Discharge mechanism and threshold in second harmonic generation by periodically poled LiTaO3

Short ps and fs pulses are well known to produce a strong non-equilibrium between conduction band (CB) electrons and crystal lattice since laser-matter interaction time is shorter than the electron-to-lattice energy transfer time (a few ps). In contrast, it is generally accepted that for ns pulses the energy absorbed initially by CB electrons is fully dissipated to the lattice and the electron-lattice energy equilibrium is established. However, by making our experimental-theoretical study for ns pulsed second harmonics generation (SHG) in periodically poled lithium tantalate (LT) we find that in addition to this mechanism a specific inverse mechanism providing an effective kinetic pathway for electron heating and discharge exists in nonlinear crystals. In short, our study suggests the mechanism by which during nonlinear frequency conversion the initial temperature increase ΔT of few K caused by photo-absorption in non-symmetric crystal lattice induces a high gradient of the spontaneous polarization P_s across the irradiated zone resulting in the onset of the electric field E≈10 kV/cm, which accelerates CB electrons to the energy of ≈10-20 eV, followed by the discharge and crystal damage.