Plasmon-enhanced Raman scattering in graphene: Quantum nature of the enhancement mechanism

2015 
Plasmon-enhanced Raman scattering (PERS) describes the giant enhancement of the Raman cross section close to localized surface plasmons. It is used in surface-enhanced Raman spectroscopy (SERS) to detect single molecules and in tip-enhanced scattering (TERS) to increase the resolution of Raman imaging beyond the diffraction limit. Despite the wide-spread use of plasmon-enhanced Raman spectroscopy in imaging and materials characterization its fundamental principles remain strongly debated. Here we demonstrate the quantum nature of plasmon-enhanced Raman scattering and its striking consequences. We coupled graphene and carbon nanotubes to rationally designed plasmonic nanostructures.[1] The coupling increased the Raman cross section by 105 when excited under resonance. Raman ima-­‐ ging and wavelength-­‐dependent Ram-­‐ an measurements show that the en-­‐ hancement is strongly localized in space (10 nm) and frequency (15 meV). We propose a quan-­‐tum theory of plasmon-­‐enhanced Raman scatter-­‐ ing based on perturbation theory.[2] It predicts quantum interferences bet-­‐ ween scattering pathways that result in the suppression and enhancement of scattering channels, which we demonstrate experimentally. We also verify other surprising consequences of the quantum nature of PERS such as the appearance of the D-­‐mode in perfect graphene. Left: Plasmonic gold nanodimer covered by graphene. Right: Imaging of the Raman intensity close to the dimer at its resonance energy. [1] S. Heeg et al., Nano Lett. 13, 301 (2013); S. Heeg et al., Nano Lett. 14, 1762 (2014). [2] P. Kusch et al., Quantum nature of plasmon-enhanced Raman scattering (submitted, 2015).
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