Breakdown of spin-to-helicity locking at the nanoscale in topological photonic crystal edge states

TL;DR

This study investigates the complex local spin structure of topological photonic edge states at the nanoscale, revealing a breakdown in the expected spin-helicity locking due to higher-order effects, which impacts light-matter interactions.

Contribution

It provides the first detailed experimental and numerical analysis of local spin distributions in topological photonic edge states, highlighting the breakdown of spin-helicity locking at the nanoscale.

Findings

Local spin density is highly structured and not linked to a unique pseudospin.

Global spin is significantly reduced and can be flipped at certain frequencies.

Higher-order Bloch harmonics cause spin inhomogeneity and disrupt spin-helicity coupling.

Abstract

We measure the local near-field spin in topological edge state waveguides that emulate the quantum spin Hall effect. We reveal a highly structured spin density distribution that is not linked to a unique pseudospin value. From experimental near-field real-space maps and numerical calculations, we confirm that this local structure is essential in understanding the properties of optical edge states and light-matter interactions. The global spin is reduced by a factor of 30 in the near field and, for certain frequencies, flipped compared to the pseudospin measured in the far-field. We experimentally reveal the influence of higher-order Bloch harmonics in spin inhomogeneity, leading to a breakdown in the coupling between local helicity and global spin.