Past QuMat seminars in Utrecht

The QuMat seminars in Utrecht is a bi-weekly joint venture between Utrecht’s experimental and theoretical condensed matter physics groups.
Coming QuMat seminars are listed here.

Videos from previous talks are available on youtube and listed here.


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].

[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[4] unpublished

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