Array of dipoles near a hyperbolic metamaterial: Evanescent-to-propagating Floquet wave transformation

We investigate the capabilities of hyperbolic metamaterials (HMs) to couple near-fields (i.e., evanescent waves) emitted by a two-dimensional periodic array of electric dipoles to propagating waves. In particular, large-order Floquet harmonics with transverse-magnetic polarization that would be evanescent in free space, and therefore confined near the array surface, are transformed into a propagating spectrum inside the HM and thus carry power away. Because of this property, independent of the finite or infinite extent of the HM, the power generated by an array of elementary electric dipoles is strongly enhanced and is mostly directed into the HM when the array is located near a HM surface. In particular, the power coupled to the HM exhibits narrow frequency features that can be employed in detection applications. The results shown in this paper provide a clear signature on wave dynamics in HMs. A link between the results pertaining to the case of an isolated dipole on top of HM and the planar array is found to be convenient in explaining both wave dynamics and spectral power distribution. The narrow frequency emission features appear in the array case only; they depend on its spatial periodicity and remarkably on the HM thickness.