Plasmon coupling in topological insulator multilayers

Topological insulators (TIs) house two-dimensional Dirac plasmons on their surfaces. In TI thin films, plasmons on the top and bottom surfaces couple electrostatically, giving rise to an acoustic and an optical plasmon mode. By extension, a superlattice comprising TI layers and trivially insulating layers could house multiple complex plasmon modes which could be used to create a new type of Dirac metamaterial. In this paper, we synthesize TI superlattices, fabricate them into stripe arrays, characterize their optical and plasmonic properties, and model the samples using transfer matrices. We excite plasmon modes that couple to the phonons in the superlattice, resulting in hybrid plasmon-phonon polaritons. These modes are modeled using an analytical Fano resonance model and the extracted resonant positions are reproduced by transfer matrix modeling. We also excite an epsilon near-zero mode in the top dielectric material ${({\mathrm{Bi}}_{0.5}{\mathrm{In}}_{0.5})}_{2}{\mathrm{Se}}_{3}$. Understanding the behavior of the polariton modes in this complex system will lend insight into the many-body interaction in low-dimensional systems.