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Photo-tunable photonic crystals were prepared from three dimensional (3D) colloidal crystal templates using a photoresponsive azopolymer. For the preparation of azopolymer infiltrated photonic crystals, silica colloidal crystals were fabricated by gravity sedimentation, a self-assembly technique. The interstitial voids between colloidal particles were filled with azopolymer and azopolymer inverse opals were produced by treatment with aqueous hydrofluoric acid. These photonic crystals exhibited stop bands in their transmission spectra measured in the normal incidence to the (111) plane of face centered cubic (fcc). The photonic bandgap of the azopolymer infiltrated opal and inverse opal could be controlled by the refractive index change due to the photoinduced orientation of azobenzene chromophores. When the azopolymer photonic crystals were irradiated with linearly polarized light, their bandgap positions were shifted to shorter wavelength regions with increasing irradiation time. This behavior experimentally produced a photoinduced orientation of the azobenzene groups in parallel with the incidence of the excitation light. Through such an out-of-plane orientation of azo chromophores, parallel to the [111] fcc crystallographic axis, the effective refractive index of the photonic crystal medium was decreased. Therefore, a blue-shift in bandgap positions was consequently induced with 20-40 nm tuning ranges. The out-of-plane orientation was confirmed by angular resolved absorption spectral measurements.

For use in hybrid solar cells consisting of TiO2 and poly[2-methoxy, 5-(2 � -ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV), a TiO2 nanostructure which has periodic hexagonal hole arrays was fabricated using surface relief gratings on azobenzene-functionalized polymer films as a template in the sol–gel reaction of a Ti-precusor. The ordered bulk heterojunction solar cells, prepared with the TiO2 nanostructure and MEH-PPV, have a higher power conversion efficiency of 0.21% compared to bilayer and random bulk heterojunction solar cells fabricated with thin-dense TiO2 films and randomly networked TiO2 nanoparticles, respectively, under one sun with air mass 1.5 global illumination. © 2007 Elsevier B.V. All rights reserved.

Well-ordered TiO2 nanostructures such as 1-dimensional nanowire arrays, 2-dimensional square and hexagonal hole arrays, and 3-dimensional wire junction arrays were fabricated by a simple, controllable, economical, and reproducible method using surface relief gratings on azobenzene-functionalized polymer films as a template in the sol–gel reaction of a Ti precusor. In addition, as an application of TiO2 nanostructure, transparent electrodes were patterned by selective dry etching with the TiO2 nanowire array etch mask. Scanning electron microscope and atomic force microscope images confirmed the successful formation of a variety of well-ordered TiO2 nanostructures and nanopatterning of transparent electrodes.

A three-dimensionally ordered array of close-packed colloidal spheres, a photonic crystal structure in which the refractive index of the medium interstitial lattice in a colloidal crystal spatially changes in the [111] crystallographic axis, is demonstrated. The colloidal photonic crystal structure with refractive index chirping was produced by infiltration of a monomer and organic dopants with a high refractive index into a silica opal, followed by interfacial gel polymerization. The resulting photonic crystal structure has a gradually varying stop band at each different (111) plane in the face-centered cubic (fcc) crystal structure at a normal incidence. This novel structure exhibited optical characteristics that have band-gap broadening by the superposition of stop bands at each plane of the crystal with different dielectric functions. Moreover, the refractive index perturbation in the [111] fcc opal also showed a defect state within a pseudo-photonic band gap. This new type of photonic crystal structure should be useful for the band-gap engineering of photonic-band-gap materials.

We report a novel all-fiber, maskless lithograpic technology to form various concentric grating patterns for micro zone plate on azo polymer film. The proposed technology is based on the interference pattern out of the cleaved end of a coreless silica fiber (CSF)-single mode fiber (SMF) composite. The light guided along SMF expands into the CSF segment to generate various circular interference patterns depending on the length of CSF. Interference patterns are experimentally observed when the CSF length is over a certain length and the finer spacing between the concentric rings are obtained for a longer CSF. By using beam propagation method (BPM) package, we could further investigated the concentric interference patterns in terms of intensity distribution and fringe spacing as a function of CSF length. These intereference patterns are directly projected over azo polymer film and their intensity distrubution formed surface relief grating (SRG) patterns. Compared to photoresist films azo polymer layers produce surface relief grating (SRG), where the actual mass of layer is modulated rather than refractive index. The geometric parameters of the CSF length as well as diameter and the spacing between the cleaved end of a CSF and azo polymer film, were found to play a major role to generate various concentric structures. With the demonstration of the circular SRG patterns, we confirmed that the proposed technique do have an ample potential to fabricate micro fresnel zone plate, that could find applications in lens arrays for optical beam formings as well as compact photonic devices.