Atoms in Intense Laser Fields: Classical Models and Bicircular Light

Nearly 60 years ago, and just a few years after the invention of the laser, it was found that the intense light from a pulsed ruby laser could be used to ionize xenon atoms. This demonstration of so-called "many-photon ionization" changed the way we would think about the interaction of light with matter, and ushered in the study of strong-field physics. Some years later, the related phenomena of nonsequential double ionization (NSDI) and high harmonic generation (HHG) were discovered, and a simple and physically intuitive model known as rescattering was developed. In this three-step process, a single electron tunnels through the distorted Coulomb barrier, is accelerated by the oscillating laser field, and is driven back to impact the parent ion, leading either to double ionization (release of both electrons) or high-harmonic generation (release of a high-energy photon). The presence of suitable returning trajectories is critical in the rescattering mechanism, and as a result, linear polarization is typically used to study NSDI or HHG. But recently, bicircular laser fields (composed of two colors of circularly polarized light) have been shown to be effective in driving rescattering. We present our latest computational work on the study of ionization by intense bicircular laser fields. We utilize a completely classical atomic model, generating ion yield curves, electron momenta distributions, and trajectory maps across a range of atomic species. We find that the presence of NSDI is universal, but that the underlying dynamics evolve from the traditional "tunneling" to "multiphoton" regimes, even within a purely classical model.