Ultrastrong coupling of vibrationally dressed organic Frenkel excitons with Bloch surface waves in a one-sided all-dielectric structure

We demonstrate a transition from weak, to strong, to ultrastrong coupling of Frenkel molecular excitons and Bloch surface wave photons at room temperature using a one-sided, all-dielectric optical structure. The all-dielectric structure comprises an organic semiconductor thin film on the surface of a distributed Bragg reflector. We investigate the evolution of multiple vibronic polariton branches and their dominant absorption peaks as a function of coupling and in-plane momentum, which is absent in previous ultrastrong coupling systems. Measurements are interpreted using both the transfer matrix method and a coupled-oscillator model without the rotating wave approximation. The dependence of Rabi splitting on the number of excitons and electrical field amplitude is also modeled showing a transition to ultrastrong coupling at film thicknesses $\ensuremath{\ge}50$ nm. This low-loss polaritonic structure enables us to study phenomena such as organic exciton-polariton dynamics, ultralong range polariton propagation, and high efficiency energy transport in the ultrastrong coupling regime at room temperature.