# Dr. Walker awarded grant from the National Science Foundation to support his Optics research

"The Ultimate Limit of Rescattering in Strong Laser Fields"

One of the most significant recent discoveries in laser science is high harmonic generation and high energy electrons from the interaction of strong laser fields with matter. These are driven by the rescattering process, now clearly understood for single electron processes within a three-step model of ionization. As the field drives the interaction to higher energies, either with longer wavelengths or higher intensities, the interaction becomes more complex and the interaction becomes relativistic and the Lorentz force from the laser magnetic field enters into the dynamics. It was recognized these new aspects to the interaction would eventually reduce the rescattering, high harmonic generation, and high energy photoelectron scattering. Some schemes have been proposed to help circumvent the end to rescattering, such as creating a standing wave of ultrahigh intensity light or controlling the shape of the driving field. With due consideration to these possibilities, the end of rescattering is inevitable for most ultraintense light-matter interactions and it will be supplanted by new dynamics including the laser magnetic field and relativity. We use relativistic Monte-Carlo trajectory ensemble calculations to model photoelectron dynamics in strong and ultrastrong fields. The model allows us to use three dimensional focused laser fields and realistic ion core potentials to model how rescattering will change in atomic species as the interaction leaves the rescattering regime. Our study for noble gases at multiple laser wavelengths and intensities reveals the “end to rescattering” energy beyond which it becomes essentially impossible to collide a photoionized electron with a parent ion. This energy of this ultimate cutoff is 3,000 Hartree (82,000 eV).