Long-propagating ghost phonon polaritons enabled by selective mode excitation

Abstract

The ability to precisely control the excitation of phonon polaritons (PhPs) provides unique opportunities for various nanophotonic applications, such as on-chip optical communication, quantum information processing, and controlled thermal radiation. Recently, ghost hyperbolic phonon polaritons (g-HPs) have been discovered, which exhibit in-plane hyperbolic dispersion on the surface and oblique wavefronts in the bulk. These g-HPs exhibit long-range, ray-like propagation, which is highly desirable. However, selective excitation of polaritonic modes and flexible control over the directionality of g-HPs remains an open problem. In this work, we experimentally demonstrate that changing the shape of the launching micro/nano antenna allows for control over the polariton mode excitation. Using a single asymmetric triangular gold antenna fabricated on a calcite crystal surface, we showcase highly directional g-HP excitation through selectively exciting desirable polariton modes. Our near-field imaging experiments verify that the g-HP excited by the triangular antenna can propagate over 80 microns, which is consistent with our numerical predictions. Overall, by combining g-HP theory with structural engineering, our work has further developed the potential of such anisotropic materials, enabling unexpected control over g-HPs, thus opening opportunities for various applications in mid-IR optoelectronics.