2.7 THz Balanced Waveguide HEB Mixer

We report on the development of a waveguide-based balanced superconducting mixer for operation near 2.7 THz. The mixer employs a pair of NbN hot-electron bolometers defined on 6 μm-thick silicon substrate that follows a 90 ^° hybrid coupler. To produce the critical structures of the coupler and waveguide embedding circuit, we have utilized silicon micromachining techniques based on deep reactive ion etching. Operating near 4.2 K bath temperature, we have measured a minimum uncorrected DSB receiver noise temperature of less than 2000 K using Callen-Welton formula and local oscillator sideband noise rejection better than 13±3 dB at 2.74 THz. The concept is suitable for building arrays, readily scalable for higher frequencies up 5 THz, and could accommodate other mixer technologies, such as room-temperature Schottky diode mixers.

I. Introduction

Sensitive terahertz-frequency heterodyne receivers are needed to detect and resolve spectral lines that originate from the interstellar medium and from the atmospheres of planets, including the Earth's. Beyond 1.25 THz, the most sensitive heterodyne receivers are based on superconducting hot-electron bolometer (HEB) mixers. In several practical implementations both waveguide [1]–[3] and quasioptical architectures [4] are successfully used, typically using an interferometer diplexer when local oscillator (LO) power is scarce or a simple dielectric beam splitter when there is sufficient LO power margin. Looking toward operation at higher frequencies, we must contend with the combination of shortage of local oscillator power from frequency-agile electronic sources and the difficulty of efficiently coupling it to the mixer. Further, the desire for imaging capability with multi-pixel systems makes further demands on the LO diplexer as it should not affect mixer performance that would otherwise be obtained with a single-pixel receiver.