Imaging Light with Photoelectrons on the Nano-Femto Scale

Abstract

The interaction of light with solid state quasiparticles, such as excitons and plasmons, on the nanometer-femtosecond spatio-temporal scale illuminates ultrafast physical and chemical processes on surfaces. In this thesis, I report on the generation, control, and spatio-temporal evolution of 2D evanescent electromagnetic waves confined at the silver (Ag)/vacuum interface; such fields, known as surface plasmon polaritons (SPPs), have joint particle-wave nature. SPPs are generated by the interaction of light with the collective response of conduction band freeelectrons of metals. I image and study the ultrafast dynamics of SPP fields by interferometric timeresolved multi-photon photoemission electron microscopy (ITR-mP-PEEM). First, I report on the generation and propagation of SPPs excited on epitaxially grown Ag nanocrystals. The PEEM images record an interference pattern between SPPs and vacuum light, defined by a mismatch in their propagation wave vectors. Next, I explore the light polarization as a control parameter for the SPP generation, where the in-plane and out-of-plane components of optical electric fields couple differently. For equilateral triangle Ag island samples, the SPP interference patterns strongly depend on both the linear and circular polarizations. For circularly polarized light, the SPP coupling depends on the matching between spin angular momenta (SAM) of light and SPPs. The SAM of evanescent waves like SPPs is transverse and points oppositely when the propagation wave vector is reversed; this is known as the photonic quantum spin Hall effect (QSHE). I demonstrate that QSHE affects the function of an SPP lens coupling structure through a vectorial superposition of longitudinally and transversely coupled SPP waves that are launched by TE waves v (s-polarized) and TM waves (p-polarized), respectively. Finally, I combine my understanding of SPP generation and imaging in a normal-incidence PEEM measurement to explore SPP dynamics when formed by an Archimedean spiral coupling structure. The geometrically defined phase structure of such SPP fields generates plasmonic vortices, whose singularities and time evolution are imaged by PEEM. Based on simulations, I conclude that the SPPs SAM distribution at the vortex core has a stable topological texture of a Néel type Skyrmion, and experimentally locate it by imaging the SPP field singularities.

Coupled Plasmon Wave Dynamics beyond Anomalous Reflection: A Phase Gradient Copper Metasurface for the Visible to Near-Infrared Spectrum

“…The huge difference between the theoretical SPP propagation wavelengths of the air-Cu and SiO2-Cu interfaces, along with the SPP wavevector, is shown in Figure S2.1b,c. Theoretically, the SPP propagation lengths for the Cu-SiO2 interface were 1.77 μm and 3.8 μm Thanks to the phase gradient metasurface, light could be coupled to the SPP mode from a normal incidence beam with a flat two-dimensional geometry [23][24][25][26]. The detailed mechanisms are explained in Section S2.1 of the Supplementary Materials [27][28][29][30][31].…”

“…The nature of this guided surface wave was related to its plasmonic counterpart through the metal-insulator-metal cavity, especially when it was preferentially unidirectional due to phase gradient metallic nanoantenna. Waveguide mode coupling using phase gradient metasurfaces has been reported in the literature, in which the asymmetric optical power transmission and mode conversion occur on an oppositely propagating beam in the cavity [14,22,25,29,40,41]. When the incidence waveguide mode propagated toward the phase gradient direction of the metasurface, the wave vector increases continuously and the optical power coupled to the propagating surface wave.…”