Quantum cascade (QC) lasers achieve population inversion by selecting quantum wells (QW) thicknesses so that the inherent scattering mechanisms ensure a higher population of electrons in the upper laser state compared to the lower laser state. Previously, longitudinal optical (LO) phonons have been considered the fastest, most significant scattering process in QC lasers. Recently, it has been shown that interface roughness (IFR) can have substantial effects in determining the effective lifetimes within QW systems . Simulations have shown that IFR scattering lifetimes can be the dominant scattering process for selected QW configurations .
Here we have designed and fabricated three QC structures, which differ in the positioning of a strategically placed monolayer barrier to selectively affect the IFR scattering lifetimes of the energy levels in the QC structures. Initial current-voltage characteristics suggest a shorter carrier transit time through the QC structure due to increased interface roughness interactions. We also observed an expected narrowing of the EL spectra based on these same interactions. Using these results, we have also designed a QC laser using IFR scattering as the determining process for maintaining population inversion. By using IFR scattering, we were able to design an energy separation between the lower laser level and subsequent injector levels much greater than the LO phonon energy without compromising fast carrier depopulation from the lower laser level. This in effect opens doors for completely new intersubband design techniques. This work is supported in part by MIRTHE (NSF-ERC).
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.