The terahertz (THz) frequency range of the electromagnetic spectrum has a broad number of potential applications across the physical, medical, biological and astronomical sciences. Yet its full potential has historically not been realized owing to the lack of compact, solid state, sources and detectors.
The demonstration of the first terahertz (THz) frequency quantum cascade laser (QCL) in 2002 opened up a plethora of new opportunities. Peak output powers exceeding 1 W were first demonstrated in 2014, and these have subsequently been increased to ~2.4 W at 10 K, and ~1.8 W at 77 K. Furthermore, THz QCLs now operate over a frequency range of ~1 – 5.5 THz, and at temperatures of up to 200 K, with a breadth of photonic patterning techniques being used to select specific frequencies and control output beam profiles.
Despite this success, and the flourishing research field that has developed, THz QCLs remain extremely challenging opto-electronic devices to grow epitaxially, each typically consisting of over 1000 separate interfaces defined to atomic monolayer precision. I will briefly review the growth of GaAs-AlGaAs THz QCLs by molecular beam epitaxy, and explain why, despite the maturity of this materials system and growth process, there remain challenges to be resolved. However, I will also show that it is possible to achieve repeatable THz QCL growth, not only on a day-to-day but also a year-by-year basis.
I will then discuss how THz QCLs can be incorporated into optical systems. I will explain the development of self-mixing imaging systems, which use the QCL itself as both a source and coherent detector, enabling: reflection imaging at distances exceeding 7 m; the demonstration of swept-frequency interferometry; coherent three-dimensional imaging; and, near-field microscopy. I will also outline the potential of THz QCLs for use as local oscillators in satellite-based instrumentation for Earth observation and planetary science.
Giles Davies, Lianhe Li, Paul Dean, Alexander Valavanis, and Joshua Freeman
School of Electronic and Electrical Engineering, University of Leeds, Leeds LS2 9JT, UK