Pulse-width and focal-volume dependence of laser-induced breakdown

Abstract

The laser-induced breakdown fields at 1.06 μm of fused Si${\mathrm{O}}_2$, single-crystal NaCl, and air were measured as a function of focal volume and laser pulse width while keeping all other parameters, including the specimen, constant. The laser pulse width was varied from 40 psec to 31 nsec, and the focal volume was varied by over two orders of magnitude. The dependence of the breakdown field for NaCl and Si${\mathrm{O}}_2$ on the laser pulse width $t_p$ and the focal volume $V$ was empirically determined to be $E_B=A{V}^{-1\mathrm{}}{t}_p^{\frac{-1\mathrm{}}{4}}+C$ and the dependence for air to be $E_B=(A{V}^{-1\mathrm{}}+C)t_p^{\frac{-1\mathrm{}}{4}}$, where $A$ and $C$ are material-dependent constants. Current theories of laser-induced breakdown are carefully compared with these present measurements and are found to be inconsistent.