Exact Degeneracies in Guided Modes of Biaxial Dielectric Waveguides*

Lifting of degeneracies in a composite system through interaction among its constituents is a generic theme of all physical systems - from splitting of s-orbital energies when two hydrogen atoms are brought nearby to the frequency splitting of propagating modes in optical waveguides. In this work we challenge this well-accepted notion by presenting a new mechanism of mode coupling that does not lift degeneracies. The mechanism is illustrated through the interaction of recently discovered ghost waves (Adv. Photon. 1(4), 046003(2019)), which are a special class of nonuniform electromagnetic waves within biaxial anisotropic materials. The resulting “ghost coupling” offers a fundamentally new way of controlling light-matter interaction with a broad range of potential applications, from integrated waveguides to nonlinear optics and optical sensing.