Using surface plasmonics to turn on fullerene's dark excitons

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Using our recently proposed Bethe-Salpeter G₀W₀ formulation, we explore the optical absorption spectra of fullerene (C₆₀) near coinage metal surfaces (Cu, Ag, and Au). We pay special attention to how the surface plasmon ωₛ influences the optical activity of fullerene. We find the lower energy fullerene excitons at 3.77 and 4.8 eV only weakly interact with the surface plasmon. However, we find the surface plasmon strongly interacts with the most intense fullerene π exciton, i.e.the dipolar mode at ℏω+≈ 6.5 eV, and the quadrupolar mode at ℏω-≈ 6.8 eV. When fullerene is close to a copper surface (z₀≈ 5.3 Å) the dipolar mode ω+ and "localized" surface plasmons in the molecule/surface interface hybridize to form two coupled modes which both absorb light. As a result, the molecule gains an additional optically active mode. Moreover, in resonance, when ωₛ≈ω±, the strong interaction with the surface plasmon destroys the ω- quadrupolar character and it becomes an optically active mode. In this case the molecule gains two additional very intense optically active modes. Further, we find this resonance condition, ωₛ ≈ ω±, is satisfied by silver and gold metal surfaces.