Abstract

Laser induced breakdown by ultrashort pulses in optically transparent materials has become increasingly interesting. As has been shown[1], avalanche ionization is the dominant mechanism for laser induced breakdown in such materials for pulsewidth down to ~ 100 fs, and multiphoton ionization is responsible for the initial electron generation. The breakdown threshold for the avalanche process is a function of the initial free electron density n₀. By controlling the initial seed electron density, one may vary the breakdown threshold value. We have performed a pump-probe experiment on fused silica (SiO₂) to test this hypothesis. A UV “pump” pulse, illuminating the same location, generates seed electrons for the later-arrived IR pulse by two-photon absorption. By varying the delay between the two pulses so that the UV pulse arrives before and after the IR pulse, we can observe the difference in the breakdown thresholds.

© 1996 Optical Society of America