275–500-GHz Wideband Waveguide SIS Mixers

We have successfully demonstrated low-noise and wideband SIS mixers for the 275-500-GHz radio-frequency (RF) band, aiming to cover Atacama Large Millimeter/submillimeter Array band 7 (275-373 GHz) and band 8 (385-500 GHz) simultaneously. The waveguide SIS mixers integrate a multisection impedance transformer with a parallel-connected-twin-junction or single-junction circuits based on Nb/AlN/Nb trilayers with a critical current density of 31 kA/cm
²
. In order to achieve the targeted broadband performance, we designed an asymmetric one-side waveguide probe placed across a reduced height waveguide with dimensions 100 μm × 580 μm. The shape of the waveguide probe was optimized to have nearly constant impedance at around 35 Ω in the desired RF range. For evaluation of the receiver performance, we have established a dual-band noise temperature measurement system. The system implements a broadband corrugated horn antenna and a 15-dB RF/local oscillator (LO) waveguide coupler based on WR-2.3 (290 μm × 580 μm) waveguide size. The target bandwidth is too wide for typical LO sources at these high frequencies, and, therefore, we applied a dual-band LO injection system utilizing two different LO sources and an LO diplexer. The IF chain uses a cryogenic isolator and a low-noise amplifier with a typical noise temperature of 2.1 K over the 4-8-GHz range. The measured double sideband receiver noise temperature is two to three times the quantum noise at all LO frequencies for both the twin-junction and the single-junction circuits.