Fig. 1. Cavity-free strong light-matter coupling.

a Sample design: SPI/MC film on silicon substrate. b Ellipsometer schematic. c Complex permittivities ϵ of SPI and MC, as derived from ellipsometry for a film of thickness 109 nm. d, e Dispersion plots constructed using the ellipsometric parameter Ψ for d SPI and e MC films over a range of thicknesses at fixed angle θ = 65^°. The dashed lines in d, e indicate the positions of the uncoupled TE (black) and TM (white) leaky modes. The green dashed line in e at E = 2.22 eV indicates the position of the MC molecular resonance. The solid lines show the predicted positions of polariton branches using the 2N coupled oscillator model. The coupled TE polariton branches (black) were fit with a coupling strength g = 225 meV. The TM₂ and TM₃ polariton branches (white) were fit with g = 185 meV and g = 200 meV, respectively. We did not perform a coupled oscillator fit for the TM₁ mode since it does not show any clear anticrossing.