Spatiotemporally resolved photoemission from plasmonic nanoparticles

Abstract

Streaked photoemission from nanostructured surfaces and nanoparticles by attosecond extreme ultraviolet (XUV) pulses into an infrared (IR) or visible streaking pulse allows for sub-femtosecond resolution of the plasmonically enhanced streaking-pulse electric field. It holds promise for the temporally and spatially resolved imaging of the dielectric response near nanostructures. In this dissertation, I present four distinct yet interconnected aspects of numerically modeling plasmonic reconstruction by the photoemission from nanoparticles. First, I present a theoretical model of simulating the IR-streaked XUV photoemission spectra, by calculating (a) the plasmonic field induced by IR pulses within Mie theory, and (b) the T-matrix elements for photoemission using a quantum-mechanical model. The simulation results show significant oscillation-amplitude enhancements and phase shifts, comparing to calculations without the induced plasmonic field. These observable effects can be traced to the dielectric properties of the nanoparticles, demonstrating the applicability of streaking spectroscopy to the investigation of induced plasmonic effect near nanoparticles and nanostructured surfaces. Second, based on this model, I propose a scheme for the reconstruction of plasmonic near-fields at isolated nanoparticles from streaked photoelectron spectra. The success of this proposed scheme is demonstrated by the accurate imaging of the IR-streaking-pulse-induced plasmonic fields at the surface of gold nanospheres and nanoshells with sub-femtosecond temporal and sub-nanometer spatial resolution. Third, I further improve the physical accuracy of the model, by developing a semi-classical approach, ACCTIVE, to solve the time-dependent Schrödinger's equation in spatially inhomogeneous electromagnetic fields. I demonstrate the validation of this method by studying electron final-state wavefunctions in Coulomb and laser fields, before applying these improved final photoelectron states to streaked photoemission from hydrogen atoms. The results show excellent quantitative agreement with direct solution of the Schrödinger's equation. Implementing this method to simulating the streaked photoemission from Au nanospheres shows better agreement in plasmonic-field reconstruction for low energy photoelectrons than previous strong-field-approximation simulations. Finally, I extend the previous work and explore the non-linear optical response of nanoparticles observed in momentum imaging experiments at the Kansas State University Department of Physics. My Mie simulations, by including intensity-dependent index of refraction, show a significant non-linear effect in SiO₂-core-Au-shell nanoparticles in response to 10¹⁰ - 10¹² TW/cm² intensity and 780 nm central wavelength IR pulses. This effect is responsible for the change in the experimentally observed photoelectron "cut-off" energies, as a function of the external pulse intensity, suggesting the non-linear optical response to be a significant factor in strong-field photoemission from plasmonic nanoparticles.