Nobuhiro Kawatsuki

Orientational photorefractive effects observed in polymer-dispersed liquid crystals

We present a novel type of photorefractive polymer-dispersed liquid crystal (PDLC) that contains a polymer, low-molecular liquid crystals (LCs), and a photoconductive sensitizer. Photorefractivity is demonstrated by two-beam-coupling experiments with an external dc field. This material shows highly sensitive photorefractivity, and a purely refractive-index grating (Dn >/= 3.0 x 10(-3)) is generated by irradiation of interference light (lambda=632.8 nm) . The grating in the photorefractive PDLCs shows strong anisotropy, and the dynamics depends strongly on the composition. In the case of a 40:60 weight ratio of polymer:LCs, the photorefractive PDLCs present a stable memory effect.

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.

Orientational holographic grating observed in liquid crystals sandwiched with photoconductive polymer films

We presented a novel type of orientational photorefractive device, consisting of liquid crystals (LCs) sandwiched with photoconductive polymer films across the insulated polymer films. The photoexcited charges were generated in the photoconductive layers and eventually become trapped in the dark areas at the insulated polymer surfaces. The resulting space charge field reoriented the LC molecules and the holographic gratings were generated. The strong beam coupling effects were observed and the refractive index modulation Δn was estimated to be of the order of 3.6×10−3.

Photorefractive Bragg diffraction in high- and low-molar-mass liquid crystal mixtures

Orientational photorefractive Bragg diffraction is observed in high- and low-molar-mass liquid crystal mixtures doped with fullerene (C60). These novel materials are in a nematic phase without phase separating. In the functionalized materials, we observe a high two-beam coupling gain coefficient (Γ=75 cm−1) with a low applied field of 4 V/μm, low total losses (32 cm−1 including scattering, reflection, and absorption), a high diffraction efficiency (9%), and response time of 200 ms.

High-performance photorefractivity in high- and low-molar-mass liquid crystal mixtures

High-performance photorefractivity in a thick-grating regime was observed in high- (H-LC) and low-molar-mass liquid crystal (L-LC) mixtures (HL-LCMs) doped with fullerene. The photorefractive performance strongly depended on the concentration ratio of H-LC and L-LC. A high net gain coefficient over 600 cm−1 was achieved with low applied dc fields (0.7 V/μm) and a fringe spacing of 2.8 μm. The gain coefficients and the resolution increased with increasing the concentration of the H-LCs. The photorefractive grating formation time was varied between 0.12 and 3.5 s, which depended on the concentration of the H-LCs.

Photoinduced alignment control of photoreactive side-chain polymer liquid crystal by linearly polarized ultraviolet light

Alignment control of mesogenic groups in a photocrosslinkable polymer liquid crystalline film was achieved by linearly polarized (LP) ultraviolet (UV) irradiation followed by heating. The alignment direction was parallel to the electric vector of the incident LPUV light and the tilt angle of the mesogenic groups could be controlled by a one-step slantwise LPUV irradiation. The photoinduced birefringence dn was 0.07 and the order parameter of the mesogenic group was 0.28.

Optical anisotropy of a photoreacted side-chain liquid-crystalline polymer induced by Linearly polarized UV light

Linearly polarized (LP) UV photoreaction of a photo-crosslinkable side-chain liquid-crystalline polymer (SLCP) containing photoreactive cinnamoyl and biphenyl mesogenic groups (1) was studied. The optical anisotropy of the polymer film was induced by the LP-UV photoreaction and was investigated by the temperature-controlled polarized UV absorption spectroscopy and polarized FT-IR measurements. The reorientation of the nonreacted mesogenic groups along to the E direction of the incident LP-UV light during the photoreaction occurred at the LC temperature range of the polymer, and the induced birefringence Δn was about 0.02. Because of the high-density photo-crosslinking, the LP-UV photoreacted film showed orientational stability up to 160°C.

Conformational polymorphism and optical properties in the solid state of 1,4,7,10-tetra(n-butyl)tetracene

Two conformational polymorphs of 1,4,7,10-tetra(n-butyl)tetracene showed different characteristics of molecular structure, packing arrangement and optical properties in the solid state.

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.

Orientational photorefractive gratings observed in polymer dispersed liquid crystals doped with fullerene

We presented orientational photorefractive materials based on polymer dispersed liquid crystals (PDLCs) doped with fullerene ( C60) as photoconductive sensitizer and discussed the grating formation mechanism originating in the space charge field and resulting in reorientation of liquid crystal molecules. A large refractive index modulation of the order of 10-3 was obtained in the Raman-Nath region for orientational photorefractive PDLCs. A stable memory effect of the reorientation was obtained for a 40 wt/60 wt (polymer/liquid crystal) sample.