Multiphoton and Strong-Field Processes

The excitation of atoms by intense laser pulses can be divided into two broad regimes: the first regime involves relatively weak optical laser fields of long duration, and the second involves strong fields of short duration. In the first case, the intensity is presumed to be high enough for multiphoton transitions to occur. The resulting spectroscopy is not limited by the single-photon selection rules for radiative transitions. However, the intensity is still low enough for a theoretical description based on perturbations of field-free atomic states to be valid, and the time dependence of the field amplitude does not play an essential role. In the second case, the field intensities are too large to be treated by perturbation theory, and the time dependence of the pulse must be taken into account. In addition to a detailed picture of the two subjects described above, we include a discussion on the generation of attosecond pulses and applications of both high-order harmonic generation (HHG) and above-threshold ionization (ATI). Furthermore, we incorporate a brief summary about the incipient field of atto-nano physics.