Photo-induced charge density distribution in metal surfaces and its extraction with apertureless near-field optics

Electromagnetic (EM) waves impinging on finite metallic structures can induce non-uniform electrical currents and create oscillating charge densities. These local charges govern the important physical processes such as plasmonic behavior or enhanced Raman scattering. Yet the quantitative calculation and probing of the spatial distribution of the charge density still remain challenging at the subwavelength scale. This is especially the case if one considers the boundary effect, where the charge density can become divergent and conventional finite element methods fail to obtain accurate information. With an approach we recently developed, we calculate this charge density for subwavelength structures with and without sharp corners: gold disks and equilateral triangles. We also devise an independent way to extract the surface charge density distributions from experiments using scattering-type scanning near-field optical microscope (s-SNOM). We found that the charge density σ is related to the near field signal S_n by σ_element ∞ (S_n - <Sn>)/<S_n>. With no adjustable parameters, the extracted surface charge distribution from the experiments matches well with that from the theoretical prediction, both in magnitude and phase. Our work provides a quantitative study of the surface charge distributions and a systematic and rigorous treatment to extract surface charge distributions at the nanoscale, opening opportunities for mining the near-field data from s-SNOM.