Mechanism of refractive index increase in silica glass for the development of fiber-optic photodevices

点击翻译按钮获取中文摘要

The refractive index change induced in a transparent material by UV photon or ion irradiation promises direct drawing of a waveguide that can be used as optical gratings. Structural changes in silica glass induced by ion microbeam were evaluated using microscopic photoluminescence and Raman scattering, and optical and atomic force microscopes (AFM) measurements. The AFM measurements on the microbeam irradiated area show the formation of a groove on the surface. In addtion, a cross sectional observation on the surface parallel to the incident plane reveals surface deformation along the ion tracks, which is deepest at the projected range of ions. Taking into account the possible structural changes of silica induced by energy deposition, the measured topological changes at the front and side surfaces result from internal compaction of silica glass. Refractive index changes were estimated from the Lorentz-Lorenz relationship using the distribution of the internal compaction estimated by the AFM measurements. A small refractive index change was observed along the ion tracks besides a much stronger index change at the projected range, suggesting that energy depositions by the ionization as well as by the atomic collision should be taken into account.It has also become clear that the refractive index change can be induced by UV photons from a KrF excimer laser in a-SiO_x N_y : H films prepared by PECVD. Fabrication of a diffraction grating was performed using a phase mask made of a high quality fused silica plate. The surface of the fabricated grating observed by scanning electron microscopy (SEM). A clear square-toothed pattern with periodicity of around 1 μm can be observed. Furthermore, from the Fraunhofer diffraction pattern, the periodicity of the grating pattern was estimated to be around 1 μm, which agrees with the SEM image. This simply demonstrates that a-SiO_xN_y : H can be processed by UV photon irradiation.