Strong nonlinear growth of energy coupling during laser irradiation of transparent dielectrics and its significance for laser induced damage

The interaction of high power nanosecond laser pulses with absorbing defects, located in the bulk of transparent dielectric materials and having a multilevel electronic structure, is addressed. The model assumes a moderate localized initial absorption that is strongly enhanced during the laser pulse via excited state absorption and thermally driven generation of new point defects in surrounding material. This model is applied to laser induced damage initiation in the bulk of potassium dihydrogen phosphate crystals (KH2PO4 or KDP) and addresses how during a fraction of the pulse duration the host material around the defect cluster is transformed into a strong absorber that leads to the sufficiently large energy coupling resulting in a damage event. This scenario is supported by time resolved imaging of material modification during the initial phases of laser induced damage in KDP and fused silica.