|
Light confinement is of fundamental importance in science and technology. In recent years, different groups have investigated a unique approach to confine and trap light in open structures, based on the concept of bound states in the radiation continuum, or embedded eigenstates. While conventional bound states are forbidden from coupling to the radiation continuum by symmetry, momentum mismatch, or direct suppression of outgoing waves, embedded eigenstates are compatible with free-space radiation, but remain confined due to destructive interference between different radiation channels. More generally, embedded eigenstates correspond to non-radiating eigenmodes of an open system, namely, radiationless eigenmodal distributions of conduction/polarization currents. In our talk, we will discuss our recent efforts on this exciting topic [Doeleman, H. M., Monticone, F., den Hollander, W., Alù, A. and Koenderink, A. F., “Experimental observation of a polarization vortex at an optical bound state in the continuum,” Nat. Photonics 12(7), 397–401 (2018); Monticone, F., et al., “Trapping Light in Plain Sight: Embedded Photonic Eigenstates in Zero-Index Metamaterials,” Laser Photon. Rev. 12(5), 1700220 (2018)], with particular focus on our recent demonstration of topologically-protected embedded eigenstates in dielectric metasurfaces at optical frequencies. We show experimental evidence that the embedded eigenstate of this structure corresponds to a vortex of the polarization state in wavenumber space, characterized by a quantized topological charge, which determines an inherent robustness under continuous deformations. We also present a dipole model that explains the embedded eigenstate in terms of destructive interference between two radiation channels and fully accounts for its topological nature.
|
