Strong exciton–plexciton coupling in nickel phthalocyanine thin films on gold

Strong light–matter coupling is demonstrated in a simple, cavity-free structure consisting of nickel phthalocyanine thin films deposited on gold. Variable-angle spectroscopic ellipsometry, recorded in both conventional and total internal reflection geometries, reveals hybrid exciton–plasmon (plexciton) states with a Rabi splitting up to 0.5 eV. From the ellipsometric data, the transition dipole moment, coupling strength, and Hopfield coefficients are evaluated. Strong coupling is evidenced by abrupt phase shifts in the ellipsometric parameter Δ coinciding with minima in the amplitude parameter Ψ, as well as by topologically distinct ρ-trajectories in the complex reflection plane exhibiting winding numbers of two. Two-dimensional and three-dimensional representations of ρ as a function of the angle of incidence further highlight these signatures of strong coupling. The observed effects depend sensitively on both film thickness and molecular density. The results establish a platform for exploring polaritonic phenomena and suggest potential applications in nanoscale sensing, light-harvesting, and organic optoelectronics.