Generalized Josephson plasmons in bilayer superconductors

Layered superconductors like high-Tc cuprates display out-of-plane plasma oscillations between layers sustained by the weak Josephson coupling among the superconducting sheets, the so-called Josephson plasmons. Bilayer cuprates hosts two of such modes. However, due to the anisotropy of the electronic response, their description at generic wavevector cannot be separated from that of the in-plane oscillations. In this paper, we provide an analytical theoretical framework to describe the dispersions and the polarizations of the generalized plasma modes of such systems, an aspect partly addressed in previous literature. We employ this framework to explain the peculiar characteristics of their linear optical response, by providing a fully microscopic explanation for the appearance of a finite-frequency peak in the real part of the optical conductivity. From a wider perspective, the complete characterization of the Josephson plasma modes provided by our approach represents a groundwork to address open issues raised by recent experiments with strong THz pulses, able to drive them beyond the linear-response regime.