Topological Bound States in the Continuum: A New Paradigm for Light Confinement

Authors: H.M. Doeleman, W. den Hollander, A. Al├╣, A.F. Koenderink, F. Monticone

Source: FERMAT, Volume 30, Communication 4, Nov.-Dec., 2018

Abstract: Light confinement is one of the most common and important operations in optics, photonics, and electromagnetics. Recently, a fundamentally new approach to light confinement has been explored by different groups, based on the concept of bound states in the radiation continuum (BICs), or embedded eigenstates. While conventional bound states in, e.g., fibers, waveguides, and resonators are created by preventing their coupling to radiation modes through symmetry incompatibility, momentum mismatch, or by directly suppressing outgoing waves, BICs are compatible with free-space radiation, but remain confined due to the destructive interference between different radiation channels. Two recent works experimentally demonstrated, for the first time, a defining feature of BICs in planar structures, namely, the fact that the light radiated by these modes, around the BIC, exhibits a vortex in its polarization far-field profile. This radiation feature is associated to an integer winding number, or topological charge, which is unaffected by continuous deformations of the structure. Hence, the presence of the polarization vortex bestows topological robustness to the BIC. For our experimental demonstration, we fabricated a dielectric metasurface supporting a BIC at optical frequencies. Using momentum-space polarimetry, we measured the full polarization state of the reflected light, highlighting the presence of the topological vortex. The essential physics can be captured in a general, new, theoretical framework that identifies BICs as radiationless distributions of induced currents of electric and magnetic types. This model fully accounts for the topological nature of the BIC, showing how the vortex must appear right at the point of destructive interference between two radiation channels.

Index Terms: Bound states in the continuum, embedded eigenstates, topological electromagnetics, optical vortices, scattering.

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Topological Bound States in the Continuum: A New Paradigm for Light Confinement