THz Schottky diode mixers for high resolution FIR spectroscopy

Research Project, 2018 – 2019

The aim of this project is to explore room temperature and broadband Schottky diode mixers for atmospheric sensing and for use in phase-locking of far infrared optical sources (e.g. quantum cascade laser), with the main objective to improve spectral resolution in heterodyne instrumentation up to 4.7 THz for air/space-borne applications. Today, Schottky diode technology is the workhorse for local oscillator chains up to ca 1.9 THz (Herschel HIFI) and the preferred choice up to ca 1.2 THz for low noise and broadband submillimetre wave heterodyne radiometers (ODIN, EOS MLS, JUICE, ISMAR, STEAM-R). For Earth observation, the future scientific objectives and instrument requirements are driven by the need to accommodate the atmospheric measurement of Hydroxyl at 1.8, 2.5, and 3.5 THz (Harwell SUPRA-THz sounders). Moreover, for space science following the success of the Herschel telescope, there is now a strong effort towards a new generation of heterodyne THz receiver instruments based on ultra-low noise Hot Electron Bolometer (HEB) mixer arrays (Origins Space Telescope, Millimetron) with up to hundred times more pixels per frequency channel, with an overall significant improvement in terms of sensitivity. These missions call for the development of powerful and efficient LO sources and low-noise broadband mixers in the frequency range of 1 – 5 THz.
The Schottky diode is robust and can operate over a wide temperature range and is hence attractive for space instrumentation. Since passive cooling is sufficient, the Schottky diode is better suited for long lifetime missions compared to cryogenic technology. Nevertheless, the output power of Schottky frequency multipliers drops with increasing frequency and heterodyne receivers beyond 2.5 THz require a mix of optical & electronic techniques in order to provide enough LO power to drive the mixer. For instance, heterodyne receiver arrays at 4.7 THz are now possible thanks to compact and powerful QCLs for the LO generation. However, for sufficient line width resolution, the QCL require phase and frequency stabilisation, which can be accomplished with a THz Schottky diode harmonic mixer locked to a microwave reference signal (Phased Locked Lope – PLL). Schottky diode-based sub-systems are therefore of strategic importance for submillimetre wave and FIR instruments dedicated to the study of the Earth’s atmosphere, and the study of planets and astrophysics.