Submicrometer Space Control of Metal Oxides for Creating Optical Functions Based on Spontaneous Optical Resonator Formation

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An attempt has been made to form well-defined macropores in high-refractive-index-metal oxides such as alumina (Al_O_3), zirconia (ZrO_2), and titania (TiO_2) in order to obtain strong light scattering in the visible region. In addition, the transport mean free path and diffusion constant of light in the resultant porous media have been evaluated by means of coherent backscattering and timeTesolved transmission measurements, in which the continuous wave and untrashort pulsed lasers ar used as the light sources, respectively. The results can be summarized as follows.(1) It is well established that silica (SiO_2) with well-defined macropores can be successfully prepared via the sol-gel process accompanied by the phase separation. However, it has been very hard to extend this synthetic technique to other metal oxides because of the high reactivity of precursors. In this study, it is shown that bicontinuous macroporous structures can be formed in binary-component oxides such as Y_3Al_5O_<1 … More 2> (YAG) and Y_2O_3 -doped ZrO_2 (YSZ) as well as single-component oxides such as Al_2O_3, ZrO_2, and TiO_2 by selecting the starting materials and compositions in the sol-gel systems adequately. In particular it is Sound that the SS of macropores in TiO_2 monoliths can be controlled over a wide range of 100 nm to 10 μm.In strongly scattering media, coherent backscattering peaks exhibit very wide angular dependence of scattering light. Home-made equipment has been built in order to measure the angle dependence of scattering light and also to obtain the transport mean free path. In addition, optical systems to measure the time-resolved transmission has been built by combining femtosecond laser pulses and a streak camera so that the diffusion constant can be evaluated. It is found that the transport mean free path varies with the size of macropores, indicating that the scattering strength can be systematically controlled by the macroporous morphology. In some media, the transport mean free path is shorter than the wavelength of light. Analysis of time-resolved transmission profiles shows the slight deviation from classical light diffusion model, suggesting the signature of Anderson localization. The detailed experiments and analysis are required to clarify the phase transition between the diffused and localized states of light. Less