Origin and Properties of Second-Order Optical Nonlinearity Induced by Crystallization in GeO₂-SiO₂ Glass Films

Abstract

We report, for the first time, on a direct observation of second-harmonic generation (SHG) caused from crystalline particles in thermally crystallized GeO₂-SiO₂ thin films. GeO₂-SiO₂ glass films with a thickness of approximately 5 μm were fabricated by chemical vapor-phase deposition (CVD). Thermally treatments for crystallization have been performed, and crystalline particles with 20-50 μm diameters were induced in the glass films. Using a SHG microscopic technique, it has been successfully found that SH emitting with wavelength of 532 nm, corresponding to wavelength conversion from a fundamental Nd:YAG laser with 1.06 μm, was clearly observed directly from the crystalline particles. In addition, we have found strong correlation between SHG and stress-induced phase transition based on crystobalite-like structures. These results clearly indicate the origin of second-order optical nonlinearity space selective induced by crystallization in silica-related glasses with centrosymmetry.