Extreme Pulse Duration Scaling of Strong Field Ionization of Nanoparticles

Strong-field physics has been at the forefront of scientific research for multiple decades, mainly focusing on atoms and molecules, and only recently becoming focused on nanoparticles and solids. Recent developments in nanotechnology pushed this research to more complex systems, where the distinction of ionization regimes via the Keldysh parameter is insufficient. Here we report on pulse duration-dependent strong-field ionization of dielectric nanoparticles at low intensities. We find that despite the inverse relation to field intensity, increasing the pulse duration results in an increased photoelectron cutoff energy. This cutoff energy enhancement is seen both in absolute energy (units of eV) and when represented in terms of the ponderomotive potential. Further, our findings implicate nonsequential ionization as being the dominant underlying mechanism to this observation. In this new regime of strong field science, the intuitive approach of higher peak intensity yielding a higher photoelectron cutoff no longer applies. Furthermore, comparable results are found for ionization by circularly polarized fields, indicating that recollision is not significantly suppressed in nanoparticles as it is in atoms and molecules.