Single-step Photo fabrication of Kinoforms in use of Azobenzene-containing Polymer Films
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Single-step Photofabrication of Kinoforms in use of Azobenzene-containing Polymer Films Shin Yasuda, Jiro Minabe, Katsunori Kawano, Tatsuya Maruyama, and Hidenori Yamada* Corporate Research Center *New Business Center, Fuji Xerox Company, Ltd., 430 Sakai, Nakai-machi, Ashigarakami-gun, Kanagawa 259-0157, Japan ABSTRACT We propose a new method of optically fabricating kinoforms using polyester containing cyanoazobenzene units in the side chain. This method utilizes the surface relief structures induced optically on the azopolymer films. Using a gray-tone amplitude mask with black and white lines as a spatial light modulator, we constructed the surface relief structures on the azopolymer films with an argon-ion laser beam (488nm). The relief depths constructed were deep enough to provide a wide range of visible light with a phase difference of 2π. In addition, irradiation on the azopolymer film with a spatially modulated light that had multilevel intensities inscribed the multilevel relief structure reflecting the intensity modulation. This property is applicable to the fabrication of diffractive optical elements, especially kinoforms, because the multilevel relief structure provides an incident light with phase modulation. We designed a phase pattern for a kinoform by computation and expressed it as a gray-tone amplitude mask. Irradiation on the film through the gray-tone mask with the pump beam inscribed a multilevel relief structure that diffracted an incident light to produce a desired image. The desired image appeared without the effect of the internal refractive index modulation of the film. We, therefore, confirmed a possibility of fabricating a kinoform with a multilevel relief structure on the azopolymer film. Since our fabrication method for kinoforms requires only exposure, it has a significant advantage over other techniques that require additional processes such as developing and etching. INTRODUCTION The kinoform is a type of diffractive optical element designed by computation, which modulates only the phase of incident light; it transforms a known wavefront into the wavefront needed to form a desired image [1]. For this transform, the kinoform is required to provide a maximum phase difference of 2π with the incident light. The kinoform also requires the phase-controlling surface to vary smoothly. For the fabrication, binary lithography involving a series of processes such as photoresist coating, exposure, developing, etching and photoresist removal is a powerful method [2,3]. Arbitrary structures composed of 2N levels in height are fabricated by N iterations of these processing steps with N masks. On the other hand, it has been reported that surface relief gratings (SRGs) can be induced optically on azobenzene-containing polymer films [4,5]. Formation of the SRGs is highly relevant to the trans-cis photoisomerization of azobenzene. This phenomenon involves unusual large-scale photo-driven mass transport process at a temperature much below the glass transition temperature of the azopolymer. Alth
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