Using surface plasmonics to turn on fullerene's dark excitons

TL;DR

This study uses a new theoretical approach to show how surface plasmons can activate dark excitons in fullerene, revealing strong interactions and new optically active modes near metal surfaces.

Contribution

The paper introduces a Bethe-Salpeter $G_0W_0$ formulation to analyze surface plasmon effects on fullerene excitons, highlighting how plasmon coupling activates dark excitons.

Findings

Surface plasmons strongly interact with the $ ext{π}$ exciton at 6.5 eV.

Hybridization creates additional optically active modes.

Resonance with silver and gold surfaces enhances exciton activity.

Abstract

Using our recently proposed Bethe-Salpeter $G_0W_0$ formulation, we explore the optical absorption spectra of fullerene (C$_{60}$) near coinage metal surfaces (Cu, Ag, and Au). We pay special attention to how the surface plasmon $\omega_S$ 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 $\pi$ exciton, i.e.\ the dipolar mode at $\hbar\omega_+\approx$ 6.5 eV, and the quadrupolar mode at $\hbar\omega_-\approx$ 6.8 eV. When fullerene is close to a copper surface ($z_0\approx$ 5.3 \AA) the dipolar mode $\omega_+$ 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 $\omega_S\approx\omega_\pm$, the strong interaction with the surface plasmon destroys the $\omega_-$ 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, $\omega_S \approx \omega_\pm$, is satisfied by silver and gold metal surfaces.