Development of 1.5 THz waveguide NbTiN superconducting hot electron bolometer mixers

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 a commercial solid state tunable local oscillator (LO) source. The NbTiN HEB mixer is made from 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 to ensure good contact. The uncorrected DSB receiver noise temperature is measured to be 1700 K at 1.5 THz, whereas the mixer noise temperature is derived to be 1000 K after corrections for losses of the input optics and the intermediate frequency (IF) amplifier chain. The required LO power absorbed in the HEB mixer is evaluated to be 340 nW by using an isothermal technique. The IF gain bandwidth is supposed to be about 1.3 GHz or higher. The present results show that good performance can be obtained at 1.5 THz even with a relatively thick NbTiN film (12 nm), as in the case of 0.8 THz. In order to investigate the cooling mechanism of our HEB mixers, we have conducted performance measurements for a few HEB mixers with different microbridge sizes both at 1.5 and 0.8 THz. The noise performance of the NbTiN HEB mixers is found to depend on the length of the NbTiN microbridge. The shorter the microbridge is, the lower the receiver noise temperature is. This may imply a contribution of the diffusion cooling in addition to the phonon cooling.

1.5 THz heterodyne receiver with waveguide superconducting NbTiN hot electron bolometer mixer

“…The simulated IF gain bandwidth is 2.7 GHz at the optimum bias point. Hence we have confirmed that fairly good performance can be obtained at 1.5 THz even with the 10.8 nm NbTiN film which is thicker than that used by the other groups (a few nm) for the phonon-cooled mixers [14], improving our previous result at 1.5 THz after optimizing the RF matching circuit [6]. By using the in situ fabrication process, we obtain the above performance by use of a 10.8 nm NbTiN film.…”

“…In the fabrication process of the mixer, an in-situ technique is employed to avoid the oxidation and contamination of the surface of the NbTiN film. Therefore, the optimized hot spot model incorporates in the phonon-and diffusion-cooling mechanism together [6]. In a typical HEB mixer, a microbridge is made up of an ultrathin superconducting film placed between two metal contact pads.…”

“…Niobium nitride (NbN) phonon-cooled hot electron bolometer (HEB) mixers are currently the most sensitive heterodyne detectors at frequencies above 1.2 THz [3]. However, the niobium titanium nitride (NbTiN) film is a good alternative option for the HEB mixers [4][5][6], since its physical and chemical properties are quite similar to those of the NbN film. In contrast to the NbN film, the NbTiN film can readily be deposited on a quartz substrate, where the waveguide mixer is easily fabricated.…”

Effect of the Critical and Operational Temperatures on the Sensitivity of HEB Mixers

“…Two tested HEB mixers differed, not only in terms of the critical temperature, but also MgB2 film thickness. As it follows from the HEB mixer theory, and some experiments with NbTiN HEB mixers [10], the film thickness affects the GBW and NBW of the device due to a longer phonon escape time. However, it should not affect the noise temperature (at IF << GBW) unduly.…”

“…ot-electron bolometer (HEB) mixers have been proven to be a class of highly sensitive terahertz (THz) detection elements (from 1.3THz to 5.3THz) employed in many receivers for astronomical and atmospheric science observation programs launched in recent years, including RLT [1], APEX [2], [3], Herschel [4], [5], TELIS [6], [7], SOFIA [8], [9]. They are also chosen for different programs under development, such as ASTE [10], DATE5 [11].…”