Transverse mode control in quantum cascade lasers via lossy lateral constrictions

Quantum Cascade (QC) lasers are semiconductor devices operating in the mid-infrared and terahertz regions of the electromagnetic spectrum. Since their first demonstration in 1994, they have evolved rapidly into high power devices. However, they also have intrinsic challenges, such as beam steering at high power. Such phenomenon has been observed in QC lasers and attributed to the interaction between the two lowest transverse modes in the laser cavity. In this project, we have used COMSOL Multiphysics simulations to first investigate how transverse mode propagation can be controlled with cavity spoilers. We have modeled this effect by creating short and lossy lateral constrictions from the top of the laser ridge to perturb the modes distributed more toward the sides of the laser ridge, while leaving the fundamental mode intact. After obtaining optimized dimensions for the constrictions, we have utilized focused ion beam (FIB) milling to etch two small trenches from the top of several laser ridges to create the simulated effect on our devices. We, then, filled them with platinum in an effort to completely suppress the propagation of higher order transverse modes in the cavity. The results obtained show minimal effect on threshold and a Gaussian far-field distribution at various current levels, indicating a complete suppression of the higher order transverse modes.