Wavelength Dependence of Laser-Induced Bulk Damage Morphology in KDP Crystal: Determination of the Damage Formation Mechanism

Wet etch process is applied to expose the bulk damage sites in KDP crystals to the surface for the examination by scanning electron microscopy (SEM) and optical microscopy. The damage sites induced by 1064 nm laser consist of three distinct regions: a core, an outer region of modified material, and some oriented cracks. Laser irradiated with 355 nm results in an increase of damage density, a decrease of core diameter and, rarely, occurrence of the crack. Wavelength dependence of the damage feature suggests that a repulsive force exists among the adjacent plasmas, which prevents further expansion of plasma and decreases the size of plasma. The deposited energy absorbed by the smaller plasma may not be able to generate the crack.