|
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
30-300μm 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.
|
