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 cutoff 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 9 U-p and 14.5 U-p, respectively, where U-p 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 bunches via near-field induced photoemission and high harmonic generation at nanostructures.