Insights into the pinpoint damage mechanism of DKDP crystals: from the measurement of the intrinsic three-photon absorption coefficient.

We measured the intensity-dependent transmittance of type-II phase-matched, 70%-deuterated potassium dihydrogen phosphate (DKDP) crystals (θ = 61°, φ = 0°) using linearly polarized, single-pulse excitation at a central wavelength of 420 nm, a pulse duration of 173 fs, and a maximum average intensity of 800 GW/cm2. To the best of our knowledge, this is the first experimental determination of the intrinsic three-photon absorption (3 PA) coefficient of DKDP using sub-200-fs, single-pulse irradiation. Under the experimental conditions optimized in this work, the combined contributions of laser-induced point-defect-related transient absorption (PDRTA) and other secondary nonlinear dissipative processes were estimated to be less than 0.7% of the intrinsic 3 PA loss. The measured nonlinear transmittance was therefore dominated by 3 PA, allowing the intrinsic o-polarized 3 PA coefficient at 420 nm to be determined as γ3 = (5.83 ± 0.23) × 10-4 cm3/GW2. No statistically resolvable difference in the 3PA-dominated nonlinear transmittance was detected among the investigated samples and regions within the experimental uncertainty, consistent with the intrinsic nature of 3 PA in DKDP. We also compared the nonlinear response before and after laser conditioning. Although conditioning increased the pinpoint-damage threshold from 11.8 to 15.5 J/cm2, no measurable change in the 3PA-dominated response was detected. In conjunction with the defect-assisted multistep/multiphoton model, these results suggest that laser conditioning does not significantly modify the bulk electronic structure of DKDP, but instead reduces the population of damage-relevant point defects within the precursors responsible for pinpoint damage. These findings provide a refined physical basis for understanding nonlinear absorption in DKDP and offer insight into the microscopic mechanisms of laser-induced pinpoint damage.