Unveiling Rabi splitting in a hybrid plasmon–exciton polaritonic system for refractometry

We computationally report strong plasmon–exciton coupling and quantitative Rabi splitting in a hybrid plasmon–exciton polariton platform designed for ultrasensitive refractometric sensing. Strong coupling between surface plasmon polaritons in metal and excitonic transitions in an organic semiconductor gives rise to plexcitons, i.e., upper and lower plexciton modes, causing pronounced anti-crossing. Rabi splitting energies ranging from 0 to 250 meV are achieved by tuning the organic semiconductor layer thickness and analyte refractive index, confirming operation in the strong-coupling regime. The highly dispersive upper plexciton mode (at wavelength, λ = 532 nm) enables refractive index sensing across gaseous and aqueous environments. The incorporation of an ultra-thin silicon (Si) interlayer (∼5 nm) atop silver (Ag) results in maximum sensitivity of ∼252° per refractive index unit, with further tunability achieved by varying the Si thickness and the analyte refractive index. Therefore, this device establishes a tunable, high-performance polaritonic sensing platform and provides insight into plasmon–exciton coupling for advanced nanophotonic device applications.