Biomedical Engineering Reference
In-Depth Information
when the direction of the incident beam is oriented by the same
angle to the wire axis. However, the end emission of the
m
=
1
mode is mainly in the direction along the wire axis [27] so that end-
fire excitation can be a more optimal configuration for this mode.
Therefore,iftheincidentangleisvaried,thee
ciencyofin-coupling
of different SPP modes changes so that the ellipticity of the chiral
SPP is modified.
In the excitation configuration depicted in Fig. 6.2, the retar-
dation effect is a critical issue to excite the
m
=
1modein
the
z
-direction. Thus, the ratio between the incident wavelength
and the nanowire radius is critical in the formation of chiral
SPPs. Figure 6.5 shows the calculated transmission spectra of a
Ag nanowire for inclined (
θ
=
45
◦
) and perpendicular (
θ
=
90
◦
)
incidentpolarization.Forperpendicularexcitation,thetransmission
spectrum shows Fabry-Perot resonances due to the interference
of the
m
=
1mode(
y
-direction) in short wavelength range. For
longer wavelengths, the excitation e
ciency is reduced due to the
smaller overlap between the mode profile and the incident light.
For inclined polarization, the excitation of the
m
=
0modeinthe
long wavelength region dominates the transmission spectrum. The
Figure 6.5
FEM calculated transmission spectra of an Ag nanowire
normally excited by a paraxial Gaussian beam with inclined (
θ
=
45
◦
)or
perpendicular(
θ
=
90
◦
)polarization.Thelengthofthenanowireis5.0
μ
m,
and
R
=
60 nm,
ε
45
◦
are taken from Ref. [19],
with permission of the American Physical Society.
D
=
2.25. The data for
θ
=
