Long-propagating ghost phonon polaritons enabled by selective mode excitation.

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.