High-power (> 1 Watt) terahertz frequency quantum cascade lasers for stand-off imaging

Terahertz frequency quantum cascade lasers (THz QCLs) are compact electrically-driven semiconductor sources of radiation in the 1–5 THz (λ = 300–60 μm) band. Through a systematic optimisation of the device design and processing, we have recently demonstrated a world-record peak output power in excess of 1 W from a single facet at 10 K (more than double the previous record) and \textgreater 400 mW at 77 K. We review the development and performance of this laser, and consider a range of novel imaging techniques that could exploit high-powered THz emission. The first such technique senses the relatively weak scattered radiation from a powdered material, thus obtaining sensitivity to both the particle geometry and the bulk spectral absorption properties of the material, in good agreement with conventional THz time-domain spectroscopy. An alternative self-mixing technique is discussed, in which feedback of radiation into the laser cavity from an external object causes perturbations to the laser voltage, allowing ‘detector-free’, highly sensitive, confocal and phase-sensitive imaging of concealed three-dimensional surface morphologies