Formation of micromodifications in a KDP crystal irradiated by tightly focused femtosecond visible laser pulses

The formation of micromodifications in the bulk of a KDP crystal irradiated by tightly focused 600-nm, 100-fs and 200-fs, 0.02–10 μJ femtosecond laser pulses is studied. A theoretical model describing the initial stage of formation of a plasma channel taking into account field ionisation and heating of the electron component of the plasma is proposed. The laser pulse intensity (10¹³ W cm^-2), the electron concentration (10²⁰ cm^-3) and the average electron temperature (5 eV) in the plasma channel are estimated.