Fabrication-Tolerant Design for High-Power, Single-Lobe, Surface-Emitting Quantum Cascade Lasers

Grating-coupled, surface-emitting (GCSE) quantum-cascade lasers (QCLs) of linear geometry generally operate in an antisymmetric mode, which is undesirable due to its double-lobed far-field beam pattern. The antisymmetric mode can be suppressed by designing 2
^nd
-order metal/semiconductor gratings that favor symmetric-mode operation, but using such an approach requires tight tolerances in grating duty cycle that impair fabrication yields. Alternatively, the grating can be designed to favor antisymmetric-mode operation with single-lobe emission by using a central π-phaseshift. Here we show that such a design is highly fabrication tolerant. Simulations using COMSOL Multiphysics are performed followed by finite-length analysis using coupled-mode theory and the transfer-matrix method, as well as a semi-empirical QCL model for predicting CW performance. The analysis reveals a device of significantly higher tolerance for fabrication errors, operating in a single-lobe beam pattern to 0.63 W CW, although the emitted power scales less efficiently with length than for symmetric-mode operating devices.