Ultrashort pulse damage of Si and Ge semiconductors

Abstract

An experimental and theoretical investigation of ultrashort pulse damage thresholds of Si and Ge semiconductors has been carried out. As the source of laser radiation, a commercial sub picosecond Ti:Sapphire laser system has been used. It produces laser pulses of 0.5 mJ pulse energy at 1 kHz repetition rate at a wavelength of 775 nm, providing a Gaussian-like beam profile. Compressor tuning allowed for varying the pulse duration from 150 fs to 5.5 ps. The laser damage thresholds were measured in air and for this pulse duration range. The experimental results supported for both materials an asymptotic temporal behaviour in the sub picosecond regime. For 200 fs pulses, a threshold of 0.17 J/cm^2 and 0.10 J/cm^2 was found for Si and Ge, respectively. The results of our theoretical description for Si were consistent with the experimental findings. The damage morphologies were investigated with various microscopic inspection techniques like Nomarski DIC, atomic force and white light interference microscopy. The surface phenomena were ablation and melting of the crystal lattice. The used online diagnostics were sensitive on the occurrence of melt. Therefore, the given experimental thresholds are specific thresholds on melting. Further surface morphologies were identified.