Carrier dynamics in KDP and DKDP crystals illuminated by intense femtosecond laser pulses

Abstract

The dynamics of electrons and holes in potassium dihydrogen phosphate (KH 2 PO 4 or KDP) crystals and its deuterated analog (KD 2 PO 4 or DKDP) induced by femtosecond laser pulses is investigated at λ = 800nm. To do so, experiments based on a femtosecond time-resolved interferometry technique have been carried out. It is shown that two relaxation dynamics exist in KDP and DKDP crystals. In particular, it appears that one dynamics is associated with the migration of proton/deuteron in the crystalline lattice. Both of the dynamics correspond to physical mechanisms for which the multiphoton order required to promote valence electrons to the conduction band is lower than the one of a defect-free crystal. These results suggest the presence of states located in the band gap that may be due to the presence of defects existing before any laser illumination or created in the course of interaction. In order to interpret the experiments, a model based on a system of rate equations has been developed. Modeling results are in good agreement with the experimental data, and allow one to obtain fundamental physical parameters governing the laser-matter interaction as multiphoton absorption cross sections, capture cross sections, recombination times, and so forth. Finally, it will be shown how these results can be used to the understanding of laser-induced damage by nanosecond pulses in inertial confinement fusion class laser aperture.

Self-trapped holes (small polarons) in ferroelectric KH2PO4 crystals

“…Their performance in nonlinear optical applications is often degraded when one or more unwanted transient absorption bands are produced during exposures at room temperature to short pulses of highpower laser radiation. These broad transient absorption bands, representing optical damage, have been extensively investigated in recent years [19][20][21][22][23][24][25][26][27] and are believed to be associated with specific charge states of point defects. STHs are assigned a major role in the underlying damage mechanism [21,22,25], since they are an intrinsic defect in this material and are among the first defects to be formed by the intense short-wavelength laser pulses.…”

“…These broad transient absorption bands, representing optical damage, have been extensively investigated in recent years [19][20][21][22][23][24][25][26][27] and are believed to be associated with specific charge states of point defects. STHs are assigned a major role in the underlying damage mechanism [21,22,25], since they are an intrinsic defect in this material and are among the first defects to be formed by the intense short-wavelength laser pulses. Attention is also focused on the STHs because they serve as the precursor to self-trapped excitons (STEs).…”