Abstract

Highly amplified femtosecond pulses permit the study of atomic ionization in superintense light fields on a time scale shorter than the escape time of electrons from the interaction region or the recombination of ionized species. Most recent studies of strong field femtosecond ionization have utilized photoelectron spectroscopy at low gas pressures (~10^-7 torr)¹. We utilize a complementary experimental method with unique capabilities, namely the spectral blue shift which is caused by the ultrafast refractive index decrease which occurs when a plasma is created within 1 to 20 optical cycles during the pulse^2,3. Accurate analysis of such blue shifts requires modelling based on ionization rates calculated from a Keldysh theory⁴. Because of the ultrashort pulse duration (< 90 fs.), atomic collisions and plasma expansion play no role, compared to experiments with picosecond and longer pulses. Moreover, we have shown³ that in a sufficiently tight focus geometry, contributions from non-ionizing χ⁽³⁾ processes (e.g.white light continuum generation⁵, self focusing), which cause both red and blue shifts, are suppressed relative to plasma generation effects². Thus detailed observation of blue shifts provides a clean diagnostic of strong field ionization which complements photoelectron spectroscopy in testing theories of superintense laser-atom interactions. In addition this technique offers two additional capabilities not easily achieved by photoelectron spectroscopy: 1) It lends itself readily to femtosecond pump-probe experiments, which we report here for the first time. Such experiments allow us to map ionization rate within the temporal profile of a strong pump pulse. In addition, they allow us to measure plasma dynamics (e.g. recombination, expansion) following the ionizing pulse, manifested as a time-delayed probe red shift. 2) In addition this technique is compatible with gas pressures ranging from 0.1 atm. to supercritical densities (> 100 atm.), allowing us to study ionization, recombination, and expansion dynamics at densities in the range (10¹⁹ to 10²² cm^-3) relevant for X-ray recombination lasers⁶. At supercritical densities, both forward and backward propagating blue-shifted waves, as well as plasma-confined DC magnetic fields, are generated⁷. In addition plasma heating⁷ and relativistic loss mechanisms⁸ become important. Initial results in this high density regime will be presented.

© 1989 Optical Society of America