Starting times of laser−induced intrinsic damage in NaCl

The starting time for laser−induced intrinsic damage in NaCl is calculated as a function of the rms optical field strength at the peak of the laser pulse. Three different damage mechanisms are considered: (i) avalanche ionization for λ⩾1 μ, (ii) five−photon assisted avalanche ionization at λ=0.694 μ, and (iii) three−photon assisted avalanche ionization at λ=0.347 μ. It is shown that the probability to measure the true damage threshold E*th is zero. (E*th is the optical peak field strength of a laser pulse that heats a small sample volume so that the melting point is reached exactly at the end of the pulse.) For this reason a ’’practical’’ damage threshold E*T is defined which is the peak field strength of a pulse that causes damage by melting at the time the laser flux reaches its peak.