In-Plane and Out-of-Plane Investigation of Resonant Tunneling Polaritons in Metal–Dielectric–Metal Cavities

[Image: see text] Polaritons can be generated by tuning the optical transitions of a light emitter to the resonances of a photonic cavity. We show that a dye-doped cavity generates resonant tunneling polaritons with Epsilon-Near-Zero (ENZ) effective permittivity. We studied the polariton spectral dispersion in dye-doped metal-dielectric-metal (MDM) cavities as a function of the in-plane (k(||)) and out-of-plane (k(⊥)) components of the incident wavevector. The dependence on k(||) was investigated through ellipsometry, revealing the ENZ modes. The k(⊥) dependence was measured by varying the cavity thickness under normal incidence using a Surface Force Apparatus (SFA). Both methods revealed a large Rabi splitting well exceeding 100 meV. The SFA-based investigation highlighted the collective nature of strong coupling by producing a splitting proportional to the square root of the involved photons. This study demonstrates the possibility of generating ENZ polaritons and introduces the SFA as a powerful tool for the characterization of strong light–matter interactions.