ATOMS IN ULTRA-INTENSE LASER FIELDS

The interaction of an atom with an ultra-intense radiation field is characterized by the involvement of many photons in absorptions and emissions. Of course the word 'intense' has to be compared with some atomic reference: if the induced transition coupling between bound states exceeds inherent widths, then the dressed atom Rabi oscillations which dominate the atomic evolution are typical intense field effects, even though laser intensities may actually be quite modest. When laser intensities exceed 10(13) W cm-2, and infrared frequencies are employed, then free electrons are dressed by interaction energies which exceed the photon energies. Non-perturbative continuum processes such as above-threshold ionization then occur, combined with the emission of very high order harmonics of the pump laser frequency. At higher laser intensities, the optical electric field can exceed the Coulombic binding electric field and allow over-the-barrier ionization, which defines a new regime of high intensity physics. In this region (or at higher intensities) the atomic electron charge cloud oscillates in the laser field with large amplitude excursions from the nucleus, during which time it is unable to absorb further photons. This stabilization regime is predicted to persist until the electron dressing energy approaches the rest-mass energy when wholly unexplored regions remain to be investigated. In this topical review, we examine theoretical models of atoms dressed by intense fields. We review the breakdown of lowest-order perturbation theory and those 'essential states' methods adopted to include Rabi frequencies, Stark shifts, induced widths and continuum dressing. Newer methods more suitable for super-strong fields are described, such as Floquet and Volkov methods and the direct numerical integration of the Schrodinger equation. Such methods are used to provide completely non-perturbative strong field descriptions of atomic dynamics. We conclude with a brief examination of the relativistic effects expected to be important when new high intensity ultrashort pulse lasers currently under development are employed in strong field physics.