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Under extreme conditions, when the coupling between photon and exciton is sufficiently strong, a hybridised quantum state called polariton is formed. Polaritons exhibit intriguing features, such as Bose-Einstein condensation and Rabi splitting, and have applications in many areas, including molecular sensors, light harvesting and quantum optical devices.
Previously polaritons are often produced in microcavities at low temperatures, with a cavity volume at the order of m3. Plasmonic junctions provide extreme confinement and enhancement of optical fields within ~nm3 cavity, about 8-9 orders smaller than that of microcavity, thus producing extremely strong Purcell effect, which renders the observation of strong-coupling between plasmon and exciton at room temperature, so called plexciton.
Here we report the observation of strong coupling between localised surface plasmon (LSP) and the excitons of fluorescent graphene quantum dots. We adopt a nanoparticle-on-mirror (NPoM) plasmonic structure, comprised of a Au nanoparticle on top of a reflective Au substrate (the 'mirror'). Extremely strong field enhancement is produced within the nanometer-scale junction. The Au nanoparticle is encapsulated with a thin layer of graphene shell. The measured scattering spectra of Au nanoparticles show distinct splitting double peaks, a characteristic feature of strong coupling. In addition, we demonstrate that the strong coupling is configurable. The splitting can be tuned with a low-power laser irradiation, exhibiting typical anti-crossing behaviour as a result of tuned LSP resonance and the oscillator strength of nano graphene. Our results demonstrate a new avenue for investigating strong-coupling at room temperature and provide opportunities for developing tuneable quantum optical devices.
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