Tunable Third-Harmonic Generation from Polaritons in the Ultrastrong Coupling Regime

Strong interparticle interactions between polaritons have traditionally stemmed from their exciton component. In this work, we impart a strong third-order nonlinearity to a polaritonic mode by embedding a nonlinear polymethine dye within a high- Q all-metal microcavity. We demonstrate nonlinear microcavities operating in the ultrastrong coupling regime with a normalized coupling ratio of 62%, the highest reported to date. When pumping the lower polariton branch, we observe tunable third-harmonic generation spanning the entire visible spectrum, with internal conversion enhancements more than 3 orders of magnitude larger than in bare films. Transfer matrix calculations indicate that the observed enhancements are consistent with the enhanced pump electric field.