Thomas Brabec

A novel equation for light propagation in underdense field-ionizing media is derived by extending concepts of nonlinear optics into the strong-field domain. The equation is first order in the propagation coordinate and is valid for arbitrarily short pulse durations. Solutions of the first-order wave equation are found to be in excellent agreement with solutions of the scalar wave equation. Furthermore, the polarization response of a field-ionizing medium is derived from semiclassical considerations. The polarization reveals, in agreement with experiments, a previously unrecognized contribution that is shown to significantly affect the absorption loss in the presence of ionization.

The interaction of noble gases with laser pulses in the near-infrared and visible wavelength regime is dominated by above barrier ionization while tunnel ionization plays a subordinate role. We develop a theory of ionization in this parameter range. A condition for the applicability of the quasistatic approximation in the above barrier ionization regime is obtained. Ionization of arbitrary atoms by a laser pulse may then be calculated from static ionization rates. The static ionization rate of He is obtained by solving the full two-electron Schrödinger equation. Our analysis yields a verification of the single active electron approximation and indicates the possibility to create keV radiation by high harmonic generation in a He gas.