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.

European Research Council (ERC) , The Netherlands Organisation for Scientific Research (NWO) , Philips
Photonic Forces

Arora, S., Bauer, T., Parappurath, N., Barczyk, R., Verhagen, E., & Kuipers, K. (2022). Breakdown of Spin-to-Helicity Locking at the Nanoscale in Topological Photonic Crystal Edge States. Phys.Rev.Lett., 128(20), 203903: 1–6. doi:10.1103/PhysRevLett.128.203903