Yutaka Yamagishi

Mid-infrared wire-grid polarizer with silicides

An infrared (IR) polarizer with tungsten silicide (WSi) wire grid was fabricated by two-beam interference exposure and reactive ion etching. To enhance TM transmittance, silicon monoxide was deposited between the WSi wire grid (400 nm period) and a Si substrate. The transmittance was over 80% in the 4-5 microm wavelength range. The ratio of TM and TE transmittances was over 100 (20 dB) in the 2.5-6 microm wavelength range. The fabricated polarizer has higher durability and better compatibility with microfabrication processes compared with conventional IR polarizers.

Transmittance enhancement of a wire-grid polarizer by antireflection coating

We examined the effect of an antireflection (AR) coating to enhance the TM transmittance of the wire-grid polarizer. The polarization transmission spectra were calculated using the rigorous coupled-wave analysis. As a result, we verified that an AR film should be inserted between a wire-grid and a Si substrate as regards the TM transmittance and the polarization function. Based on the simulation results, we fabricated a tungsten silicide (WSi) wire-grid polarizer with SiO films on both sides of the Si substrate. The transmittance exceeded 80% at a 4-5 microm wavelength range, although the theoretical transmittance of Si substrate is 54% and the ratio of the TM and TE transmittances reached 24 dB at a 3 microm wavelength when the WSi grating has a 300 nm thickness, a 400 nm period, and a fill factor of 0.6. Wire-grid polarizers with higher transmittance and larger extinction ratio can be obtained by adjusting the AR film thickness, the fill factor, and the thickness of the WSi grating.

Driving-Voltage Reduction of Electrostatically Tunable Infrared Filter

Infrared Fabry–Perot filters were fabricated by stacking two silicon plates etched in KOH solution. When we applied a voltage between the plates, the spacing between them was decreased by an electrostatic force, which caused a shift in interference wavelength. The silicon plates were etched to 34 µm thickness to reduce the driving voltage. When the voltage was increased from 0 to 20 V, the wavelength of the interference peak shifted from 7.9 to 5.5 µm, corresponding to the decrease in the spacing from 7.9 to 5.5 µm. The peak transmittance increased to ~90% by coating an antireflection film on the outer surface of the filter. This coating was effective in suppressing the interference inside the silicon plates, which created a complicated spectrum.

Electrostatically tunable infrared filter that uses etched thin Si plates

We fabricated infrared Fabry-Perot filters by stacking two wet-etched Si plates. When an electric voltage was applied between the plates, the spacing between the plates changed due to an electrostatic force, which caused a shift of interference peaks. The Si plates were etched in a KOH solution to 34-μm thickness in order to reduce the driving voltage. When the voltage was raised from 0 to 20 V, an interference peak shifted from 7.9- to 5.5-μm wavelength, corresponding to the decrease in the spacing from 7.9 to 5.5 μm. The peak transmittance increased to 91% by an antireflection coating on the outer surface of the filter. This coating was effective to suppress the interference inside the Si plates that created a complicated spectrum.