Holographic recording using azo-polymers

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Polymeric materials are the most promising organic materials for electro-optic, and memory devices. In the past several years, direct fabrication of relief structures in azo-polymers has been reported. This method is a one step fabrication technique. A surface relief structure is fabricated by means of irradiation of interference fringes. We propose a new reversible surface relief hologram using photomduced surface deformation on azo-polymer films. The diffraction efficiency of the hologram is dramatically increased by corona charging, and it is controlled by applied voltage of the corona charging.The side-chain azo-polymer, poly orange tom-1 isophoronediisocyanate, is used in this study. Figure 1 shows the chemical structure the material. The glass transition temperature T_g is 136℃. The absorption peak and the cut-off wavelength of the dye are 440 nm and 560 nm, respectively. This polymer is dissolved in cyclohexanone. Samples of l〜5μm thickness are prepared by spin-coating on a slide glass plate. The refractive index of the film is measured as 1.65 at a wavelength of 633 nm by the m-line technique. The surface relief grating is fabricated by the irradiation of two-beam interference fringes.The mechanism of increasing the diffraction efficiency and the relief depth of a surface relief structure by corona charging at temperatures near or above its T_g is not clearly understood, but we consider that the relief depth increases as a result of the Coulomb force exerted by electric charge ; therefore the diffraction efficiency is increased. In the corona charging process, the voltage was applied at a temperature near T_g and the polymer film was heated to the corona charging temperature with the voltage applied, because the surface relief structure is thermally erased above T_g without the applied voltage.