Wavelength Dependence of Laser-Induced Damage: Determining the Damage Initiation Mechanisms

A novel experimental approach is employed to understand the mechanisms of laser induced damage. Using an OPO (optical parametric oscillator) laser, we have measured the damage thresholds of deuterated potassium dihydrogen phosphate (DKDP) from the near ultraviolet into the visible. Distinct steps, whose width is of the order of $k_{B}T$, are observed in the damage threshold at photon energies associated with the number of photons ($3\to 2$ or $4\to 3$) needed to promote a ground state electron across the energy gap. The wavelength dependence of the damage threshold suggests that a primary mechanism for damage initiation in DKDP is a multiphoton process in which the order is reduced through excited defect state absorption.