Ultrafast Laser Induced Bulk Damage Dynamics and Pulse ‐ Width Criticality Mechanism on KDP Crystals

ABSTRACT Potassium dihydrogen phosphate (KDP) crystals play significant roles in strong‐field laser systems applied as the electro‐optical switches and nonlinear frequency convertors. However, the intrinsic ultrafast laser induced bulk damage is still a challenging issue and severely restricts the further improvement of ultrafast laser performance. Here, the transient evolution of bulk damage in KDP crystals is systematically investigated by employing the multi‐view pump‐probe ultrafast imaging technique. Based on the observations from top and side views, a dynamic model of damage is established involving laser propagation, carrier excitation, energy transfer, and structural effects. Moreover, through simulations and dynamic behavior comparisons under varying pulse widths, a pulse‐width‐dependent transition mechanism is unveiled, separating non‐thermal and thermal damage regimes at around 1 ps. This conclusion is further supported by ex situ characterization using optical microscopy and Raman spectroscopy. Femtosecond pulses produce cavities surrounded by micro‐cracks with partial amorphization, while picosecond pulses cause complete lattice destruction through ablation and re‐melting. These findings provide critical guidance for advancing the applications of KDP‐based components in ultrafast optics field.