Strong Coupling of Molecular Vibrational Resonances

Abstract

Strong coupling of molecules placed in an optical microcavity may lead to the formation of hybrid states called polaritons; states that inherit characteristics of both the optical cavity modes and the molecular resonance. This is possible for both excitonic and vibrational molecular resonances. Previous work has shown that strong coupling may be used to hybridize different excitonic resonances, this can be achieved when more than one molecular species is included in the cavity. In this thesis I show that under suitable conditions different molecular vibrational resonances of the same molecular unit may also be coupled together, the resulting polariton having characteristics of all vibrational resonances. I will also demonstrate strong coupling between surface plasmon resonances and molecular vibrational resonances of polymethylmethacrylate (PMMA) molecules in the mid-infrared range through the use of grating coupling, complimenting earlier work using microcavities and localised plasmon resonances. Many experiments involving strong coupling make use of metal-clad microcavities, ones with metallic mirrors. Metal-clad microcavities are well known to support coupled plasmon modes in addition to the standard microcavity mode. However, the coupled plasmon modes associated with an optical microcavity lie beyond the light-line and are thus not probed in typical experiments on strong coupling. I will investigate, through experiment and numerical modelling, the interaction between molecules within a cavity and the coupled plasmon mode and I will show that such modes do undergo strong coupling, making use of grating coupling to provide an experimental demonstration. Overall, light−matter hybridization offers many new opportunities for the molecular and materials sciences. It works in the absence of light, it is simple to implement, and its full potential is waiting to be explored.