Multi-layer coatings designed for optical power enhancement of long-wavelength infrared quantum cascade lasers

This work presents the design, fabrication, and experimental validation of multilayer optical coatings for enhancing the optical power of long-wavelength infrared (LWIR) quantum cascade lasers (QCLs). A high-reflectivity (HR) coating based on a Y₂O₃/Ti/Au structure and a novel partial-reflectivity (PR) coating using a Y₂O₃/Ge bilayer were developed. Optical constants were determined by ellipsometry and used for numerical optimization, while FTIR measurements confirmed the coatings’ optical performance.Applying HR and PR coatings to the rear and front facets of QCLs (λ = 9.3 μm) resulted in a significant increase in output power (from 974 mW to 1653 mW per facet) without affecting the threshold current density. The extracted reflectivities were 96% (HR) and 9% (PR). This work demonstrates, for the first time, a Y₂O₃/Ge-based PR coating on QCL facets, enabling substantial performance improvement.