ABSTRACT
The optical hybridization of localized surface plasmons and photonic modes of dielectric nanostructures provides us wide arenas of opportunities for designing tunable nanophotonics with excellent spectral selectivity, signal enhancement, and light harvesting for many optical applications. Graphene-supported Au nanoprisms on a periodic Si nanopillar array will be an ideal model system for examining such an optical hybridization effect between plasmonic modes and photonic modes. Here, through the measurement of the reflectance spectra as well as graphene phonons by surface-enhanced Raman scattering (SERS), we investigated both the far-field and near-field properties of these optically hybridized modes. The effects of photonic modes and Mie resonances of the Si nanopillars on the localized surface plasmons of the Au nanoprisms and on their near-field enhancement were experimentally elucidated through the measurements of graphene phonons using two excitation lasers with wavelengths of 532 and 785 nm. The wavelength-dependent SERS intensities of monolayer graphene are clearly understood in terms of the optical hybridization, and the SERS enhancement factor estimated from finite-difference time-domain simulations exhibited good agreement with the measurements. The elucidated spectral tunability in the near-field light-matter interaction would be useful for potential applications in various types of graphene-based photonics.
ABSTRACT
We studied the effects of relative orientation of bowtie nanostructures on the plasmon resonance both experimentally and theoretically in this work. Specifically, we fabricated gold bowtie nanoantennas with rotated nanoprisms, measured the near-field and the far-field resonance behaviors using Raman spectroscopy and scattering microspectroscopy, and simulated the effects of the rotation angle on the localized surface plasmonic resonance using finite-difference time-domain simulations. In addition to the widely-discussed dipolar resonance in regular bowtie nanostructures, defined as tip-mode resonance in the present study, the excitations of edge-mode resonance were discovered under certain rotation angles of nanoprisms. Because of the resonances of different modes at different wavelengths, two different incident laser sources were used to measure the Raman spectra to provide evidence for the evolution of different resonance modes. Also, both the tip-mode and edge-mode resonances were verified by the simulated charge density distribution and their trends were discussed. Based on the discovered trend, a plasmon protractor was created with a near-exponential decay relationship between the relative resonance wavelength shift and cosine of the rotation angle. A plasmon hybridization model was also proposed for rotated bowties to explain the coupling between nanoprisms during rotation.
