Abstract
Abstract Plasmonic radial breathing mode (RBM), featured with radially oscillating charge density, arises from the surface plasmon waves confined in the flat nanoparticles. The zero net dipole moment endows the RBM with an extremely low radiation yet a remarkable intense local field. On the other hand, owing to the dark mode nature, the RBMs routinely escape from the optical measurements, severely preventing their applications in optoelectronics and nanophotonics. Here, we experimentally demonstrate the existence of RBM in a hexagonal Au nanoplate-on-mirror nanocavity using a far-field linear-polarized light source. The polarization-resolved scattering measurements cooperated with the full-wave simulations elucidate that the RBM originates from the standing plasmon waves residing in the Au nanoplate. Further numerical analysis shows the RBM possesses the remarkable capability of local field enhancement over the other dark modes in the same nanocavity. Moreover, the RBM is sensitive to the gap and nanoplate size of the nanocavity, providing a straightforward way to tailor the wavelength of RBM from the visible to near-infrared region. Our approach provides a facile optical path to access to the plasmonic RBMs and may open up a new route to explore the intriguing applications of RBM, including surface-enhanced Raman scattering, enhanced nonlinear effects, nanolasers, biological and chemical sensing.
Highlights
Metallic nanoparticles with localized plasmons modes [1, 2] could provide a unique way to squeeze light into nanoscale volume and profoundly enhance the light field near the particles
Plasmonic radial breathing mode (RBM) having a vanishing dipole moment are almost unattainable in the optical measurements
We have demonstrated an optical way to probe this dark mode in a plasmonic nanocavity composed of the closely spaced hexagonal Au nanoplate and ultrasmooth Au film
Summary
Metallic nanoparticles with localized plasmons modes [1, 2] could provide a unique way to squeeze light into nanoscale volume and profoundly enhance the light field near the particles These plasmonic modes fall into two categories, i.e., the bright and the dark modes. Due to the dark mode nature, the plasmonic breathing mode is mainly characterized using the electron beam, for instance, the cathodoluminescence [14, 25] and the scanning transmission electron microscopes equipped with the electron energy loss spectroscopy [3, 11, 14, 26] These approaches provide unambiguous spectral analysis of the RBM with the ability to image the mode profile with unprecedented spatial resolution. Our results shed light on the physics of plasmonic RBM and pave the way for its applications in optical sensing, surface-enhanced Raman scattering and enhanced optical nonlinearity
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