Ghost Line Waves

Time-harmonic electromagnetic plane waves in anisotropicmediacan exhibit complex-valued wavevectors (with nonzero real and imaginaryparts) even in the absence of material dissipation. These peculiarmodes, usually referred to as "ghost waves", hybridizethe typical traits of conventional propagating and evanescent waves,displaying both phase accumulation and purely reactive exponentialdecay away from the direction of the power flow. Their existence hasbeen predicted in several scenarios and has been recently observedexperimentally in the form of surface phonon polaritons with complex-valuedout-of-plane wavevectors propagating at the interface between airand a natural uniaxial crystal with slanted optical axis. Here, wedemonstrate that ghost waves can arise also in lower-dimensional flat-opticsscenarios, which are becoming increasingly relevant in the contextof metasurfaces and in the field of polaritonics. Specifically, weshow that planar junctions between isotropic and anisotropic metasurfacescan support "ghost line waves" that propagate unattenuatedalong the line interface, exhibiting phase oscillations combined withevanescent decay both in the plane of the metasurface (away from theinterface) and out-of-plane in the surrounding medium. Our theoreticalresults, validated by finite-element numerical simulations, demonstratea novel form of polaritonic waves with highly confined features, whichmay provide new opportunities for the control of light at the nanoscaleand may find potential applications in a variety of scenarios, includingintegrated waveguides, nonlinear optics, optical sensing, and subdiffractionimaging.