Abstract
We study localized surface plasmons in strong coupling with excitons of organic semiconductors. The plasmons are localized in lithographed silver nanodisks organized in square array. A giant Rabi splitting energy of 450 meV is obtained, and typical behaviors of mixed states, i.e., anticrossing of their energies and crossing of their linewidths, are observed. Three-dimensional finite-difference time-domain simulations and coupled oscillator calculations are used to analyze and corroborate the experimental results.