Spin-orbit interactions in noncentrosymmetric plasmonic crystals probed by site-selective cathodoluminescence spectroscopy

TL;DR

This paper investigates spin-orbit coupling in noncentrosymmetric plasmonic crystals using combined theoretical and experimental cathodoluminescence spectroscopy, revealing energy band splitting and polarization-dependent scattering linked to surface plasmon spin angular momentum.

Contribution

It provides the first combined theoretical and experimental analysis of spin-orbit interactions in noncentrosymmetric plasmonic crystals, highlighting the dependence on excitation position and polarization.

Findings

Energy band splitting due to broken symmetry observed

Polarization-dependent scattering of surface plasmons demonstrated

Spin angular momentum influences plasmon propagation direction

Abstract

The study of spin-orbit coupling (SOC) of light is crucial to explore the light-matter interactions in sub-wavelength nanostructures with broken symmetries. In noncentrosymmetric photonic crystals, the SOC results in the splitting of the otherwise degenerate energy bands. Herein, we explore the SOC in a noncentrosymmetric plasmonic crystal, both theoretically and experimentally. Cathodoluminescence (CL) spectroscopy combined with the numerically calculated photonic band structure reveals an energy band splitting that is ascribed to the broken symmetries in the noncentrosymmetric plasmonic crystal. By shifting the impact position of the electron beam throughout a unit cell of the plasmonic crystal, we show that the emergence of the energy band splitting strongly depends on the excitation position of the surface plasmon (SP) waves on the crystal. Moreover, we exploit angle-resolved CL and dark-field polarimetry to demonstrate polarization-dependent scattering of SP waves interacting with the plasmonic crystal. The scattering direction of a given polarization is determined by the transverse spin angular momentum inherently carried by the SP wave, which is in turn locked to the direction of SP propagation. Our study gives insight into the design of novel plasmonic devices with polarization-dependent directionality of the Bloch plasmons. We expect spin-orbit plasmonics will find much more scientific interests and potential applications with the continuous development of nanofabrication methodologies and uncovering new aspects of spin-orbit interactions.