Biomedical Engineering Reference
In-Depth Information
Figure 14.13
The NRR as a function of the EHD location along the positive
x
-axisforthesuper-resonantAg,Au,andCu-basedCNPs.(a)
z
-orientedEHD
and (b)
x
-oriented EHD.
Although the behaviors of the NRR for the two EHD orientations
both qualitatively and quantitatively resemble one another for
locations inside the nano-core (where an almost constant NRR is
observed), this is not the case for other locations. In particular, two
notable differences are observed. First, the NRR of the
x
-oriented
EHD is well below that of the
z
-oriented EHD when the EHDs are
inside the respective nano-shells. For the
x
-oriented EHD, some of
its field gets trapped inside the nano-shell (i.e., the nano-shell acts
as awaveguideexcitedbytheEHDorientedorthogonalto itswalls),
andthusdoesnotgetradiated,resultinginreducedNRRvalues.The
second notable difference between the two orientations occurs for
theEHDlocationsoutsidetheCNPs.Fortheselocations,theNRRfor
the
z
-oriented EHD drops below its values obtained when the EHD
is inside the CNPs, whereas the NRR for the
x
-oriented EHD attains
its maximum as the
x
-oriented EHD moves just outside the CNP;
this maximum is also larger than any of the NRR values obtained
with the
z
-oriented EHD. The
x
-oriented dipole not only excites the
resonant mode of the electrically small core-shell cavity, but it also
directly drives the SPPs on the outer shell. Because the shell is thin,
these surface waves extend into the core and in turn are amplified
by the presence of the gain medium, and this explains the increased
NRR values for the
x
-oriented EHD outside the CNP. In summary,
the strongest excitation of the resonant dipole mode requires the
z
-
oriented EHD to be inside the nano-cores, while the locations of the
