Biomedical Engineering Reference
In-Depth Information
Figure 6.1
The real (a) and imaginary (b) part of the propagation constant
ofthe
m
-thorderSPPonaAgnanowireasafunctionofthewireradius.Inset
to (b) shows an enlarged region for the
m
0 mode for small wire radius.
The permittivity of the surrounding medium is
=
D
ε
=
2.25.
k
0
=
2
π
/
λ
0
and
λ
0
=
632.8 nm.
Figure 6.1(b) shows the attenuation constants of different SPP
modes. As the radius of the nanowire decreases, the attenuation
constant of the
m
=
0 mode increases, meaning that the mode is
getting lossy. Therefore, this mode can be regarded as a short-range
SPP for thin wires. On the contrary, the
m
=
1modehasmuch
smaller loss in the thin wire limit due to the extension of its mode
profile into the non-absorbing dielectric region. Accordingly, this
modecanberegardedasalong-rangedSPP.Thehigherordermodes
(
m
=
2and3)haveevensmallerattenuationconstantsthanthe
m
=
1mode,andalsoshowreducedlossesastheradiusofthemetalwire
decreases.
6.3 Generation of Chiral Surface Plasmon Polaritons
In analogy with circularly polarized light, the superposition of two
orthogonal degenerate modes with a
π
/2 phase shift can result in a
circularly polarized state, which is chiral. Further interference with
thecoexcitedfundamentalmode(
m
=
0) generates a time-averaged
helically distributed near-field energy flow along the metal surface
(see Fig. 6.2), which is chiral as well. If not specified, chiral SPPs
refer to the helically distributed SPPs in the following discussions.
