Abstract
Being an enabling technology for applications such as ultrasensitive biosensing and surface enhanced spectroscopy, enormous research interests have been focused on further boosting the local field enhancement at Fano resonance. Here, we demonstrate a plasmonic Fano resonance resulting from the interference between a narrow magnetic dipole mode and a broad electric dipole mode in a split-ring resonator (SRR) coupled to a nanoarc structure. Strikingly, when subjected to an azimuthally polarized beam (APB) excitation, the intensity enhancement becomes more than 60 times larger than that for a linearly polarized beam (LPB). We attribute this intensity enhancement to the improved conversion efficiency between the excitation and magnetic dipole mode along with improved near-field coupling. The APB excited Fano structure is further used as a nanoruler and beam misalignment sensor, due to the high sensitivity of intensity enhancement and scattering spectra to structure irregularities and excitation beam misalignment. Interestingly, we find that, regardless of the presence of structural translations, the proposed structure still maintains over 60 times better intensity enhancement under APB excitation compared to LPB excitation. Moreover, even if the APB excitation is somewhat misaligned, our Fano structure still manages to give a larger intensity enhancement than its counterpart excited by LPB.
Highlights
Localized surface plasmon resonances (LSPRs), resulting from the collective electron oscillations in metal-dielectric interfaces of nanostructures, have attracted tremendous interests recently[1]
We start our discussion with scattering spectrum of the split-ring resonator (SRR)-nanoarc structure under the excitation of an linearly polarized beam (LPB) at normal incidence, whose polarization is indicated in the lower insets of Fig. 2
The electric dipole mode is highly radiative, and it is treated as a “continuum” state. The interference between these two modes leads to a pronounced spectral dip located at 1.37 μm in the scattering spectrum of the combined structure, which indicates the occurrence of Fano resonance
Summary
Localized surface plasmon resonances (LSPRs), resulting from the collective electron oscillations in metal-dielectric interfaces of nanostructures, have attracted tremendous interests recently[1]. This popularity can be mainly attributed to the ability to concentrate and manipulate light in nanoscale regime. CVBs exemplified by radially and azimuthally polarized beams (APBs) can be generated and reconfigured by a spatial light modulator (SLM) at ease[38,39,40] It can provide a much more flexible and effective approach to excite a giant local field by matching the polarization states of the illumination to the eigenmodes of the plasmonic structures. Our results provide a new possibility to control the near-field properties of Fano resonance, and could find applications in SERS and biosensing
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