Strong coupling beyond the light-line

Many experiments on strong coupling of molecules make use of metal-clad microcavities. Coupling is usually investigated between the excitonic or vibronic molecular resonance (as appropriate) and the lowest order cavity mode. However, metal-clad microcavities also support a coupled surface plasmon mode, something that appears to have been ignored in previous work, probably because it exists beyond the light-line. Here we show that this coupled plasmon mode also interacts with molecular resonances to produce hybrid polariton modes.
We use a vibrational resonance in the polymer PMMA, incorporated into a microcavity with gold mirrors. Although the asymmetrically coupled surface plasmon (TM_-1 ) mode has no cut-off, it lies outside the air light-line, we bring that mode inside the air light line by incorporating a grating structure into the lower gold mirror. The grating-scattered coupled plasmon mode is observed to strongly couple with the vibrational resonance. Our results indicate that this coupled plasmon mode should be taken into account when looking at how strong coupling may be used to alter/create molecular properties via strong coupling.