Hidekazu Ishitobi

Use of two-photon absorption in a photorefractive crystal for three-dimensional optical memory

We describe the use of two-photon absorption in a photorefractive crystal for recording bit data in multilayered optical memory. A short-pulse near-infrared laser is used for generating the photorefractive effect by two-photon absorption. We succeeded in recording and reading seven layers of data in a LiNbO(3) crystal with a lateral resolution (distance between bits) of 5microm and an axial resolution (distance between layers) of 20 microm .

Polarization storage by nonlinear orientational hole burning in azo dye-containing polymer films

We present experimental evidence of polarization storage by nonlinear orientational hole burning in films of poly(methylmethacrylate) containing disperse red one (DR1). The data are written by polarized nonlinear excitation of DR1 by a tightly focused Ti: Sapphire laser and read by polarized confocal laser scanning microscopy in the reflection mode. The bits that are imaged with a light polarization which is parallel to the excitation laser polarization reflect light ∼2.5 times more efficiently than those which are imaged with a light polarization which is perpendicular to it. This finding is consistent with orientational hole burning of chromophores by two-photon excitation.

Controlling the plasmon resonance wavelength in metal-coated probe using refractive index modification.

We present a novel technique to tune the plasmon resonance of metal-coated silicon tips in the whole visible region without altering the tips original sharpness. The technique involves modification of the refractive index of silicon probe by thermal oxidization. Lowering the refractive index of silicon tip coated with metal shift the PRW of the metallic layer to shorter wavelength. Numerical simulation using FDTD agrees well with the empirical results. This novel technique is very useful in tip-enhanced Raman spectroscopy studies of various materials because plasmon resonance can tuned to a specific Raman excitation wavelength.

Two-photon isomerization and orientation of photoisomers in thin films of polymer

Abstract We present experimental evidence of two-photon isomerization and orientation of a diarylethene derivative in films of poly-methyl-methacrylate. The changes of the linear absorption spectrum of the photochromic sample before and after infra-red, high power, pulsed, excitation are consistent with chromophores isomerization induced by two-photon absorption. The slopes of the real-time dynamics of two-photon isomerization showed the square law dependence on the pump light intensity. Two-photon molecular orientation was induced in the polymer samples by polarized infra-red excitation, and it was observed by dichroic absorption on the entire ultraviolet–visible spectrum of the isomers. Molecular reorientation by photoisomerization is inferred as a possible mechanism of the observed two-photon orientation.

Photo-orientation by multiphoton photoselection

We present evidence of photoselection by polarized multiphoton absorption (MPA) in an azo dye, i.e., Disperse Red 1 (DR1), dispersed in films of poly(methyl-methacrylate). MPA of DR1 at a fundamental of 785 nm, from a pulsed, regeneratively amplified, Ti:sapphire laser, results in photobleaching of the dye, and linearly polarized MPA of DR1 creates anisotropy, a feature that is indicative of orientational hole burning by MPA. The slopes of the early-time evolution of the isotropic absorbance and anisotropy show a dependence on the 3.65±0.15 power of the excitation laser intensity, and the dynamics of MPA-induced orientation are described by a theoretical model for MPA selective bleaching.

The anisotropic nanomovement of azo-polymers

Nanoscale polymer movement is induced by a tightly focused laser beam in an azo-polymer film just at the diffraction limit of light. The deformation pattern that is produced by photoisomerization of the azo dye is strongly dependent on the incident laser polarization and the longitudinal focus position of the laser beam along the optical axis. The anisotropic photo-fluidity of the polymer film and the optical gradient force played important roles in the light induced polymer movement. We also explored the limits of the size of the photo-induced deformation, and we found that the deformation depends on the laser intensity and the exposure time. The smallest deformation size achieved was 200 nm in full width of half maximum; a value which is nearly equal to the size of the diffraction limited laser spot.

Nanomovement of azo polymers induced by metal tip enhanced near-field irradiation

Nanomovement of azo polymers induced by metal tip enhanced near-field illumination was studied. A protrusion with 47 nm full width at half maximum was induced with a resolution beyond the diffraction limit. At the top of the protrusion, an anisotropic movement occurs in a direction nearly parallel to the polarization of the incident light, and suggests the existence at the tip end of not only a longitudinal but also a lateral component of the electric field of light. The anisotropic photofluidity and the optical gradient force played important roles in the process of the light induced polymer movement.

Ordering of azobenzenes by two-photon isomerization

We report on light induced orientation by two-photon isomerization of azobenzenes in films of polymer. The dynamics of isomerization and orientation by one-photon absorption and two-photon absorption (TPA) are similar, and TPA creates a degree of molecular orientation which is comparable to that achieved by single-photon isomerization, in agreement with the theoretical predictions of two-photon isomeric orientation.

Orientational Imaging of Single Molecules by Using Azimuthal and Radial Polarizations

Three-dimensional molecular orientations of single fluorescence molecules in polymeric thin films were measured by focused azimuthally and radially polarized light, in which we found that the fluorescence intensity was dependent on the depth position of the molecule with respect to the film surface. We found that the fluorescence intensity for a molecule which is 80 nm deep in the film excited by radial polarization is appreciably larger when compared with the fluorescence intensity for a molecule which is also excited by radial polarization but which is closer to the polymer/air interface, a feature which leads to different fluorescence intensities, under excitation by radial polarization, for molecules with the same polar orientation but with different depths inside the film. We also found that the variation of fluorescence intensity from a molecule inside an 80 nm film in radial polarization is appreciably larger compared with one in azimuthal polarization. These findings were confirmed by comparing experiments using different thickness films with theoretically calculated electric field distributions.

Two-photon induced polymer nanomovement

We present the first report of two-photon induced plastic surface deformation in solid polymer films. Exposure of azo polymer films, which absorb in the visible range (lambda(max) = 480 nm), to intense 920 nm irradiation leads to polarization dependent photofluidic polymer nanomovement caused by photoselective two-photon trans <-->cis isomerization. The deformations were induced by a gradient of light intensity; and strongly depend on the wavelength and the polarization direction of the incident laser light and the position of the focused spot with respect to the plane of the polymer film.