Itsunari Yamada

Fabrication of a mid-IR wire-grid polarizer by direct imprinting on chalcogenide glass

A mid-IR wire-grid polarizer with a 500 nm pitch was fabricated on a low toxic chalcogenide glass (Sb-Ge-Sn-S system) by the thermal imprinting of periodic grating followed by the thermal evaporation of Al metal. After imprinting, deposition of Al on the grating at an oblique angle produced a wire-grid polarizer. The fabricated polarizer showed polarization with TM transmittance greater than 60% at 5-9 μm wavelengths and an extinction ratio greater than 20 dB at 3.5-11 μm wavelengths. This polarizer with a high extinction ratio can be fabricated more simply and less expensively than conventional IR polarizers.

Infrared wire-grid polarizer with Y2O3 ceramic substrate.

Using two-beam interference lithography and dry etching, we fabricated a mid-IR wire-grid polarizer consisting of a 350 nm pitch WSi grating on an Y(2)O(3) ceramic substrate, which has wider transparency than sapphire. The transmittance of TM polarization was greater than 70% in the 3-7 μm wavelength range without antireflection films, and the extinction ratio was over 20 dB in the 2.5-5 μm wavelength range. The wire-grid polarizer with the Y(2)O(3) ceramic substrate provides high durability and good IR transparency.

Fabrication of Achromatic Infrared Wave Plate by Direct Imprinting Process on Chalcogenide Glass

An achromatic infrared wave plate was fabricated by forming a subwavelength grating on the chalcogenide glass using direct imprint lithography. A low toxic chalcogenide glass (Sb?Ge?Sn?S system) substrate was imprinted with a grating of 1.63-?m depth, a fill factor of 0.7, and 3-?m period using glassy carbon as a mold at 253 ?C and 3.8 MPa. Phase retardation of the element reached around 30? at 8.5?10.5 ?m wavelengths, and the transmittance exceeded that of a flat substrate over 8 ?m wavelength. Fabrication of the mid-infrared wave plate is thereby less expensive than that of conventional crystalline wave plates.

Reflective waveplate with subwavelength grating structure

A reflective waveplate with subwavelength grating structure of the photoresist was fabricated using two-beam interference technology. From the optical measurement, it is found that the phase retardation of the fabricated reflective element (period: 400 nm, fill factor: 0.5, depth: 280 nm) was almost the same as that of the transmission waveplate with the around twice deeper grating depth (period: 400 nm, fill factor: 0.5, depth: 450 nm). This is because the optical length of the reflective element was twice of the transmissive one by using reflection. By changing the period and depth to 285 nm and to 300 nm, respectively, it is also confirmed from the experimental result that the phase retardation of the reflective waveplate exceeded 150° for 450 nm wavelength at the incident angle of 45°.

Design and fabrication of an achromatic infrared wave plate with Sb–Ge–Sn–S system chalcogenide glass

We designed and fabricated an achromatic infrared wave plate. To examine its phase retardation characteristics, the birefringence was calculated using the effective medium theory. A wave plate with a subwavelength grating was fabricated by direct imprint lithography on a low toxic chalcogenide glass (Sb-Ge-Sn-S system) based on calculated results. As a result of imprinting onto chalcogenide glass by a glassy carbon mold, a grating with 1.63 μm depth, a fill factor of 0.7, and a 3 μm period was obtained. The phase retardation of the elements reached around 30° in the 8.5-10.5 μm wavelength range. The fabrication of the infrared wave plate is less costly compared with conventional crystalline wave plates.

Fabrication of infrared wire-grid polarizer by sol–gel method and soft imprint lithography

An infrared wire-grid polarizer was fabricated by forming a subwavelength grating of zirconia by imprinting techniques, a sol–gel method, and Al shadow thermal evaporation. After imprinting on a dropped zirconia nanoparticle dispersion using a silicone mold, 100-nm-thick Al was deposited on the fabricated zirconia grating with 400 nm period by evaporation. The extinction ratio of the fabricated element was 27.5 dB at 5.4 µm wavelength. The TM polarization transmittance was higher than that of the Si plate in the 4.2–5.3 µm wavelength range because the zirconia film acted as an antireflection film.

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.

Infrared Polarizer Fabrication by Imprinting on Sb--Ge--Sn--S Chalcogenide Glass

We fabricated infrared wire-grid polarizers consisting of a 500-nm pitch Al grating on a low toxic chalcogenide glass (Sb–Ge–Sn–S system) using the direct imprinting of subwavelength grating followed by a deposition of Al metal by thermal evaporation. To fabricate the subwavelength grating on a chalcogenide glass more easily, the sharp grating was formed on the mold surface. The fabricated polarizer with Al thickness of 130 nm exhibited a polarization function with a transverse magnetic transmittance greater than 60% in the 5–9 µm wavelength range, and an extinction ratio greater than 20 dB in 3.5–11 µm wavelength range. The extinction ratio of the element with Al wires of 180-nm thickness reached 27 dB at 5.4-µm wavelength. The polarizer can be fabricated at lower costs and simpler fabrication processes compared to conventional infrared polarizers.

Near-Infrared Polarizer with Tungsten Silicide Wire Grids

We fabricated a near-infrared wire-grid polarizer consisting of a 230-nm-pitch tungsten silicide (WSi) grating on a SiO2 substrate using two-beam interference lithography and dry etching. The transverse magnetic (TM) polarization transmittance of the fabricated polarizer exceeded 80% in the 1000–1600-nm wavelength range. The extinction ratio was higher than 20 dB in the 650–1500-nm wavelength range. We also measured the extinction coefficient κ of WSi and verified that WSi is a suitable polarizing material in the near-infrared range.

Retardation of sol–gel titanium oxide with imprinted grating structure

Abstract. We fabricated a titanium oxide grating using an imprinting technique and a sol–gel method and evaluated phase retardation. A titanium oxide grating with 75 nm depth, fill factor of 0.3, and 400 nm period was obtained by imprinting a titanium oxide sol solution on a silicone (polydimethylsiloxane; PDMS) mold at 150°C. As a result of phase retardation evaluation, it reached 9.2 deg at the 532-nm wavelength.