We develop a many-body theory for the properties of exciton-polaritons interacting strongly with a Bose-Einstein condensate (BEC) of exciton-polaritons in another spin state. Interactions lead to the presence of a two-body bound state, the biexciton, giving rise to a Feshbach resonance in the polariton spectrum when its energy is equal to that of two free polaritons. Using the minimal set of terms to describe this resonance, our theory recovers the main findings of two experiments probing interaction effects for upper and lower polaritons in a BEC. This strongly supports that Feshbach physics has, indeed, been realized, and we furthermore extract the energy and decay of the biexciton from the experimental data. The decay rate is predicted to be much larger than that coming from its dissociation into two free polaritons, indicating that other decay channels are important.

• Received 29 August 2019
• Revised 18 October 2019

DOI:https://doi.org/10.1103/PhysRevB.100.195301