Abstract

Absorption and redistribution of ultrafast pulsed laser energy in silicon is examined with 800 nm radiation for pulse durations ranging from 80 fs to 7 ns. The results are successfully interpreted in terms of an avalanche ionization process. Thresholds for vaporization are measured and compared to a theoretical model that incorporates a two temperature electron-lattice heating process. Ionization layer thickness and time resolved melting and vaporization events are described in terms of a thermodynamic process involving the absorption of ionization energy by the lattice and its subsequent phase transformation to the liquid and vapor state. Nominal solid density ionization heating layers of 75 nm are extracted from the model and compared to results obtained from DC high field avalanche formulations. Our experimental results are found to provide a fit with, and continuation of, the avalanche ionization rate curve for silicon ID for electric field strengths extending to 4e7 volts/cm.

© 1996 Optical Society of America