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 four-period a-Si/SiO₂ DBR with an Al₂O₃ bonding layer, and HR2, a six-period a-Si/Ta₂O₅ 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 µm, with cavity finesse up to 396,000. We used IBS-deposited a-Si directly as a bonding layer, for the first time, to our knowledge, verified its sub-angstrom roughness, and demonstrated excellent optical performance. This establishes a clear path toward scalable coatings for longer mid-infrared wavelengths, building on prior results in Nat. Commun.14, 7846 (2023).NCAOBW2041-172310.1038/s41467-023-43367-z.