Ge/BaF2 thin-films for surface micromachined mid-wave and long-wave infrared reflectors

High performance distributed Bragg reflectors (DBRs) are key elements to achieving high finesse MEMS-based Fabry–Pérot interferometers (FPIs). Suitable mechanical parameters combined with contrast between the refractive indices of constituent optical materials main requirements. In this paper, Germanium (Ge) and barium fluoride (BaF2) thin-films have been investigated for mid-wave infrared (MWIR) long-wave (LWIR) filter applications. Thin-film deposition fabrication processes were optimised achieve properties that provide flat suspended structures uniform thickness maximum reflectivity. Ge-BaF2-Ge 3-layer solid-material DBRs fabricated matched predicted simulation performance, although a degradation in was observed wavelengths beyond 10 μm is associated absorption BaF2 material. Ge-Air-Ge air-gap DBRs, which air rather than served as low index layer, realized exhibit layer flatness at level 20 nm across lateral DBR dimensions several hundred micrometers. Measured reflectance found be ≳90 % over entire wavelength range MWIR band LWIR up 11 μm. Simulations based on measured indicates FPIs able peak transmission band, corresponding spectral passband ≲50 <80 LWIR.