Yanan Dai




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 free-electrons of metals. I image and study the ultrafast dynamics of SPP fields by interferometric time-resolved 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 SPPs waves that are launched by TE waves (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.




Graduate Advisor

Hrvoje Petek