Biomedical Engineering Reference
In-Depth Information
powerflow in the vicinity of the metal surfaces reveals formation
of localized areas of circulating powerflow that recycles optical
energy between metal and dielectric volumes. Such recirculation
translates into the tight light localization of the SPP mode field on
the surface, the large in-plane wavevector of SPP waves, and the
resultingslowingofthewavepropagationalongthemetal-dielectric
interfaces.
In the following sections, we will discuss how the insights
into the spatial structure of the localized topological features in
the near field of plasmonic components and materials help to
better understand their properties, to predict and exploit new
plasmonic effects, and to design next-generation plasmonic devices
with improved performance.
8.2 Back to Basics: Surface Plasmon Polariton
The most studied and written about plasmonic effect is the excita-
tionofSPPwavespropagatingalongmetal-dielectricinterfaces.SPP
modes are TM polarized surface waves, which are characterized by
large wavevectors parallel to the interface and low group velocities.
As a result, they create strong electric field and high local DOS
(LDOS) within the sub-wavelength thick layer adjacent to the inter-
face. These waves can only exist on interfaces between materials
having dielectric permittivity values of opposite signs. Dielectric
permittivities of materials with a high density of free charge
carriers—such as metals and highly doped semiconductors—are
defined by the Drude model,
ε
m
(
ω
)
=
ε
∞
−
ω
p
(
ω
2
2
+
i
γω
), where
p
is the plasma frequency,
ε
∞
is the high-frequency permittivity
limit, and
γ
is the electron collision frequency. The real part of
the Drude permittivity becomes negative in the frequency range
belowtheplasmafrequencyofthematerial.ThismakespossibleSPP
propagation along their interfaces with other materials.
An eigensolution of the electromagnetic boundary problem for
theMaxwellequationsonsuchaninterface(showninFig.8.3a)that
describes the SPP mode has the followingform [15]:
ω
