Probing the in-Plane Near-Field Enhancement Limit in a Plasmonic Particle-on-Film Nanocavity with Surface-Enhanced Raman Spectroscopy of Graphene.
When the geometric features of plasmonic nanostructures approach the subnanometric regime, nonlocal screening and charge spill-out of metallic electrons will strongly modify the optical responses of the structures. While quantum tunneling resulting from charge spill-out has been widely discussed in the literature, the near-field enhancement saturation caused by the nonlocal screening effect still lacks a direct experimental verification. In this work, we use surface-enhanced Raman spectroscopy (SERS) of graphene to probe the in-plane near-field enhancement limit in gold nanosphere-on-film nanocavities where different layers of graphene are sandwiched between a gold nanosphere and a gold film. Together with advanced transmission electron microscopy cross-sectional imaging and nonlocal hydrodynamic theoretical calculations, the cavity gap width correlated SERS and dark-field scattering measurements reveal that the intrinsic nonlocal dielectric response of gold limits the near-field enhancement factors and mitigates the plasmon resonance red-shift with decreasing the gap width to less than one nanometer. Our results not only verify previous theoretical predictions in both the near-field and far-field regime but also demonstrate the feasibility of controlling the near- and far-field optical response in such versatile plasmonic particle-graphene-on-film nanocavities, which can find great potential in applications of graphene-based photonic devices in the visible and near-infrared region.
['Liu|Danjun|D|', 'Wu|Tingting|T|', 'Zhang|Qiang|Q|', 'Wang|Ximiao|X|', 'Guo|Xuyun|X|http://orcid.org/0000-0003-0365-7545', 'Su|Yunkun|Y|', 'Zhu|Ye|Y|', 'Shao|Minhua|M|http://orcid.org/0000-0003-4496-0057', 'Chen|Huanjun|H|http://orcid.org/0000-0003-4699-009X', 'Luo|Yu|Y|http://orcid.org/0000-0003-2925-682X', 'Lei|Dangyuan|D|http://orcid.org/0000-0002-8963-0193']
surface-enhanced Raman spectroscopy graphene nonlocal screening charge spill-out nanoparticle-on-film nanocavities