### QuMat seminar

## Local investigations of topological edge states: robustness and breakdown of spin-momentum locking Speaker: Kobus Kuipers – Delft University of Technology |

Abstract:

Edge states between topologically distinct, non-trivial photonic crystals hold the promise of robustness against backscattering. The origin of this robustness lies in the spin-momentum locking of the edge states, i.e., counterpropagating edge states have an opposite spin. As a result, only spin-flipping scattering events can result in a reversal of the propagation direction. Combined with the inherent optical spin properties of the edge states they hold a large promise for quantum information processing. In this talk I will show that the edge states are indeed highly robust against certain types of backscattering defects, resulting in two orders of magnitude reduced back scattering as compared to non-optimized W1 photonic crystal waveguides [1]. For the Spin-Hall mimicking edge state, we find a direct one-to-one relation between the far-field optical spin of the state, which is close to unity, and its direction of propagation [2]. However, in the near-field the simple spin-momentum locking breaks down: the spatial distribution of the spin density is highly heterogeneous and the unit-cell averaged spin is close to zero. Moreover, for certain frequencies its spin has an opposite sign to the far-field radiated light. This behavior can be understood by taking higher order Bloch harmonics into account [3]. Finally, we’ll address the question what happens to an edge state when disorder is introduced which, by definition breaks the symmetry-induced, topological protection or when it is confronted with a symmetry-preserving mirror [4].

References

[1] S. Arora, et al., njp Light: Science & Applications 10, 1-7 (2021)

[2] N. Parappurath, et al., Science Advances 6, eaaw4137 (2020)

[3] S. Arora, et al., Phys. Rev. Lett. 128, 203903 (2022)

[4] unpublished