The pursuit of wave control in artificial media has been expanded with tools borrowed from topology, enabling exotic phenomena such as unidirectional transport, robustness to fabrication imperfections, and immunity to back-scattering from sharp corners. To this end, the photonic valley Hall effect is particularly promising for photonic applications as it enables unidirectional and topologically protected propagation of light and requires no magnetic effects but only needs inversion symmetry breaking and is realized using low-loss all-dielectric materials. However, implementing such topological waveguiding structures in compact dimensions on a photonic chip requires particular concern for radiative losses. In this connection, the combination of the valley Hall effect with non-radiative states of light, also known as bound states in the continuum, is of particular interest. Yadian Feng et al. propose bound valley edge states in the continuum formed at the interface between air and a single valley photonic crystal with topological features. These states do not require the adjoint photonic crystal of another topological charge to prevent radiation and hence allow reduction of the structure’s size, making it feasible for photonic chip applications. Further development of the approach is possible by exploring topological modes of other nature.

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