Coherent coupling in light-matter systems

Abstract

The strength of interaction between light and matter states is characterised by the Rabi frequency Ω which describes the rate at which they exchange energy. When Ω exceeds the system linewidth then the system is correctly described by dressed eigenstates of the coupled light-matter system. This is the strong coupling regime. Strong coupling has been demonstrated at the surface of polar dielectrics between phonon oscillations of the ionic species and light resulting in evanescent field confinement in modes termed surface phonon polaritons (SPhPs ). Recent fabrication advances allow for creation of user defined SPhP resonators that support sub-diffraction SPhPs confined in three dimensions. We present a numerical study of these localised SPhP resonances in subwavelength cylindrical SiC resonators finding modes with long lifetimes and strong electric field enhancements. Additionally we investigate numerically and experimentally interactions between the localised SPhPs and the delocalised SPhP of a planar SiC interface demonstrating that this can also be coherent. Strong light-matter coupling is also possible when a material with an resonant elec- tronic transition such as an organic semiconductor is sandwiched in a planar optical microcavity. By employing a molecule with a broad electronic transition it is possible to reach the ultrastrong coupling (USC) regime where the coupling frequency becomes an appreciable fraction of the bare frequency and new physics is predicted. We investigate experimentally the electroluminescence properties of an optical microcavity in this regime utilising for the active layer the oligomer TDAF. We demonstrate light-matter couplings exceeding 20% of the bare resonant frequencies. Our bias resolved measure- ments allow for conclusions about the process of polariton formation under electrical pumping to be drawn. Finally we present a method to describe the coupled eigenstates in arbitrary, inhomo- geneous dielectric environments. By following Hopfield procedure we show that it is possible to diagonalise and correctly quantise the modes of the system. We also show how this model can be extended to include losses and illustrate an application to the specific case of the SPhP at a planar interface.