  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.
