Background DKDP (deuterated potassium dihydrogen phosphate) crystals are key harmonic conversion elements in high-power laser facilities, yet their surface damage growth under repetitive laser irradiation severely limits component lifetime and system stability. The damage growth behavior is highly wavelength-dependent, but systematic studies comparing infrared and ultraviolet laser effects on surface damage growth kinetics remain insufficient.

Purpose This study aims to comparatively investigate the surface damage growth characteristics of DKDP crystals under 1ω (1064 nm) and 3ω (355 nm) laser irradiation, focusing on the morphological evolution, growth thresholds, and underlying mechanisms governing wavelength-dependent growth behaviors.

Methods A Nd:YAG laser system was employed with pulse durations of～12 ns (1ω) and～9 ns (3ω). The 1-on-1 irradiation method was first used to obtain damage probability curves, and then S-on-1 irradiation with stepwise increasing fluence was applied to initiate and propagate damage on the rear surface. In-situ online microscopy and ex-situ optical microscopy were used to monitor and characterize the damage morphology. Seven representative initial damage sites were selected for detailed growth tracking.

Results Three types of damage precursors were identified: surface defects, bulk defects, and surface scratches. Under 1ω irradiation, the damage growth threshold was significantly higher than the initiation fluence, and growth exhibited a "delayed burst" pattern — a long stable period followed by abrupt explosive expansion, attributed to plasma formation and subsequent shock-wave mechanical effects. Under 3ω irradiation, the growth threshold was lower than the initiation fluence, and growth proceeded in either slow linear or stepwise modes, driven by defect-assisted absorption and multiphoton absorption. The stepwise growth was associated with crack-network propagation, leading to mechanical spallation, while linear growth corresponded to surface ablation.

Conclusions The wavelength-dependent damage growth originates from different absorption mechanisms: 1ω-driven growth relies on plasma-mediated shock waves, whereas 3ω-driven growth is governed by absorption-induced ablation and crack propagation. The spatial location of precursors determines energy deposition depth and subsequent growth patterns. These findings provide experimental guidance for lifetime prediction and damage-mitigation strategies in DKDP components, emphasizing the need to eliminate bulk defects and subsurface cracks, and to optimize surface finishing processes to suppress scratch-induced growth.