Study of MgB2 HEB mixers at THz frequencies

Licentiate thesis, 2016

The terahertz (THz) range is very attractive for astronomical observations. Spectroscopy and photometry of remote space objects in the THz range allows for a study of their chemical composition, because this range covers rotational lines from simple molecules and electron transition lines from atoms and ions. Heterodyne receivers, due to their high spectral resolution, in the THz range allows for a studying of dynamical behaviour of space objects manifested in doppler-shifted emission lines. Niobium nitride (NbN) hot-electron bolometer (HEB) mixers currently used at frequencies >1 THz provide a typical gain bandwidth (GBW) of 3–4 GHz which limits the number of astronomical applications. Moreover, the low (8–11 K) critical temperature (Tc) of NbN ultra-thin films necessitates a use of liquid helium (LHe) for the device cooling, which reduce a lifetime of spaceborn missions. MgB2 HEB mixers, introduced recently, can solve both of these problems. The high (39 K) Tc and the short (3 ps) electron-phonon interaction time of MgB2 could provide a bandwidth up to 10 GHz and operation at temperatures >20 K, where compact cryocoolers are available. However, the sensitivity of HEB mixers made from MgB2 thin films with such a high Tc operating at these temperatures is still in question. This thesis presents the noise performance study of novel submicron size MgB2 HEB mixers. Noise temperatures and conversion gains at 1.6 THz local oscillator (LO) of devices with different Tc were investigated with respect to the bath temperature. The minimum Double sideband (DSB) noise temperature for a “low” Tc HEB was 700K with a 3.2 GHz noise bandwidth (NBW) and 1150K with a 3.5 GHz NBW for 2.7K and 4.2K bath temperatures, respectively. The operation at a bath temperature of 12K was demonstrated showing a 2150K noise temperature and a 5 GHz NBW. At a bath temperature of 4.2K the noise temperature reduced to 1700K keeping a NBW of 5 GHz. The same 3.5 GHz GBW was measured for both bath temperatures. The voltage responsivity of such a device was estimated to be 1–2kV/W at 1.6 THz. The results of preliminary noise measurements at a 2.6 THz LO as well as modeling of HEB conversion gain using the standard model are presented. Based on this research operation of MgB2 HEB mixers with Tc >30K at bath temperatures >20K is suggested with no or acceptable sensitivity reduction.