Recent Developments of Terahertz Quantum Cascade Lasers

Terahertz (THz) quantum cascade lasers (QCLs) are semiconductor-based electrically pumped laser sources that are based on electron transitions between the subbands in the conduction band of a multiple-quantum-well heterostructure. Being such, the wavelength of THz QCL can be tuned by varying the thickness of the quantum wells, covering the frequency range of similar to 1-5 THz. THzQCLs are also high-power light sources in the THz region, with highest output power in the 100 mW and watt levels for the continuous mode and pulsed mode operation, respectively. However, there are also limitations that hinder their widespread applications, which include low operating temperature, large beam divergence, the intrinsic carrier dynamics, etc. Nevertheless, continuous efforts have been devoted to this area and pushing the frontier further and further. In this paper, we will review both the aforementioned limitations and some of the key recent developments in THz QCLs in the past few years.