Akira Emoto

Highly stable polarization gratings in photocrosslinkable polymer liquid crystals

Highly stable polarization gratings have been prepared in photocrosslinkable polymer liquid-crystal films by the use of two orthogonally polarized He–Cd laser beams and subsequent annealing. The resulting pure polarization gratings exhibiting thermal stability up to 150 °C diffract the beam and convert the polarization state at the same time according to the theoretical expectation.

Multiplex diffraction from functionalized polymer liquid crystals and polarization conversion.

We report on polymer liquid crystals with periodically oriented mesogenic side chains and demonstrate that the resulting two-dimensional polarization gratings multiplex-diffract the laser beam and convert the polarization state at the same time. Two-dimensional diffraction patterns with various kinds of polarization states can be successfully generated by designing a combination of one-dimensional polarization gratings. This study is a considerable advance towards the realization of highly functionalized passive optical devices that can control both the beam propagation direction and the polarization state.

Self-organized phase gratings in photoreactive polymer liquid crystals

Optical phase gratings have been prepared in a photo-cross-linkable polymer liquid crystal substituted with a 4-methoxycinnamoyloxybiphenyl side group by the use of linearly polarized ultraviolet interference light and subsequent annealing. The diffraction was almost invisible after irradiation, and was successfully enhanced during the subsequent annealing. Recorded phase grating involves both linear birefringence and surface-relief modulation, which is generated by thermally enhanced reorientation of the mesogenic groups and molecular migration. The sign of the induced birefringence was dependent on the degree of photoreaction and the reversion of the reorientation direction of mesogenic groups has been observed.

Polarization holograms in azo dye-doped polymer dissolved liquid crystal composites

We have investigated holographic recordings in azobenzene-containing amorphous polymers (AP) and polymer-dissolved liquid-crystalline composites (PDLCC) with two waves that have orthogonal linear polarizations. Only linear photobirefringence was induced in AP, while both linear and circular photobirefringences were induced in PDLCC. The value of the photoinduced linear and circular birefringences had an effect on the diffraction properties in the polarization holographic gratings. The diffraction properties in AP and PDLCC samples were well explained by theoretical calculation on the basis of the Jones analysis.

Transmission and reflection phase gratings formed in azo-dye-doped chiral nematic liquid crystals

Transmission and reflection gratings were simultaneously formed in azo-dye-doped chiral nematic liquid crystals (N∗LCs) with planar alignment. The formation process is based on a phototuning of the Bragg reflection band of the N∗LC. The helical pitch of the photoreactive N∗LC was spatially controlled with intensity variation of interference light. The resultant periodic structure showed both transmissive and reflective diffractions due to the spatially modulated light intensities. The observed dependence of diffraction efficiencies on the polarization states of the probe beam was well explained by considering a spatial modulation of the helical pitch.

Vector holograms using radially polarized light

We have demonstrated the vector holograms using the radially polarized light. The inhomogeneous polarized light well controlled the spatial distribution of the optical anisotropy in the solid state polymeric materials. The theoretical calculation revealed formation mechanism and optical properties of the vector holograms.

Polarization gratings in twisted-nematic liquid-crystal composites doped with azobenzene dye

Highly efficient and functionalized polarization gratings have been recorded in azobenzene-containing mesogenic composites with twisted nematic cell configurations. The polarization gratings formed in azobenzene-containing mesogenic composites show a high diffraction efficiency of more than 45% and convert the polarization state of light at the same time. The polarization direction of the diffracted laser beams can be controlled by the twisted angle of the nematic cell. These characteristics of the polarization gratings are well explained by means of Jones calculus.

Photorefractive liquid crystal-polymer dispersion with different morphology

The orientational photorefractive properties of the liquid crystal/polymer composites with different morphology are studied experimentally. The composites consist of the similar chemical components, but one shows phase-separated structure (polymer dispersed liquid crystal) and another one, miscible structure (polymer-dissolved liquid crystal composite). Particular attention is given to the observation and qualitative and/or quantitative modeling of the resolution, dependence of the applied dc field, dynamics of grating generation, and photocurrents, which is strongly dependent on the morphology of the composites.

Anisotropic photonic gratings formed in photocross-linkable polymer liquid crystals

We demonstrated anisotropic photonic gratings in photocross-linkable polymer liquid crystals irradiated with three mutually coherent beams. Both birefringence and surface relief modulation were induced by reorientation and/or migration of mesogenic molecules. Anisotropy and optical properties of photonic gratings were strongly dependent on the combination of polarization states of the three beams. Atomic force microscopy revealed that topological patterns of gratings formed according to interference patterns of the three beams and diffraction efficiencies of recorded gratings depended on polarization direction of the linearly polarized probe beam. These diffraction properties including efficiency and optical anisotropy could be explained by the theory based on Jones calculus.

Effects of thermal modulation on diffraction in liquid crystal composite gratings

A microperiodic structure composed of polymer and liquid crystal (LC) phases, called holographic polymer dispersed LC, is fabricated by a photo-induced phase separation technique using LC composites with different physical properties, such as refractive indices and clearing points. Effects of thermal modulation on diffraction properties of LC composite gratings are experimentally investigated in the viewpoints of polarization and temperature dependences. The diffractions based on the change of refractive index induced by the nematic-isotropic transition of LCs with the increase of temperature are applied for a holographic image reconstruction.