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Diffractive optical elements (DOE) and holograms based on diffractive optics use diffracted light generated by the interaction between light and the periodic structures such as gratings. For the fabrication of these structures, the researchers have widely used the materials that can modulate the refractive index. Recently, the photochromic materials have attracted a surge of interests for reversible holograms or DOE that can be controlled by light or heat. Especially, azobenzene molecules, which can undergo photo-isomerization under the specific wavelength of the light, are widely used for fabrication of surface relief gratings (SRGs). When the interference light is illuminated, azobenzene-containing polymer (azopolymer) films can form the SRGs via mass migration rather than lithographic etching. More strikingly, these already recorded SRGs as depending on mass migration, can be further inscribed via second mass migration, which can be also guided via another interference light. However, it has been accepted that this overwriting of SRGs needs to be performed after the erasing process of the already recorded SRGs. This limitation has in turn significantly restricted the hierarchical integration of the various SRGs with different structural features. In this work, we demonstrate that overwriting of SRGs on azopolymeric film can be possible without erasing of the previously recorded SRGs. By using this method, we can greatly advance not only the versatility of the process but also the accessible design of the SRGs and the resultant range of DOE-based light molding. In particularly, the pixelated SRGs with the unprecedented structural integrity can be achieved with a successive overwriting of SRGs without erasing process, which can serve to promote the augmented reality (AR) or virtual reality (VR) technologies.
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