Monopole embedded eigenstates in nonlocal plasmonic nanospheres

In recent years, the confinement of light in open systems with no radiation leakage has raised great interest in the scientific community both due to its peculiar and intriguing physics and due to its important technological applications. In particular, materials with near-zero permittivity offer a unique opportunity for light localization, as they enable the formation of embedded eigenstates in core-shell systems with suppressed radiation loss. For all the solutions presented thus far in the literature, the exact suppression of the radiation leakage can occur only when the size of the resonator is delicately tuned. Surprisingly, here, it is shown that the tuning of the resonator radius may be unnecessary, and nonlocal metal spherical nanospheres of any size may support multiple embedded eigenstates with monopole-type symmetry.