Plasmon Mediated Multiphoton Photoemission Microscopy of Au Nanoholes and Nanohole Dimers

Multiphoton photoernission from "nanoholes" and "nanohole dimers" fabricated by nanosphere shadow lithography in a thin Au film are investigated by ultrafast scanning photoionization microscopy (SPIM), which provides detailed information on laser polarization and local electric field enhancement effects for novel 2D plasmonic architectures. The net photoemission signals (at 800 nm) from isotropic nanohole monomers both (i) below (d << lambda) and (ii) comparable to (d approximate to lambda) the diffraction limit are insensitive to laser polarization, though SPIM images of the larger species clearly reveal strongly enhanced electron emission from nanohole tangent edges perpendicular to incident laser polarization. Fundamentally contrasting behavior is observed for intrinsically anisotropic plasmonic architectures such as nanohole dimers. Specifically, small nanohole dimers reveal enhanced electron emission for lambda = 800 nm light polarized perpendicular to the dimer axis, whereas larger nanohole dimers produce optimum electron yields for polarization parallel to the dimer axis at the same excitation wavelength. We find excellent agreement between the SPIM images for the four classes of nanostructures and detailed finite element simulations for photoemission, where the latter include (i) the spatial dependence of the electric field enhancement around the nanoobject and (ii) diffraction limited spatial resolution of the laser source. In general, these studies highlight multiphoton photoemission of nanoholes and nanohole aggregrates as sensitive benchmark systems for imaging local plasmonic field enhancements, which enables detailed characterization of nanobjects far below the diffraction limit.