Emergence from nonrelativistic strong-field rescattering to ultrastrong-field laser-atom physics: A semiclassical analysis

We discuss a rescattering deflection parameter to gauge the progression from strong- to ultrastrong-field rescattering dynamics. Our analytical and numerical treatments of rescattering quantify the change from nonrelativistic, dipole approximation strong-field dynamics to an ultrastrong-field interaction where v/c≠0 and Blaser≠0. At the threshold between these two regimes the Lorentz deflection reduces the returning electron flux at the core and suppresses rescattering related phenomena. Both radiation and nonsequential ionization from rescattering are studied. We show the ultrastrong-field regime can occur at intensities as low as 1014 W/cm2 for mid-IR (2 μm to 5 μm) wavelengths or as high as 1018 W/cm2 for UV radiation