Initial energy deposition and initiation mechanism of nanosecond laser damage caused by KDP surface micro-defects

TL;DR

This paper develops a defect-assisted energy deposition model to understand how micro-defects on KDP surfaces initiate nanosecond laser damage, explaining the low damage threshold and identifying critical defect sizes.

Contribution

It introduces a novel model incorporating light enhancement and sub-band gap levels, validated by experiments, to explain laser damage initiation on KDP surfaces.

Findings

The damage threshold of KDP is two orders of magnitude lower than theoretical predictions.

The model accurately predicts the initiation mechanism of laser damage.

Critical defect sizes for damage sensitivity are identified.

Abstract

To enable an exploration of the initiation mechanism of nanosecond laser damage on a potassium dihydrogen phosphate (KDP) surface, a defect-assisted energy deposition model is developed that involves light intensity enhancement and a sub-band gap energy level structure. The simulations provide an explanation on why the laser-induced damage threshold (LIDT) of the KDP crystal is two orders of magnitude lower than the theoretical value. The model is verified by use of the transient images that appear during the laser damage. In addition, the dimensions of the "dangerous" surface defects that are the most sensitive to the laser damage are proposed. This work enables clarification on the initial energy deposition (IED) and initiation mechanism of the nanosecond laser damage caused by the KDP surface defects on micro-nano scale. It is helpful in understanding the laser-matter interactions and to improve the processing technique for high quality optical components.