Biomedical Engineering Reference
In-Depth Information
with respect to the wire axis. The period of helix is 1.83
μ
mfor
thisparticularcase.ExperimentalverificationofthechiralSPPsona
chemically synthesized Ag nanowire (
R
∼
150 nm) was performed
by imaging the fluorescence from uniformly spin-coated quantum
dots around the wire, as shown in Fig. 6.4(b). In order to maintain
the cylindrical symmetry, the Ag nanowire was immersed in oil,
which matches the glass substrate in refractive index. Ignoring the
radiative lifetime modification between quantum dots at different
locations, the fluorescence intensity provides a fairly good map
of the local electric field intensity. By changing the polarization
angle from 45
◦
to
45
◦
, the handedness of the plasmon helix can
be reversed. This is because the incident polarization angle can
determine the relative phase between the two
m
=
1modes,so
that the handedness of the chiral SPP is controlled as well. For
parallel polarization shown in Fig. 6.4(b-iv), the
m
=
1modeinthe
y
-directioncanbehardlyexcited.Then,thehelicaldistributionturns
intoabeatingpatternbetweenthe
m
=
0modeandthe
m
=
1mode
inthe
z
-direction.Forperpendicularpolarization,however,boththe
m
=
0modeandthe
m
=
1modeinthe
z
-directionarenotexcitedso
thatthehelicalorbeatingpatternsaredim.Besidesthehandedness,
the incident polarization angle can also alter the relative amplitude
ofthe
m
=
1modeinthe
y
-or
z
-direction,whichfurtherdetermines
the degree of circular polarization ofthe chiral SPP.
−
6.4 Influence of External or Structual Parameters
As the chiral SPPs depend on the relative amplitude of each excited
mode, the effect of the external excitation can have a large impact
on the properties of the chiral SPPs. Besides the polarization angle
mentioned above, the incident direction of the Gaussian beam can
affect the excitation e
ciency of the different SPP modes as well
[25]. The emission of SPP into light at nanowire terminals has been
shown to be maximized at a certain angle to the wire axis [26].
Although it has not been pointed out in the original study, this
emission characteristic is actually related to the
m
=
0 mode. As it
can be anticipated from the optical reciprocity, the coupling of an
incident light beam into the
m
=
0 mode will reach its maximum
