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Download fileEffects of the Substrate Refractive Index, the Exciting Light Propagation Direction, and the Relative Cube Orientation on the Plasmonic Coupling Behavior of Two Silver Nanocubes at Different Separations
journal contribution
posted on 2016-05-30, 00:00 authored by Nasrin Hooshmand, Sajanlal R. Panikkanvalappil, Mostafa A. El-SayedIn
this paper, we presented a detailed theoretical investigation
on the effects of changing the refractive index of the substrate and
the interparticle separation distance on the electromagnetic field
distribution around the face-to-face (FF) and the edge-to-edge (EE)
oriented silver nanocube (Ag NC) dimers. We found that the contribution
of the scattering to the absorption components is markedly decreased
with increasing the substrate refractive index. This suggests that
for surface-enhanced Raman spectroscopic (SERS) applications, the
use of a substrate of very low refractive index is highly recommended.
However, for photothermal applications, the use of substrate of high
refractive index is recommended. Furthermore, we found that the wavelengths
corresponding to the dipolar and multipolar modes are nearly unaffected
by increasing the interparticle separation distances above 40 nm on
the high refractive index substrate. However, on the low refractive
index substrate, the dipolar peaks consistently red-shifted as the
separation distance decreased. We also observed that on a glass substrate,
after a separation distance above 8 nm, the value corresponding to
the maximum field in the FF oriented Ag NC dimer gradually increased
with increasing separation distance. On the contrary, the field gradually
decreased on the higher refractive index substrate (AlGaSb) above
8 nm separation. This substrate showed a larger effect on the redistribution
of the dipoles and subsequent decrease in the intensities of both
hot spots and cold spots. Additionally, studies on the effects of
substrate location with respect to the propagation direction of the
exciting light revealed that the high refractive index substrate is
likely to absorb the incident light to a larger extent, thus, greatly
diminishes the plasmonic enhancement of the exciting light if it propagates
through the substrate first. This study is pointing toward the fact
that the refractive index of the substrate has a strong effect on
the intensity ratio of the multipole to the dipole modes. Substrate
having high refractive index seems to relax the selection rules of
light interaction with the different plasmonic multipolar characters.
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Keywords
electromagnetic field distributionAg NC dimer8 nm separationSERSinterparticle separation distanceEEExciting Light Propagation DirectionFFsurface-enhanced Raman spectroscopicSubstrate Refractive IndexRelative Cube Orientationinterparticle separation distancesseparation distanceindex substrateplasmonic multipolar characters