Electron-Lattice Heating from Avalanche Ionization in Silicon with Near Infrared Ultrafast Laser Pulses

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 ionized electron energy by the lattice and its subsequent phase transformation to the liquid and vapor state. An average solid density plasma heating layer of 78 nm is extracted from the model and used to fit threshold fluence data. More detailed experimental results are found to provide close agreement with, and continuation of, the avalanche ionization coefficient curve of silicon [1] for electric field strengths extending to 4 × 10⁷ volts/cm.