Ion-Beam-Sputtered Mid-Infrared Coatings for Hybrid Supermirrors

We report on the development of low-loss ion-beam-sputtered (IBS) mid-infrared coatings for hybrid supermirrors. Two highly reflective designs were realized: HR1, a 4-period a-Si/SiO$_2$ DBR with an Al$_2$O$_3$ bonding layer, and HR2, a 6-period a-Si/Ta$_2$O$_5$ DBR with an a-Si terminating layer for bonding. Combined with a GaAs/AlGaAs crystalline mirror, HR2-based hybrids yielded a total loss of 9.3 ppm and excess loss of 6.8 ppm at 4.45 $\mu$m, with cavity finesse up to 396000. For the first time, we used IBS-deposited a-Si directly as a bonding layer, verified its sub-angstrom roughness, and demonstrated excellent performance. This establishes a clear path toward scalable coatings for longer mid-infrared wavelengths, building on prior results in Nat. Commun. 14, 7846 (2023).