High-performance liquid crystals for next-generation in formation technologies

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Holography, which enables optical storage of three-dimension images, has been of great interest for photonic applications. To obtain clear images, Bragg hologram that theoretically achieves 1O0-% diffraction efficiency is required. In the Bragg hologram, materials should be optically sensitive, meaning that refractive index is efficiently changed by light. Liquid crystals, showing large optical anisotropy and fluidity, can be a good candidate for holographic materials since a large change in refractive index can be easily induced by external fields. There is another important characteristic in the hologram : reversibility of recording. Commercially available materials are sensitive enough, while images cannot be erased, which limits the application to photonic devices. Azobenzene is one of the outstanding photochromic molecules that are capable of changing absorption, molecular shape, and refractive index reversibly upon photoirradiation. Consequently, polymer liquid crystals incorpora … More ted with an azobenzene moiety are naturally expected to be reversible holographic materials.In this study, two types of hologram, transmission-type and the Lippmann-type, were investigated using polymer azobenzene liquid crystal films. A linearly polarized beam from Ar^+ laser at 488 nm was split to two beams and interfered in the film with a crossing angle of 14゜ in transmission type and 180゜ in the Lippmann-type, respectively. Formation behavior of the transmission-type grating was probcd with a linearly polarized beam from He-Ne laser at 633 nm. The Lippmann-type grating was evaluated with transmission spectrum. In the transmission type, diffraction of the probe beam appeared upon irradiation of writing beams at 488 nm. Diffraction efficiency (η), that is defined as the ratio of diffraction intensity during irradiation to transmission intensity before irradiation, increased to 14% on photoirradiation. On the other hand, in the Lippman-type grating, the value of q for 500-nm light reached 43% when the film was irradiated at 350 mW/cm^2 for 60 min. Less