Enhanced performance of quantum cascade lasers through active region optimization and large optical cavity engineering.

This paper presents a comprehensive study of mid-infrared quantum cascade lasers (QCLs) designed with optimized active region geometries and large optical cavity (LOC) structures to achieve enhanced performance. The devices were fabricated using molecular beam epitaxy (MBE) and characterized in terms of electro-optical, spectral, and far-field properties. The introduction of LOC architectures significantly reduced waveguide losses, improved optical confinement, and enhanced beam collimation. Devices with 50 active region stages and LOC design (QCL#3_LOC) exhibited the lowest waveguide losses (∼1.9 cm-1) and the narrowest vertical far-field divergence (FWHM = 27.1°), while maintaining stable emission around 5.3 µm. Compared to conventional designs, the optimized structures achieved nearly four times higher output power (up to 3 W) and a reduced threshold current density (∼2.5 kA/cm2), with a wall-plug efficiency exceeding 4%. Spectral measurements revealed narrower electroluminescence linewidths and reduced current-induced redshift, confirming improved carrier uniformity and thermal management. These results demonstrate that combining active region optimization with LOC engineering provides a powerful route to increased performance and reliability of mid-infrared QCLs, making them promising candidates for high-power applications such as gas sensing, spectroscopy, and free-space optical communication.