Investigation of UV laser-induced damage by precursors at the surface of LBO crystal

Abstract

The LBO crystal polished by the chemical mechanical polishing process has a great number of CeO2 particles, with micrometer or submicrometer size, and SiO2 particles, with the size of several tens of nanometers, that remain buried in the polishing surface layer. These particles absorb the UV laser at 355 nm and seriously affect the resistance of the LBO crystal to UV laser damage. Based on Mie scattering theory, this paper analyzes the enhancement effect on the incident electric field because of the residual polishing particles in the polishing layer of the LBO crystal and establishes the difference equations for studying the unsteady heat transfer condition of the absorption core. For the residual CeO2 particles buried in the polishing layer of the LBO crystal, when the particle size is between 200 and 300 nm, the electric field intensity under 355 nm UV laser irradiation will enhance about 2.5–3 times, and the corresponding optical intensity will enhance 6–9 times. Using the unsteady heat transfer equation, the result indicates that when the particle size is between 300 and 700 nm, the particles that absorb the laser under 355 nm UV laser irradiation will cause the strongest transient thermal effect. The particles with sizes in this range cause the most serious destructive effect due to heat absorption, and relatively low UV laser intensity would result in damage to the LBO crystal.