Revealing Electron Spill-Out in Plasmonic Nanostructures Using Particle Simulation

Classical macroscopic electromagnetic descriptions fail to accurately predict the optical response of plasmonic nanostructures approaching electronic length scales due to the omission of nonlocality and free electron spill-out. Alternatively, microscopic first-principles approaches are accurate, yet too computationally intensive for multiscale problems. Starting from a microscopic viewpoint but more computationally efficient, particle-in-cell (PIC) simulation is probably an appropriate mesoscopic treatment for such problems. Here, we leverage on the inherent soft boundary conditions of PIC to study the spill-out effects of surface electrons in mesoscopic-scale metal nanostructures, specifically Na nanowires [1]. The relationship between the surface plasmon resonance frequency and the radius of the nanowire is investigated, exhibiting a 1/R redshift, which agrees well with the self-consistent hydrodynamic method [2] that includes spill-out of electron density. The developed methodology can be extended to model noble metals by including inter-band transitions, di-metallic nanoplasmonic structures, electron spill-out and charge transport between nearly touching nanoparticles.