Ultrastrong coupling of acoustic graphene plasmons and polar phonons via extreme near-field localization

Abstract

Acoustic plasmons in graphene offer the advantage of exceptionally small mode volumes that is challenging to achieve with conventional metal plasmonics in the mid-infrared range. In our work, we harness this unique characteristic to experimentally demonstrate ultrastrong coupling between acoustic plasmons and the surface optical phonons within silicon dioxide films that are thinner than λ/1, 000. Unlike conventional graphene plasmons, where energy splitting diminishes with decreasing thickness of a polar dielectric film, acoustic plasmons consistently maintain a strong coupling regime since the plasmon confinement increases accordingly. Our findings validate a theoretical hypothesis that energy splitting in the strong coupling regime primarily hinges on the concentration of molecules for a given mode volume. We attribute the observed Rabi splitting to the strong coupling of acoustic plasmons with the epsilon-near-zero mode originating from longitudinal optical phonons