Dispersion of third-harmonic generation in ultrastrongly coupled organic cavity polaritons

Organic cavity polaritons are bosonic quasi-particles that arise from the strong interaction between organic molecular excitons and photons within microcavities. The spectral dispersion of third harmonic generation near resonance with the cavity polariton states is studied experimentally via angle-resolved reflected third harmonic generation measurements with several pump wavelengths. In addition, a three-step nonlinear optical transfer matrix model is used to simulate the third harmonic generation using the sum-over-states dispersive nonlinear coefficients, which include hybrid exciton-photon polariton states. The angle-dependent experiment and modeling agree, revealing that the output of third harmonic generation is resonantly enhanced when the third harmonic wavelength is near the upper polariton state. The degree of enhancement is higher on the exciton-like branch of the polariton dispersion. Lower polariton enhanced third harmonic generation is not experimentally observed due to inadequate coupling at the longer wavelength, which is also consistent with the nonlinear transfer matrix modeling. These results indicate that the sum-over-states nonlinear dispersion is descriptive of the process implying that the polariton states are terms in a complete set of states forming the basis for the perturbative calculation of the nonlinear optical response. The results also indicate the possibility of wavelength agile nonlinear optical response by angle-of-incidence tuning.