Breakdown of Spin-to-Helicity Locking at the Nanoscale in Topological Photonic Crystal Edge States
S. Arora (TU Delft - QN/Kuipers Lab, Kavli institute of nanoscience Delft)
Thomas Bauer (Kavli institute of nanoscience Delft, TU Delft - QN/Kuipers Lab)
N. Parappurath (AMOLF Institute for Atomic and Molecular Physics)
R.T. Barczyk (AMOLF Institute for Atomic and Molecular Physics)
E. Verhagen (AMOLF Institute for Atomic and Molecular Physics)
L. Kuipers (TU Delft - QN/Quantum Nanoscience)
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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.