Trapping field assisted backscattering in strong-field photoemission from dielectric nanospheres

We study strong-field ionization of dielectric nanospheres and focus on the enhancement of the cut-off energies for backscattering electrons resulting from charge interaction. Though recent studies clearly demonstrated the decisive impact of a surface trapping field on the electron backscattering process, a clear picture of the underlying mechanism is lacking. Here, we provide this picture and present a simple and transparent extension of the famous three-step model of strong-field science by adding a triangular surface trapping potential. We justify this model for the case of dielectric nanospheres based on high-level transport simulations. The analysis of the trapping field assisted backscattering provides a universal scaling of the maximal recollision and backscattering energies as 9Up and 14.5Up, respectively, where Up is the local ponderomotive potential. The universal nature of the enhancement over the conventional three-step model is of particular interest for the generation of attosecond electron ...