Spectroelectrochemical measurement and modulation of exciton-polaritons

Quantum emitters strongly coupled to optical cavity modes create new hybrid states called polaritons, resulting in a vacuum Rabi splitting (omega). Strikingly, the magnitude of this splitting correlates with modified emission properties and chemical reaction rates. However, active control of this coupling strength is difficult due to the fixed properties of the coupled oscillators (both the quantum emitter and optical resonator). Here, we demonstrate active tuning of excitonic strong coupling in a system where organic dyes strongly couple to propagating surface plasmon polaritons (SPPs). After electropolymerization of a methylene blue (MB) film on a SPP-supporting Au surface, we demonstrated active control of coupling strength through reversible redox cycling of the MB film. Excitonic strong coupling was effectively cycled on and off with the electrode potential either continuously tuned (transient) or held at a fixed value (static) and was quantitatively correlated with the simultaneously measured electrochemical charge. Switching between reduced and oxidized forms of the dye resulted in omega values tuned from similar to 0 meV to similar to 280 meV, i.e., similar to 14% of the transition energy. The ability to control coupling strengths in a given emitter-cavity coupled system is a key capability for utilizing polaritonic states for cavity-mediated chemical reactions or optical devices.