Ryushi Fujimura

Analysis of a collinear holographic storage system: introduction of pixel spread function

Image formation in a collinear holographic storage system was analyzed. The wavefront from each pixel of a spatial light modulator was regarded as a plane wave in the recording medium, and its wave vector was determined by the position of the pixel. The hologram in the recording medium was treated as the summation of all gratings written by all combinations of two plane waves. The image of a data page was formed by diffraction of the reference waves by all gratings. The results of the simulation showed good agreement with experiment. We introduced the pixel spread function to describe the image formation characteristics. Analysis of the pixel spread function reveals that a radial-line pixel pattern for reference waves gave a sharper image than other reference pixel patterns. It is also shown that a random phase modulation applied to each reference pixel improved the image formation.

Photorefractive effect and photochromism in the Fe-doped relaxor ferroelectric crystal Pb(Zn1∕3Nb2∕3)O3-PbTiO3

The photorefractive properties of the iron-doped Pb(Zn1∕3Nb2∕3)O3-PbTiO3 (PZN-PT) relaxor ferroelectric single crystals were investigated by two-beam coupling experiments. The Fe:0.01wt%- and the Fe:0.1wt%-doped 0.91PZN-0.09PT single crystals were grown with the flux solution method. The Extinction spectrum, the electro-opric coefficient, the light-induced absorption change, and the photoconductivity were measured in these samples. The maximum gain coefficient reached to 21cm−1, which is about twice as large as that of undoped PZN-PT. The grating formation rate decreases with the Fe concentration.

Photorefractive and photochromic properties of Ru doped Sr0.61Ba0.39Nb2O6 crystal

Abstract We investigate the photorefractive properties of ruthenium doped strontium barium niobate by use of two-wave mixing experiments. Large gain coefficient of 19 cm −1 and fast response time of 0.4 s at 1 W/cm 2 are obtained at a wavelength of 476.5 nm. We estimate photorefractive parameters using one-center model and discuss wavelength dependence of these parameters. Furthermore, this crystal exhibits strong photochromic effect in the wide spectral range by the illumination of UV light. This photochromic effect can be bleached by red-light illumination.

Coherent random laser fluid of nematic liquid crystal emulsions

Reported here is a coherent (resonant) random laser fluid of nematic liquid crystal (LC) emulsion in water. It is a low viscosity fluid like a usual liquid. The optical loop forms among the LC droplets and its length is evaluated to be approximately 150 micrometers. The LC emulsion is useful for the investigation of the mechanism of the random laser because it is a fluid and easy to change the scatterer density.

Polychromatic reconstruction for volume holographic memory

We propose a method of reconstructing an image from a volume hologram at a wavelength different from the recording one. Spectrally broad but spatially coherent light was used as a probe beam. Each angular spectral component of the recorded hologram could be Bragg matched at one particular wavelength within the broadband spectrum. We experimentally demonstrated that a whole image could be reconstructed by using polychromatic light, whereas only a partial image was obtained by using single-mode laser light. We discuss the required bandwidth of the probe beam and the deformation of the reconstructed image.

Low-Power All-Optical Bistable Device of Twisted-Nematic Liquid Crystal Based on Surface Plasmons in a Metal–Insulator–Metal Structure

A low-power all-optical bistable device of twisted-nematic liquid crystal (TN-LC) is reported, on the basis of coupled surface plasmons (SPs) in a metal–insulator–metal (MIM) structure. The lowest threshold switching illumination was 0.3 mW/mm2, which is much lower than the value we previously reported for a similar all-optical TN-LC device based on the coupled SPs in a gold grating. The threshold illumination is lower at higher temperature up to the phase transition. The TN-LC device is promising for two-dimensional optical memories or spatial light modulators, since the structure is simple and free from electronic circuits.

Multiplexing capability in polychromatic reconstruction with selective detection method

Holographic reconstruction with polychromatic light was investigated in terms of the multiplexing capability in a volume holographic memory. The broadband spectrum of the probe beam considerably degrades the angular selectivity and causes a serious deterioration of the multiplexing capability, but this can be overcome if an additional suitable optical component is used in the imaging system. We could selectively detect the signal image alone even though the angular separation between holograms was insufficient to suppress the diffraction from other holograms. We theoretically derived the minimum angular separation for multiplexing and experimentally demonstrated the effectiveness of this selective detection method.

Shift Selectivity of the Collinear Holographic Storage System

A numerical simulations of the signal decay with media shift were performed. Shift selectivity of the page did not depend on the media thickness, but mainly determined by the width of the reference pixel areaWe propose a paraxial solution for pixel shift selectivity, which can simulate the pixel shift selectivity in two dimensions and in wide range easily. Thus, the effect of different reference patterns can be calculated in detail. From the simulation result, we conclude that the pixel shift selectivity get worse for amplitude modulation reference patterns. Making no modulation is the best reference pattern for pixel shift selectivity, however the point spread function will be worst in this case. To get an optimum system in both pixel shift selectivity and point spread function, the reference pattern with phase modulation will be the best choice.

Resonant second-harmonic generation in metal?insulator?metal structure

Optical second-harmonic generation (SHG) in a metal?insulator?metal (MIM) structure is investigated. The experimental results of the second-harmonic (SH) intensity profile as a function of the angle of incidence indicate that the SH light observed from the MIM structure is mainly from bulk nonlinear polarization (quadrupoles or magnetic dipoles) in the metallic layer rather than from the nonlinear electric dipoles at the metallic surface (surface SHG). This is because the SH fields from the metallic surfaces in contact with the insulator layer are canceled and the SH light from the bulk nonlinear polarization (bulk SHG) is dominantly observed. A comparison with the SH intensity from a quartz crystal used as a standard reveals that the susceptibility for the bulk nonlinear polarization is found be in the order of 102 pm/V.

Modeling of semi-shell nanostructures formed by metal deposition on dielectric nanospheres and numerical evaluation of plasmonic properties

A theoretical model of semi-shell structures formed by metal deposition is constructed for accurate prediction of plasmonic properties. Our model takes account of the influence of metal migration after deposition and the granular nature of the metal. Validity of our model was confirmed by the fact that the plasmonic resonances simulated by our model agreed well with those obtained by experiment. Simulations using the discrete dipole approximation revealed that the semi-shells formed by metal deposition show a larger absorption cross-section than semi-shells fabricated by reshaping of fully-covered core–shells. The deposition type semi-shell is suitable for the photothermal therapy because a larger temperature rise is expected due to greater absorption.