Strong light-matter coupling in plasmonic microcavities

We numerically study the strong coupling between quantum well (QW) intersubband transitions (ISBT) and the plasmonic resonance of metal-dielectric-metal (MDM) microcavities. In this system, the lowest-order energy state of plasmonic resonance is a hybrid mode of propagating surface plasmons (PSP) and localized surface plasmons (LSP). For a given lowest-order resonance, the mode transformation can be realized between PSP mode and LSP mode by varying the plasmonic microcavity structure, which opens a new freedom to modulate the coupling interaction of light and matter. With the cavity mode transforming from LSP mode to PSP mode, the coupling strength increases from 20.75% to 25.75%, which is mainly dominated by the polarization conversion ratio 〈Ez2〉/〈E2〉 of plasmonic modes.