Polarisation response of a gas medium in the field of a high-intensity ultrashort laser pulse: high order Kerr nonlinearities or plasma electron component?

The polarisation response of quantum systems modelling silver and xenon atoms in the field of a high-intensity femtosecond Ti : sapphire laser (photon energy ħω ≈ 1.5 eV), has been investigated by direct numerical integration of the Schrödinger equation. The applicability ranges of the perturbation theory and polarisation expansion in powers of field are determined. The contributions of excited atoms and electrons in the continuous-spectrum states to the polarisation response at the fundamental frequency, which arise as a result of excitation and photoionisation, are analysed. It is shown that specifically ionisation changes the sign of dielectric susceptibility with an increase in radiation intensity for the systems under consideration.