| Single-shot laser induced breakdown in wide band gap materials such as SiO{dollar}sb2{dollar} and MgF{dollar}sb2{dollar} has been studied over 5 orders of magnitude in duration from 55 fs to 7 ns. A Ti:sapphire Chirped Pulse Amplification system was used in this experiment, so the pulse duration could be continuously adjusted without changing any other laser parameters. The damage threshold was detected by looking at the plasma formation and the change of material transmission coefficient. A strong departure from the conventional fluence threshold scaling law ({dollar}Fsb{lcub}rm th{rcub}simsqrt{lcub}tausb{lcub}p{rcub}{rcub}{dollar}) is observed for pulses shorter than 10 ps, where beyond this point the fluence threshold increases. The avalanche mechanism was found to dominate over the entire pulse-width range even for 55 fs pulses, where one would expect the multi-photon processes to take over. The multiphoton ionization mechanism is found to be suppressed in dielectric materials. The independence of the breakdown threshold on the linearly and circularly polarized laser light further confirms that the standard multiphoton ionization theory can not be applied to solids where electron collisions are very frequent. Numerical integration of the time-dependent Schrodinger equation with collisions included has shown a substantial reduction of the photoionization rate due to collisions. Also, it is observed for the first time that for short pulses the damage threshold becomes deterministic and less statistical than that for longer pulses, which may have important potential applications in micro-machining and laser surgery where high accuracy and least amount of collateral damage are desired. |
