Low-Temperature Synthesis and Optical Properties of Gel and Ceramic Photonic Crystals Made up of High Dielectric Constant Materials

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The objective of this study is to establish a low temperature synthesis method for fabricating two-dimensional TiO_2 and BaTiO_3 gel and ceramic photonic crystals by sol-gel processing using resist molds and also to evaluate the optical properties of the obtained crystals.The obtained results are summarized as follows:1) Synthesis of precursor sol solutions : We prepared a TiO_2 precursor solution by mixing titanium isopropoxide, Ti(O-i-Pr)_4, poly(ethylene glycol) (PEG: molar weight=400), ethylene glycol monomethyl ether (EGMME) as a solvent and water for hydrolysis of the alkoxide to have the composition Ti(O-i-Pr)_4: EGMME : H_2O = 1:3:1 (in molar ratio), and also prepared a BaTiO_3 precursor solution by using the same procedure with barium ethoxide, Ba(OC_2H_5)_2. These precursor solutions gave crack-free gel films of the respective materials when spin-coated on Si and MgO substrates.2)Preparation of resist molds : We first prepared resist films of around 1μm by spin coating on Si … More and MgO substrates, and then formed hexagonal arrays of air holes with various values of air holes radius r and spacing between holes (lattice constant) a in the resist alms using electron beam lithography. The values of r and a of the resist molds produced were theoretically determined from photonic band structure calculations to allow the photonic crystals to be fabricated exhibit complete photonic band gaps in the visible region.3)Fabrication of TO_2 and BaTiO_3 gel photonic crystals : We succeeded in fabricating two-dimensional photonic crystals consisting of hexagonal arrays of TiO_2 and BaTiO_3 precursor pillars on Si and MgO substrates, respectively, by casting the precursor sol solutions into the resist molds, drying, then followed by removing the resist molds chemically with a removal agent Prior to the removal of resist molds it is necessary to remove completely a gal film that the present method unavadably form on the resist mold, and we succeeded in ding it with a dry etching technique using CF_4/C_3F_8 gas.4) Fabrication of TiO_2 and BaTiO_3 ceramic photonic crystals : TiO_2 and BaTiO_3 ceramic photonic crystals were obtained by sintering the precursor get photonic crystals at temperatures of 500 to 800℃ in air. The obtained photonic crystals showed no appreciable lattice disorder of the crystals, with little collapse of the precursorpillars, though significant changes in the size of the pillars due to their shrinkage during firing were observed5) Evaluation of optical properties of the obtained photonic crystals : Optical reflection measurements in the wavelength range of 400 to 1000nm were carried out for TiO_2 ceramic photonic crystals with different rand a values of (r=110nm, a=400nm), (125nm, 450nm) and (125nm, 500nm) and for a BaTiO_3 ceramic photonic crystal with a=450nm and rl=0.19 in the Γ-Mand f-Kdrections of the crystals for both TE and TM modes, in which distinct reflection peaks were observed for all the photonic crystals at around wavelengths calculated. The results dearly demonstrate that 2D ceramic photonic crystals with wall-developed photonic band gaps in the optical regime have been successfully fabricated for the first time in this study. Less