Rewritable holograms based on azobenzene-containing liquid-crystalline polymers

Abstract

The design and preparation of azobenzene-containing polymers showing a large photoinduced change in refractive index, and their applications to rewritable Raman-Nath and Bragg holograms are reviewed. Optically transparent films with a thickness of 4200 lm have been prepared with random copolymers, which contain photoresponsive and amplifying components in the side chain. Various azobenzene chromophores are used as the photoresponsive component. Cyanobiphenyl and tolane moieties are incorporated as the amplifying component, which enhances the change in refractive index triggered by the azobenzene. When two writing beams are made to overlap in the film, a periodic change in refractive index is induced, and an incident probe beam is diffracted at B100% diffraction efficiency. The recorded holograms can be erased by thermal treatment or by photoirradiation with a single writing beam. Angular multiplicity is discussed from the viewpoint of high-density information storage. Keywords: azobenzene; grating; hologram; liquid crystal; refractive index INTRODUCTION Holograms, in which we can observe natural three-dimensional (3D) images without eyeglasses, have been of great interest from the viewpoint of 3D television. [1][2][3] In addition, other photonic applications such as holographic memory, optical switch and waveguide have been proposed. [3][4][5][6][7][8][9] The holograms can be categorized into two types: (1) Raman-Nath type and (2) Bragg type. The Raman-Nath hologram recorded in a thin film causes multiple diffraction of an incident beam, leading to low diffraction efficiency (o34%). On the other hand, the Bragg hologram shows a single diffraction, which enables 100% diffraction efficiency. Furthermore, angular multiplicity can be used in the Bragg hologram. Therefore, thick films are suitable for holograms with high diffraction efficiency. The diffraction efficiency also depends on a photoinduced change in refractive index of the film. If materials in which a large change in refractive index can be induced by light can be prepared, the performances parameters such as diffraction efficiency, response and multiplicity of the holograms can be enhanced. How can we induce the change in refractive index efficiently? In an isotropic material, the change in refractive index by photoirradiation is rather small and the efficiency is not high. In addition, the material is theoretically polarization inactive. On the other hand, an anisotropic material could show birefringence. By controlling their molecular alignment by photoirradiation, a large change in refractive index and high diffraction efficiency can be obtained. Furthermore, an anisotropic material shows polarization activity. Therefore, we can expect the fabrication of high-performance holograms by photoinduced alignment of anisotropic molecules.

Volume Holography: Novel Materials, Methods and Applications

“…A Bragg hologram shows single diffraction, enabling high diffraction efficiency and good selectivity. While Raman-Nath holograms may be recorded in a thin film, thick films are required to obtain Bragg holograms with good optical functionality [24]. → are displayed.…”

New strategies and implications for the photoalignment of liquid crystalline polymers

“…Photochromic reactions have frequently been incorporated in LC media because of their repeatability for controlling material properties. This approach has provided various types of smart, light-responsive materials 1 exhibiting surface-mediated photoalignment of LC materials, [2][3][4][5][6] photoinduced phase transitions, [7][8][9][10][11][12] photoorientation/addressing of polymer thin films, [13][14][15][16][17][18] photoinduced mass migrations, [19][20][21][22][23][24][25][26][27][28] phototactic sliding motions, [29][30][31] photo-driven motions and morphology of monolayers, [32][33][34][35] and macroscopic photomechanical deformations. [36][37][38][39][40][41][42][43][44][45][46] Photoalignment research and technology started in 1988 with the discovery of the reversible alignment control of nematic LCs by the photoisomerization of azobenzene on a substra...…”

Effect of surface treatment on molecular reorientation of polymer-stabilized liquid crystals doped with oligothiophene

“…In addition, nematic LCs that have high optical nonlinearity reorient the molecular director at lower light intensity compared with conventional materials. 23 To date, applications such as data storage, 24 efficient optical switching 25 and light beam modulation 26 have been reported. This process, however, still requires relatively higher-power polarized light sources compared with photochemical processes.…”