Kunihiko Okano

Highly birefringent liquid-crystalline polymers for photonic applications: synthesis of liquid-crystalline polymers with side-chain azo-tolane mesogens and their holographic properties

We have synthesized highly birefringent liquid-crystalline polymers (LCPs) containing an azobenzene group directly connected to a tolane moiety (P3ATm), in which the alkyl spacer length (CH2)m was varied as m n = 6, 9 and 12. All LCPs showed a nematic phase in a broad temperature range (>170 °C). The homogeneously aligned LCP films showed a high value of birefringence (about 0.4) and a large change in the birefringence could be induced by photoinduced alignment change. Grating formation of LCPs has been explored in a glassy state. The diffraction efficiency (η) showed a maximum when an order–disorder change was completed in the bright areas of the interference patterns. Studies on the effects of light intensity on the grating formation suggested that the diffraction efficiency was high (η n = 20%) even though the alignment change of the azo-tolane moieties was small. Furthermore, the behavior of the grating formation was found to be dependent on the alkyl spacer length of P3ATm.

Photoresponsive behavior and photochemical phase transition of amphiphilic diblock liquid-crystalline copolymer

ABSTRACT The photoresponsive behavior of an amphiphilic diblock liquid-crystalline (LC) copolymer (P40A37) was studied in both toluene solution and spin-coated film. The LC copolymer in a film state showed slower response and relaxation than that in the solution. Significant H-aggregation in the film led to a blue shift by 16 nm at the maximum absorption. The photochemical phase transition was observed with a polarizing optical microscope (POM) upon exposure to 366-nm light. The photoinduced isotropic state remained stable at room temperature, indicating the potential application for 2-D and 3-D storage.

HIGHLY EFFICIENT HOLOGRAPHIC MATERIALS BASED ON POLYMER LIQUID CRYSTALS

Formation of gratings by means of the photoinduced alignment change of polymer liquid crystals (PLCs) has been explored in a glassy state. To obtain high diffraction efficiency (η), we synthesized highly birefringent PLCs containing an azobenzene group directly connected with a tolane moiety (3ATm), in which the alkyl spacer length (CH2)m was varied as m=6, 9 and 12. It was found that the values of birefringence in all the PLCs were about 0.4. The behaviors of the photoinduced alignment change and the formation of gratings were found to be dependent on the alkyl spacer length of 3ATm.

Synthesis and Properties of Highly Birefringent Azo-Tolane Liquid-Crystalline Polymers: Effect of the Position of the Tolane Moiety in the Side Chain

ABSTRACT We have synthesized two types of azobenzene liquid-crystalline polymers with a tolane moiety exhibiting high birefringence: one is the copolymer having an azobenzene moiety and a 3-ring tolane group in the side chain (PA-3T) and the other is the polymer containing a long mesogen core, in which the azobenzene group is directly connected to the tolane moiety (P3AT). The values of 0.35 and 0.39 in birefringence were obtained in PA-3T and P3AT, respectively. When the film was irradiated with a writing beam at 488 nm, the change in birefringence was efficiently induced with P3AT compared with PA-3T.

Synthesis and Photoresponsive Behavior of a Liquid-Crystalline Polymer with a 4-Ring Azotolane Moiety with Huge Birefringence

A novel liquid-crystalline polymer (LCP) containing an azobenzene group directly connected to a tolane moiety (azotolane moiety) was synthesized. The LCP showed a nematic phase in a broad temperature range (> 200°C). Trans-cis photoisomerization and cis-trans thermal isomerization occurred even though the LCP contains a long azotolane moiety. The homogeneously aligned film of the LCP exhibited an extremely large value of Δn (0.70 at 633 nm). Upon irradiation with a pumping beam in the LC phase, a large change in Δn was induced due to destruction of the homogeneous alignment of the azotolane moieties.

Highly efficient formation of holographic gratings in polymer liquid crystals

We studied the formation of gratings by means of the photoinduced alignment change of polymer liquid crystals (PLCs) in a glassy state. To obtain high diffraction efficiency (η), we prepared a novel PLC containing an azobenzene group directly connected with a tolane moiety (3AT) which has large birefringence (Δn). It was found that the value of birefringence in 3AT was extremely high (Δn = 0.39). When two s-polarized writing beams (Ar+ laser, 488 nm) were interfered on the surface of the 3AT film in the glassy state, multiple diffraction beams were observed immediately, resulting from the grating formation. We explored the effect of light intensity on the grating formation and the relation between the alignment change and η. The value of η showed a maximum when the change in photoinduced alignment was almost completed. Furthermore the value of η was relatively high (η = 20%) even though the alignment change of mesogens was small.

Photoresponsive behavior of azotolane liquid crystals

In this study, we irradiated homogeneously aligned films of an azotolane liquid-crystalline polymer with a single femtosecond (fs) laser pulse. Upon irradiation of the single pulse, the change in ▵n was induced by destruction of due to destroy the homogeneous alignment of the azotolane moieties was induced. The estimated value of the change in ▵n was >0.2, and the response time was about 400 ns. When the experiment temperature was below glass transition temperature, the change in alignment induced by the pulsed irradiation was preserved without a thermal relaxation. On the other hand, the changed alignment completely recovered to the initial state with an increase in temperature.