Abstract
High performance distributed Bragg reflectors (DBRs) are key elements to achieving high finesse MEMS-based Fabry–Pérot interferometers (FPIs). Suitable mechanical parameters combined with high contrast between the refractive indices of the constituent optical materials are the main requirements. In this paper, Germanium (Ge) and barium fluoride (BaF2) optical thin-films have been investigated for mid-wave infrared (MWIR) and long-wave infrared (LWIR) filter applications. Thin-film deposition and fabrication processes were optimised to achieve mechanical and optical properties that provide flat suspended structures with uniform thickness and maximum reflectivity. Ge-BaF2-Ge 3-layer solid-material DBRs have been fabricated that matched the predicted simulation performance, although a degradation in performance was observed for wavelengths beyond 10 μm that is associated with optical absorption in the BaF2 material. Ge-Air-Ge 3-layer air-gap DBRs, in which air rather than BaF2 served as the low refractive index layer, were realized to exhibit layer flatness at the level of 10 to 20 nm across lateral DBR dimensions of several hundred micrometers. Measured DBR reflectance was found to be ≳90 % over the entire wavelength range of the MWIR band and for the LWIR band up to a wavelength of 11 μm. Simulations based on the measured DBR reflectance indicates that MEMS-based FPIs are able to achieve a peak transmission of ≳90 % over the entire MWIR band and up to 10 μm in the LWIR band, with a corresponding spectral passband of ≲50 nm in the MWIR and <80 nm in the LWIR.
Highlights
The studies based on SiOx and SiNx are mainly focused in the short wave infrared region since SiOx and SiNx are absorbing in the long-wave infrared (LWIR) spectral band, which renders them unsuitable for the development of narrowband Fabry–Pérot interferometers (FPIs) at LWIR wavelengths
Based on the reflectance spectra demonstrated by distributed Bragg reflectors (DBRs) in the mid-wave infrared (MWIR) and LWIR bands, we evaluated the suitability of α– and β– DBRs toward the realization of FPIs for hyperspectral sensing and imaging applications
In the case of α– series DBRs, a drop in reflectance of the DBRs is noticed toward the long wavelength end of the wavelength range in both the MWIR (3 to 5 μm) and LWIR (8 to 12 μm) spectral bands
Summary
Spectrometers[1,2,3,4,5,6,7] in the mid-wave infrared (MWIR: 3 to 5 μm) and long-wave infrared (LWIR: 8 to 12 μm) wavelength ranges are of interest for infrared (IR) spectroscopy,[8,9,10,11] multi/hyper-spectral imaging,[12,13,14,15,16] and compositional analysis[11,17] due to their inherent ability to identify the unique absorption and/or reflected spectra of elements and compounds. Gill et al.: Ge/BaF2 thin-films for surface micromachined mid-wave and long-wave infrared reflectors a high refractive index material along with low refractive index materials such as SiOx, SiNx, and ZnS for the fabrication of shortwave and longwave IR FPIs. The studies based on SiOx and SiNx are mainly focused in the short wave infrared region since SiOx and SiNx are absorbing in the LWIR spectral band, which renders them unsuitable for the development of narrowband FPIs at LWIR wavelengths. The fabrication of experimental FPIs is beyond the scope of this paper and will be addressed in future studies
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