Rayleigh scattering is usually considered to be the elastic scattering of photons from subwavelength physical objects, such as small particles or molecules. Here, we present a quantitative spectroscopic study of the scattering properties of molecules embedded in an optical cavity under strong coupling conditions, where the collective interaction between the molecules and the cavity gives rise to composite light-matter excitations known as cavity polaritons. We show that the polaritonic states exhibit strong resonant Rayleigh scattering, which depends on both the coupling strength and detuning and reaching ∼25% efficiency. Since the polaritonic wave functions in such systems are delocalized, our observations correspond to the collective scattering of each photon from a large ensemble of molecules.
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