Biomedical Engineering Reference
In-Depth Information
nanocavity is a sub-wavelength region that, in resonance condition,
experiments the presence of a very bright hot spot due to the
superposition of two strong in-phase LSPs in both dimer antenna
arms. By rotating the polarization from long to short axes direction
the hot spot intensity decreases. Although a residual EM field
remains active in the close proximity of the face-to-face apexes,
related to the excitement of short axis LSPs oscillating in-phase.
The key point in order to nullify the electric field in the gap region
consists in forcing charge distributions in both antenna arms to
contemporarily converge towards the gap in a complete out-of-
phaseplasmonmode.Thiseffectisnotpossible,innormalincidence
lightcondition,foralignedantennadimersindipolarapproximation
since all the charges in the system perceive, in every instant, the
same external electric field. Therefore, in perspective of arising
theout-of-phasemodeinantennadimer,creatingadevicemore
sensitive to light polarization, a solution consists in rotating one
of the two dimer antenna arms of 90
◦
with respect to gap center,
inducingasymmetrybreakingdirectlyonthedevicegeometry[90].
This configuration assumes the form of an L-shaped antenna
dimer. EM simulations by means of a FE code software have been
carried-out both in far- and near-field range to study the spectral
response of this device to incoming EM plane wave as function
of its polarization. In particular the study has been focused on
two specific polarization directions: along the planar inversion-
symmetry axis (
θ
=
45
◦
with respect to horizontal antenna long
axis) and along its normal (
45
◦
with respect to horizontal
θ
=−
45
◦
charge distributions in
both antenna arms are induced to oscillate in-phase similarly to
what happens in aligned configuration but, for
antenna long axis). In fact, for
θ
=−
45
◦
charges in
both arms are contemporarily pulled/pushed by external electric
field towards/far from the gap region in an out-of-phase collective
mode.Moreover,inthisdeviceconfiguration,bothin-phaseandout-
of-phase LSPs modes present non-zero associated dipolar momenta
and for this reason both resonances e
ciently radiate in far field.
From simulated far-field extinction e
ciency spectra (defined as
theratiobetweentheextinctionandthegeometricalcrosssections)
at
θ
=−
45
◦
and
θ
=
45
◦
polarization angles, it can be observed
how both spectra show a proper resonance peak, the second one
θ
=
