The Design Of Full Three-dimensional Photonic Bandgap Crystals And The Fabrication And Properties Studying Of Photonic Crystals Made Of Titania At Visible Wavelengths

Photonic crystals (PCs) are a new type of optical materials in the fields of physics and material sciences in the recent years. Due to their peculiar properties for manipulating photons transmission, PCs have been of great interest in a variety of fields of optical communications based on optical devices for high-density photonic integrated circuits. However, the general exploitation of PCs has been hindered by the difficulties in creating three-dimensional periodic dielectric structures with a photonic bandgap (PBG) at the visible wavelength region.This thesis systemically reviewed the theoretics, fabrications and applica- tions of photonic crystal. A new hollow-spherical non-close packed face- centered cubic (fcc) structure was designed. The calculations of band structure indicate that this structure possesses a robust three-dimensional complete PBG between 8-9 bands with a lower dielectric contrast. Used the titania as the dielectric, the photonic crystal of this fcc structure was fabricated successfully with the help of self-assembling, thermal sintering techniques, selective etching techniques and sol-gel process.This dissertation includes the following parts: Firstly, the mondispersed silica spheres were synthesized using the St?ber method. In this process, the reaction mechanism of synthesizing the silica spheres with different diameters was investigated. The experimental results indicated that the diameters and morphology of silica spheres were determined by the concentration of TEOS, NH3, H2O and synthesis temperature. The silica spheres were charactered with transmission electronic microscope and scanning electronic microscope.Secondly, the colloidal crystals of silica microspheres were prepared on a silicon substrate by using the modified vertical deposition method: The silicon substrate covered with a glass microslide, which was treated with the hydro- phobic water solvent, was vertically put into the ethanolic dispersion of the silica spheres. After the ethanol was evaporated for 2-3 days, the film of silica opal with a certain thickness was deposited on the silicon substrate. The experimental results indicated that the prepared colloidal crystal possessed a fcc structure. The optical test showed that the decrease of silica microspheres diameter produced a bandgap wavelength blue-shift. We explored the influ- ence of the solvent, concentration of the colloidal spheres and the evaporate rate of solvent on the quality of the colloidal crystals. We also found out the optimal condition to prepare the high quality colloidal crystal of silica spheres. The high quality colloidal crystal of silica spheres was fabricated when water or ethanol used as the solvent and the volume fraction of the colloidal spheres was 1.0%.Thirdly, the fcc structure crystal of non-close packed silica microspheres was fabricated using the colloidal crystal as template. The fabricating process was described as following: the obtained close packed structure crystal of silica microspherers was sintered at 1010 oC for 3 h. Then the sintered opal was etched by using the aqueous solution of 1.0 wt% HF at room temperature. In this structure, the silica spheres are not in contact any more, but bridged by tubular necks. As a consequence of the uniform etching process, a new opal consisted of a fcc array of silica spheres connected by cylinders was formed. In this dissertation, we also achieved the optimal conditions of fabricating high quality non-close packed structure crystals of silica microspheres: the sintered temperature should be 1010 oC, and when the concentration of HF acid was 1.0 wt%, the etching process was well-proportioned.The optical studies indicated that the bandgaps of the fcc systems of non-close packed SiO₂ microspheres becomes better than close packed systems. Based on the plane wave expansion method, theoretical calculations showed that pseudogaps opened only at point L of the Brillouin zone between 2-3 bands in close packed systems, but in the non-close packed systems, pseudogaps opened not only around point L but also around point X of the Brillouin zone.Fourthly, we designed a kind of fcc photonic crystal model of non-close packed hollow spheres. In this structure, each hollow sphere at the fcc lattice sites is connected to all of its 12 neighbors by 12 cylindrical tubes. We calculated and optimized the band structure of non-close packed structure of hollow spheres using the MPB programs based on the plane wave expansion method. The results show that it presents a band gap between the eighth and ninth bands. When the dielectric constant chose as 2.8 and the optimal parameters used as rs = 0.0132L, rc = 0.012L, t = 0.072L, the width of the band gap could reach 9.1% and the critical dielectric contrast for the existence of the band gap is only 5.3, which is lower than that of other fcc structures could be obtained by self-assembling method.We also investigated the influence of structure parameters and dielectric contrast on the gap size and the mid gap frequency between eighth and ninth bands. We found that the changes of the gap sizes were different with the parameters, but the mid gap frequency decreases with the increasing of the total volume fraction of the dielectric material.Finally,with the help of thermal sintering techniques, selective etching techniques and sol-gel process, the fcc photonic crystals of crystalline TiO₂ were fabricated using non-close packed silica opals as the templates. A simple synthesis steps describe as: (i) the sol was prepared by mixing titanium tetraisopropoxide, anhydrous ethanol, deionized water, and diethanolamine with a certain molar ratio. (ii) the non-close packed silica template adhered to a substrate was immersed in the sol for 5 min and sintered at 520 oC after pulling out from the sol. The cycle of dipping and sintering was repeated several times. (iii) the silica-titania composite was immersed in 20% aqueous NaOH solution for 48 h to remove the silica template.In this thesis, we investigated the influencing of sinter temperature, sol-gel technique and template removing technique on the quality of the non-close packed crystal of TiO₂ hollow spheres. The results indicated that the high quality colloidal template was the base to fabricate the non-close packed crystal of TiO₂ hollow spheres. The sol-gel technique and template removing technique were also important factors in the fabricate process. In order to get crystalline TiO₂, the samples should be sintered at 520 oC. It is because the NaOH could be only etching the SiO₂ but not the TiO₂, so the NaOH liquid was selected as the etching solution to remove the SiO₂ spheres.The calculation results indicate that it presents a quasi-full bandgap between the eighth and ninth bands in the non-close packed photonic crystal made of titania hollow spheres. The calculation results and the experimental results both show that the quasi-full bandgap existence at visible wavelengths.