Precision measurements of optical absorption in III-V heterostructures from 680 nm to 10.6 µm

Direct-bandgap compound semiconductors with ultralow optical absorption are crucial for classical and quantum PICs, low-noise crystalline coatings for ultrastable interferometers, and as an active medium for solid-state laser refrigeration. Device optimization entails quantifying absorption via precision measurements (CRD, PCI, etc.) to understand and ultimately minimize parasitic absorption. Our work on GaAs/AlGaAs-based substrate-transferred crystalline coatings has yielded absorption < 1 ppm from 1-1.5 µm and < 5 ppm to ~4.5 µm (extinction coefficients of < 2.5×10-7 and < 9×10-7 respectively). Recent efforts aim to extend the operating wavelength of these novel mirrors into the visible for optical clocks and to 10.6 µm for high-power-handling CO2 laser optics. For mid-IR wavelengths, AlGaAs-based materials remain relevant; however, the limited GaAs bandgap necessitates a switch to materials such as AlInGaP for wavelengths < 870 nm. The lessons learned in our supermirror development efforts are immediately translatable to high-transparency III-V materials for advanced PICs and solid-state laser refrigeration, with the ultimate goal of achieving net cooling in semiconductors.