Terahertz frequency quantum cascade lasers: Optical feedback effects and applications

Remarkable progress in terahertz (THz) technology over the past decade has been driven by the potential applications of THz waves in areas such as biomedical imaging, long-range screening, and organic materials identification [1]. This growth is in no small measure related to the success of the quantum cascade laser (QCL) which has established itself as one of the most promising radiation sources at terahertz frequencies [2]. The appeal of these novel semiconductor lasers stems from their compact size, broad spectral coverage (∼ 1–5 THz), and high output powers [3]. The ability of THz QCLs to generate coherent emission with quantum noise-limited linewidths, make them particularly suited to the development of interferometric THz sensing and imaging systems.