We theoretically study spherical cavities composed of hyperbolic metamaterials with indefinite
permittivity tensors. Such cavities are capable of electrodynamically confining fields with deep
subwavelength cavity sizes. The supported resonant modes are analogous to the whispering-gallery
modes found in dielectric microcavities with much larger physical sizes. Because of the nature of
electrodynamical confinement, these hyperbolic metamaterial cavities exhibit quality factors higher than
predicted in the electrostatic limit. In addition, confining electromagnetic fields into the small cavities
results in an extremely high photonic local density of states.
Wu, Chihhui, Alessandro Salandrino, Xingjie Ni, and Xiang Zhang. "Electrodynamical Light Trapping Using Whispering-Gallery Resonances in Hyperbolic Cavities." Phys. Rev. X Physical Review X 4.2 (2014): n. pag. http://dx.doi.org/10.1103/PhysRevX.4.021015