Anomalous unidirectional excitation of high-k hyperbolic modes using all-electric metasources

Hyperbolic metamaterials (HMMs), an important class of artificial anisotropic material with hyperbolic isofrequency contours (IFCs), have recently attracted significant attention due to their unique ability to control interactions between light and matter.^{1 – 4} Tuning the hyperbolic dispersion shape allows light propagation in HMMs to be flexibly controlled to produce all-angle negative refraction,^{5 – 8} collimation,^9,10 beam splitting,^11,12 and robust transmission.^13,14 Tuning the topological transition of the dispersion from a closed IFC to an open hyperbolic IFC significantly enhances the optical density of states (DOS). This has important consequences for the strong enhancement of spontaneous emission.^15,16 In addition, HMMs can convert evanescent waves into propagating waves with large wave vectors. This property has enabled the demonstration of superresolution imaging that overcomes the diffraction limit^17,18 and long-range dipole–dipole interactions beyond the near-field coupling limitation.^{19 – 21} Interestingly, under near-field excitation, electromagnetic (EM) waves in HMMs propagate along fixed channels at the subwavelength scale. This is similar to crossing waveguides.²² These extraordinary guided modes with strong spatial localization correspond to high-$k$ modes with large DOSs in HMMs. In 2014, Kapitanova et al.²³ experimentally demonstrated the photonic spin Hall effect (PSHE) based on spin-orbit coupling of extraordinary guided modes in HMMs. This optical spin-orbit locking phenomenon comes from the transverse spin property of evanescent waves.^{24 – 31} Unlike with the PSHE in surface plasmon polaritons at a metal–dielectric interface,^{32 – 34} the directional excitation of spin dipoles in HMMs occurs inside the bulk of the structure. This greatly broadens the near-field coupling control scope.³⁵ Although the spin dipole can achieve unidirectional excitation based on spin-orbit coupling, switchable and flexibly controlled unidirectional excitation in HMMs is a highly concerning topic.³⁶ Diversified unidirectional emission may facilitate new wireless power transfer³⁷ and information communications³⁸ applications.