Observation and Control of Photoemission and Electric Field Enhancement of Plasmonic Antennas through Photoemission Electron Microscopy

Abstract

Photoemission electron microscopy (PEEM) is an imaging method which uses electrons excited through the photoelectric effect to characterize a sample surface with nanometer-level resolution. In PEEM, a high intensity laser excites electrons from the surface of the material and electron optics are used to form an image from the intensity and spatial distribution of the photoemission from the sample. The goal of this research was to study and maximize light confinement, which was accomplished using plasmonic nanostructures. Surface plasmons represent oscillations in the electron density of a material and can occur along the transition interface between a metal and a dielectric material. Under the right conditions, both propagating and localized surface plasmon modes may be excited within the structure. Plasmonic antennas are devices that can convert incident electromagnetic fields into localized areas of high electric field enhancement within specific regions of the structures. These nano- and micron-sized devices can be created through lithography or chemical-synthesis and by varying the materials or geometries of the structures, the antennas can be tailored to a wide range of applications. This research used PEEM to excite surface plasmons in triangular gold nanoplatelets and create areas of high photoemission within the structures. The high photoemission observed in PEEM correlated to localized areas of high electric field enhancement, primarily at the tips of the triangles. Localized field enhancement was demonstrated experimentally within a tip region ~90 nm in diameter and within ~15 nm through numerical calculations, both significantly smaller than the overall size of the platelet, the wavelength of the excitation light, or the wavelength of the plasmon mode. In addition, methods for varying and optimizing the spatial distribution and strength of the field enhancement at the tips of the triangles were demonstrated experimentally through PEEM. Overall, this research involved the chemical-synthesis and deposition of thin, triangular gold nanoplatelets, as well as finite element method (FEM) numerical calculations and PEEM experiments to characterize the plasmonic effects of these structures. The nanoplatelets were evaluated for a range of experimental parameters, geometric characteristics, and surrounding materials. This required a detailed understanding of the role of surface plasmon polaritons (SPPs) and localized surface plasmons (LSPs) which may be excited within these structures. The effects of the polarization, wavelength, and angle of incidence of the excitation light on the field enhancement at the tips of the triangles were characterized in-depth. In addition, the roles of platelet size, gold thickness, edge