be340 nW by using an isothermal technique. The IF gain bandwidth is supposed to beabout 1.3 GHz or higher. The present results show that good performance can beobtained at 1.5 THz even with a relatively thick NbTiN film (12 nm), as in the case of0.8 THz. In order to investigate the cooling mechanism of our HEB mixers, wehave conducted performance measurements for a few HEB mixers with differentmicrobridge sizes both at 1.5 and 0.8 THz. The noise performance of the NbTiNHEB mixers is found to depend on the length of the NbTiN microbridge. Theshorter the microbridge is, the lower the receiver noise temperature is. This mayimply a contribution of the diffusion cooling in addition to the phonon cooling.