Kenji Kintaka

Ultrafast nonlinear effects in hydrogenated amorphous silicon wire waveguide

We, for the first time, present the ultrafast optical nonlinear response of a hydrogenated amorphous silicon (a-Si:H) wire waveguide using femtosecond pulses. We show cross-phase and cross-absorption modulations measured using the heterodyne pump-probe method and estimate the optical Kerr coefficient and two-photon absorption coefficient for the amorphous silicon waveguide. The pumping energy of 0.8 eV is slightly lower than that required to achieve two-photon excitation at the band gap of a-Si:H (approximately 1.7 eV). An ultrafast response of less than 100 fs is observed, which indicates that the free-carrier effect is suppressed by the localized states in the band gap.

Optical microscopic observation of fluorescence enhanced by grating-coupled surface plasmon resonance

On the substrate carrying a sub-wavelength grating covered with a thin metal layer, a fluorescent dye-labeled cell was observed by fluorescence microscope. The fluorescence intensity was more than 20 times greater than that on an optically flat glass substrate. Such a great fluorescence enhancement from labeled cells bound to the grating substrate was due to the excitation by grating coupled surface plasmon resonance. The application of a grating substrate to two-dimensional detection and fluorescence microscopy appears to offer a promising method of taking highly sensitive fluorescence images.

Cavity-resonator-integrated guided-mode resonance filter for aperture miniaturization.

A guided-mode resonance filter integrated in a waveguide cavity resonator constructed by two distributed Bragg reflectors is designed and fabricated for miniaturization of aperture size. Reflection efficiency of >90% and wavelength selectivity of 0.4 nm are predicted in the designed SiO(2)-based filter with 50-μm aperture by a numerical calculation using the finite-difference time-domain method. A maximum reflectance of 67% with 0.5-nm bandwidth is experimentally demonstrated by the fabricated device at around 850-nm wavelength.

Antireflection microstructures fabricated upon fluorine-doped SiO 2 films

Two-dimensional periodic structures were fabricated upon a fluorine-doped SiO(2) film in which the fluorine content changed gradually in the direction of film thickness. The films were deposited by plasma-enhanced chemical-vapor deposition. The film was periodically patterned into a 1-mum period and an ~1-mum -groove depth by inductive coupled plasma reactive-ion etching followed by chemical etching in a diluted HF solution. A surface reflectance of 0.7% was attained at 1.85-mum wavelength, a value that is one fifth as large as the 3.5% Fresnel reflection of a SiO(2) substrate with a flat surface.

Aperture Miniaturization of Guided-Mode Resonance Filter by Cavity Resonator Integration

A small-aperture wavelength filter consisting of a guided-mode resonance grating and a cavity resonator integrated in a SiO2-based waveguide was designed. The operation principle was experimentally confirmed at around 1540-nm wavelength for the first time.

Volume Grating Induced by a Self-Trapped Long Filament of Femtosecond Laser Pulses in Silica Glass

A self-trapped filament of ultrashort laser pulses can induce a 10–500 µm-long region of refractive-index change in silica glass. The translation of filament perpendicular to the optical axis induces a layer of refractive-index change with a thickness of 2 µm. We stacked the layers periodically and fabricated volume gratings. We irradiated a He–Ne laser beam at the wavelength of 632.8 nm onto the gratings at the Bragg angle. The maximum diffraction efficiency was 74.8% with the grating that had the period of 3 µm and the thickness of 150 µm.

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.

A guided-mode-selective focusing grating coupler

Guided-mode-selective focusing grating coupler, which couples out not TE/sub 0/ mode but TE/sub 1/ mode to a free space wave, was designed at around 850-nm wavelength for constructing a wavelength-division-multiplexing chip-to-chip optical interconnection board. The coupler was fabricated in a slab waveguide consisting of a Ge-SiO/sub 2/ main-guiding core, a SiO/sub 2/ subguiding core, a Si-N grating layer, and a high-reflection layer. Coupling efficiencies for TE/sub 0/ and TE/sub 1/ modes were obtained to be 5% and 46%, respectively, with 0.1-mm interaction length. The focus spot size of 3/spl times/1 /spl mu/m was obtained for TE/sub 1/ mode.

Surface-relief gratings with high spatial frequency fabricated using direct glass imprinting process

Surface-relief gratings with high spatial frequencies were first fabricated using a direct imprinting process with a glassy carbon mold at the softening temperature of phosphate glass. A grating with maximum height of 730 nm and 500 nm period was formed on the glass surface by the pressing at the softening temperature of glass under constant pressure of 0.4 kN/cm(2). Phase retardation of 0.1 lambda was observed between TE-polarized and TM-polarized light at 600 nm wavelength.

Design of resonance grating coupler

A new integrated-optic coupler was proposed for coupling a guided wave to a free-space wave propagating vertically from the waveguide surface. The coupler consists of a grating coupler in a cavity formed by two distributed Bragg reflectors, and has a small aperture. The cavity was designed to eliminate both transmission and reflection of the incident guided wave, resulting in 100% radiation by a several-micron aperture. Design consideration was theoretically discussed based on the coupled mode analysis. Predicted performance was simulated and confirmed by the finite difference time domain method.