The surface plasmon resonances of
gold contour bowtie nanostructures
were simulated in the present study. The local electromagnetic field
enhancement and the resonance wavelength for different dimensions
of contour bowtie antennas with various contour thicknesses were investigated
to find the critical conditions to induce additional enhancement compared
to the solid bowtie antenna. Both the phase of the electric field
and the bound surface charge distribution on the surface of the contour
bowtie were studied to characterize the coupled plasmon configurations
of the contour bowtie antenna. Also, a model was proposed to explain
the resonance and hybridization behavior in the contour bowtie nanoantenna,
and it was verified by examining the phase of the electric field in
the polarization direction.
The surface plasmon resonances of gold bowtie nanoring antenna arrays were simulated using the finitedifference time-domain method in the present study. Both the local field and transmission spectra of bowtie nanoring antennas with various nanohole sizes were examined to find the optimum conditions to induce the greatest local electromagnetic field enhancement and sensitivity compared to the solid bowtie antenna. With the optimized nanohole size of bowtie nanoring, the local electromagnetic field enhancement, the decay length of the electric field, and the bulk sensitivity were increased as high as about 73, 349, and 63 %, respectively, compared to the solid bowtie antenna. The electric field enhancement profile and the charge distribution of the bowtie nanoring antennas were studied to characterize the coupled plasmon configurations, and it was used to explore the mechanism of enhanced sensitivity and resonance-wavelength shift of bowtie nanoring array with different surrounding dielectric media. This highly localized electromagnetic field enhancement and sensitive bowtie nanoring array system can be applied in the field of surface-enhanced Raman scattering and bio-sensing.
Corner radius is a concept to approximate the fabrication limitation due to the effective beam broadening at the corner in using electron-beam lithography. The purpose of the present study is to investigate the effects of corner radius on the electromagnetic field enhancement and resonance wavelength for three periodic polygon dimers of bowtie, twin square, and twin pentagon. The enhancement factor of surface-enhanced Raman spectroscopy due to the localized surface plasmon resonances in fabricated gold bowtie nanostructures was investigated using both Raman spectroscopy and finite-difference time-domain simulations. The simulated enhancement factor versus corner radius relation was in agreement with measurements and it could be fitted by a power-law relation. In addition, the resonance wavelength showed blue shift with the increasing corner radius because of the distribution of concentrated charges in a larger area. For different polygons, the corner radius instead of the tip angle is the dominant factor of the electromagnetic field enhancement because the surface charges tend to localize at the corner. Greater enhancements can be obtained by having both the smaller gap and sharper corner although the corner radius effect on intensity enhancement is less than the gap size effect.
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.
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