Hybrid surface waves at microwave frequencies in superconductor–dielectric layered structures

Surface waves occur at the interfaces between distinct media, with several types identified across different paradigms, including electromagnetic and mechanical surface waves. In this work, I thoroughly investigate the occurrence of microwave surface waves at the interface between superconducting and dielectric layers. I theoretically demonstrate that a hybrid surface wave, combining electromagnetic (EM) fields and supercurrent vortices, propagates along the superconductor–dielectric interface. The associated EM fields occupy the dielectric layer, satisfying Maxwell’s equations, while the supercurrent vortices penetrate the superconducting layer, governed by Ampèr’s and London’s equations. Furthermore, I show that an extended superconductor–dielectric–superconductor (S–D–S) nanostructure can support strong nanoconfinement of these hybrid microwave surface waves, reaching spatial dimensions well below the diffraction limit. The propagating modes exhibit frequency-independent dispersion, enabling high-capacity data transmission. In addition to being intrinsically lossless, these modes can be slowed by carefully tuning the nanostructure dimensions. Importantly, our investigations reveal two key findings: (1) the divergence of the supercurrent vortices is zero, indicating no net charge (Cooper pair) transport associated with the propagating modes and (2) the energy flow of the propagating modes is carried by the associated EM fields. Hence, the proposed surface wave scheme offers a promising route for long-coherence signal transmission in superconducting circuits. I note that this paradigm of microwave surface waves has the potential to enable a new class of superconducting devices.