How resonator design improves performance of quantum cascade laser-pumped molecular lasers

The widespread application of terahertz (THz) technology remains limited by the need for compact, room-temperature, high-power sources. The quantum cascade laser (QCL) pumped molecular laser (QPML) is what we feel is a promising new THz source with great tunability and high spectral brightness, but its performance is limited by competing requirements: maintaining a stable, low-loss THz resonator while maximizing infrared (IR) absorption and minimizing IR feedback that destabilizes the QCL pump. This work systematically explores how to achieve the best of both high-pump absorption and minimal back-reflection. The lasing performance of a fluoromethane (CH₃F) gain medium in various Fabry-Perot resonator (FPR) geometries is compared using experimental measurements of lasing threshold and output power. Then, by computational modeling of each FPR geometry and a compact copper waveguide, the pumping efficiency and the amount of back-reflection are evaluated and correlated with experimental findings. Lastly, the performance of these resonators is compared for ammonia (NH₃) and carbonyl sulfide (OCS) gain media. Our optimized FPR achieved a maximum emission power of 2 mW for NH₃, the highest continuous-wave power reported for a QPML to date. These results provide a clear optimization pathway for designing stable and efficient QPMLs suitable for practical applications.