Nobuhito Takeshima

Arbitrary micropatterning method in femtosecond laser microprocessing using diffractive optical elements

We successfully developed an arbitrary micro-patterning method with femtosecond pulses using a multi-level phase type diffractive optical element (DOE) and a focusing objective lens. The large chromatic dispersion effects of DOE resulting from the spectral bandwidth of femtosecond pulses can be reduced with the appropriate DOE focal length and the proper distance between the DOE and the focusing lens. The method was verified through optical and processing experiments. A partial periodic structure was formed at the designated position. Microstructures were precisely formed on the SiO2 glass surface and inside the glass by irradiating the constructed beam. The points were evenly dispersed with a separation of 5 mum.

Fabrication of high-efficiency diffraction gratings in glass

We investigated a microfabrication process for optical gratings with periods of micrometer order that use ultrafast laser pulses in semiconductor-doped glass. ZnS- or PbS-doped SiO2-A12O3-B2O3-CaO-ZnO-Na2O-K2O glass was prepared by a melting method. Glass transmission diffraction gratings with a high refractive-index difference were fabricated with femtosecond laser pulses. The first-order diffraction efficiencies of these gratings were approximately 80%, and the first-order diffraction angles of these gratings were 8 degrees at telecommunication wavelengths.

Fabrication of a periodic structure with a high refractive-index difference by femtosecond laser pulses

A microfabrication process using ultrafast laser pulses in glass was investigated. We investigated the formation of semiconductors by the irradiation of glasses with femtosecond laser pulses. ZnS- or PbS-doped SiO(2)-Al2O(3)-B(2)O(3)-CaO-ZnO-Na(2)O-K(2)O glasses were prepared by a melting method and irradiated by femtosecond laser pulses. Periodic structures in the sample glasses with a high refractive index difference were produced by femtosecond laser pulses. The maximum relative refractive index difference between the irradiated area and the nonirradiated areas was 20%. Diffraction gratings were also fabricated inside the ZnS- or PbS-doped silicate glasses. The diffraction efficiency of these gratings was approximately 90% in the infrared region.

Fabrication of photonic crystals in ZnS-doped glass

Three-dimensional photonic crystals were fabricated in a ZnS-doped silicate glass by use of a femtosecond pulsed laser. Woodpile structures consisting of 36 layers were produced by focusing with a 100x (1.35-numerical-aperture) objective. Attenuations of approximately 3-8 dB arising from a photonic bandgap were observed in the visible and the near-infrared regions.

3D patterning method in femtosecond laser microprocessing using diffractive optical elements

We demonstrated a simple chromatic dispersion reduction method of 3-dimensional (3D) patterning of femtosecond pulses using a multi-level phase type diffractive optical element (DOE) and a focusing objective lens. Our method increases flexibility of femtosecond laser microprocessing. With appropriate focal length of the DOE and distance between the DOE and the focusing lens, large chromatic dispersion of the DOE resulting from spectral bandwidth of a femtosecond pulse can be reduced, and 3D focusing pattern of femtosecond pulse can be obtained not only controlled in focal plane but also in focal depth. The method was verified through optical and processing experiments with laser pulses of 400 fs duration and of 40 nm bandwidth. The focal length of the DOE and the objective lens was 1600 mm and 10 mm, respectively. Partially periodical structure of focusing points was formed at designed position and its focal depth were much smaller than that focused with only the DOE. By irradiating the constructed beam, microstructure was formed precisely inside SiO2 glass. The processed points are clearly separated each other with a separation of 5 mm and the spot sizes were almost same as those irradiated without the DOE.