Toward single-material multilayer interference mid-infrared filters with sub-wavelength structures for cryogenic infrared astronomical missions

摘要

We are trying to develop high performance mid-infrared (MIR) and far-infrared (FIR) optical filters with mechanical strength and robustness for thermal cycling toward cryogenic infrared astronomical space missions. Multilayer interference filters enable us to design a wide variety of spectral response by controlling refractive index and thickness of each layer, however, in longer MIR and FIR (30-300μm) wavelength regions, there are a few optical materials known to have both good transparency and physical robustness, which makes difficult to realize high performance filters because of limited refractive index values. It is also difficult to deposit thick layers required for MIR/FIR multilayer filters by conventional method. Furthermore, multilayer interference filters are realized by thin film coatings having different coefficients of thermal expansion (CTE), which makes filters fragile for thermal cycling. To clear these problems, we introduce sub-wavelength structures (SWS) for controlling the refractive index. Then, only one material is necessary for fabricating filters, which enables us to fabricate filters with mechanical strength and robustness for thermal cycling. In 30-300um wavelength regions silicon is very suitable for filter material because not only silicon has little absorption and physical robustness but also SWS are easily fabricated by micro-electro mechanical systems (MEMS) technology. As a first step, we have fabricated anti-reflection SWS layer on silicon wafers to demonstrate the refractive index control by simple SWS (periodic cylindrical holes on a silicon wafer). Comparing measured transmittance with both effective medium approximation (EMA) theory and rigorous coupled wave analysis (RCWA) simulation, we confirm that the refractive control of SWS layer is verified.