Grating duty-cycle induced enhancement of substrate emission from ring cavity quantum cascade lasers

We report on experimental and theoretical investigation of grating parameters to promote one specific emission direction from ring-cavity quantum cascade lasers. The devices show single-mode behavior at room temperature and pulsed operation. A maximized surface output power was found for grating duty cycles around 70%, while for substrate emission, the optimized grating duty cycle is ≈50%. All tested devices emit more power through the substrate, when accounting for absorption and the reflection at the substrate air interface.