Recent developments in terahertz quantum cascade lasers for practical applications

Abstract Terahertz (THz) quantum cascade laser (QCL) is an electrically pumped unipolar photonic device in which light emission takes place due to electronic transitions between subbands formed by multiple strongly coupled quantum wells. THz QCL is arguably the most promising solid-state source to realize various THz applications, such as high-resolution spectroscopy, real-time imaging, chemical and biological sensing, and high-speed wireless communication. To date, THz QCLs have covered emitting frequency from 1.2 to 5.4 THz when operating without the assistance of an external magnetic field. The highest output power is in hundreds milliwatt and watt levels continuous-mode and pulsed-mode operations, respectively. THz QCL-based local oscillators have been implemented in astronomy for the identification of atoms and ions. However, there are also limitations, including under room-temperature operation, large divergent beam, narrow single-mode frequency tuning range, incomplete polarization control, and narrow-range frequency comb operation that hinder the widespread applications of THz QCLs. Continuous efforts have been made to improve those THz QCL properties in order to satisfy the requirements of different THz applications. This report will review the key output characteristic developments of THz QCLs in the past few years, which aim to speed up THz QCLs toward practical applications.