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S. V. Boriskina
applications–Raman and fluorescence sensing being the most widespread–make use
of the electric field enhancement and high local density of states provided by plasmonic nanostructures to amplify weak molecular signals such as Rayleigh [6–8]
or Raman [9–13] scattering efficiency, absorption efficiency [14], fluorescence rate
[15–18], etc.
The electric field enhancement is a manifestation of the resonant excitation of
collective oscillations of free charge carriers in some materials (e.g., free electrons
in metals) by the external light sources or embedded emitters. In the language of
quantum mechanics, this process can be explained as the excitation of plasmons
(quanta of plasma oscillations). The collective dynamics of plasmons is driven by the
long-range correlations caused by Coulomb forces, and can be drastically modified
by engineering the boundaries of the spatial region filled by electron plasma. In
particular, bulk plasmons confined in thin metal films can couple with photons to
create surface plasmon-polariton (SPP) waves [22]. On the other hand, illuminationinduced collective response of plasmons confined in nanoparticles is manifested in
the excitation of quantized localized surface plasmon (LSP) modes with different
angular momenta [23].
Although being quantum in nature, many plasmonic effects can be well described
in the frame of the classical electromagnetic theory by using a semi-classical DrudeLorentz-Sommerfeld model to define the frequency-dependent permittivity of metals.
As a result, many fundamental principles and engineering solutions established in
the electromagnetic modeling of radio-frequency (RF) antennas, microwave transmission lines, circuit elements, etc. can be applied to study and design plasmonic
nanostructures. These include the concepts of antenna resistance, directivity and
efficiency [24] and the principle of impedance matching [25, 26] just to name a few.
Overall, the striking analogies with RF and microwave engineering–which focus on
reversible interfacing of the propagating radiation with the fields localized inside
sub-wavelength-size components–cemented the use of the design framework that
treats plasmonic nanostructures as nanoscale analogs of RF antennas [17, 24, 27–
29] and waveguides [2, 26, 30–32]. Alternative yet closely-related formulations are
based on the treatment of plasmonic components as lumped circuit elements [33, 34]
and resonators for plasmons [35–37].
The concept of an optical antenna is a very straightforward and visual analogy to
describe a plasmonic nanoparticle, which can serve as both electromagnetic transmitter and receiver capable of converting the incoming radiation into localized surface
plasmon modes, subsequently re-radiating it into the far-field, or modifying emission properties of embedded molecules. The oscillating dipole moments in individual
particles induced by external sources can in turn be treated as secondary sources of
electromagnetic radiation, i.e., dipole antennas (Fig. 12.1a). Dimer-gap nanoantenna
configuration, which provides larger radiation efficiency as well as higher spectral
tunability than an individual nanoparticle, can be constructed by analogy with a
classical radio-frequency radiator, the Hertzian dipole (Fig. 12.1b) [27, 38]. The
RF technology analogies so deeply penetrated the field of plasmonics research, that
the terms ‘noble-metal nanoparticle(s)’ and ‘optical nanoantenna(s)’ became almost
interchangeable. Furthermore, since the collective response of multi-particle struc-
S. V. Boriskina
applications–Raman and fluorescence sensing being the most widespread–make use
of the electric field enhancement and high local density of states provided by plasmonic nanostructures to amplify weak molecular signals such as Rayleigh [6–8]
or Raman [9–13] scattering efficiency, absorption efficiency [14], fluorescence rate
[15–18], etc.
The electric field enhancement is a manifestation of the resonant excitation of
collective oscillations of free charge carriers in some materials (e.g., free electrons
in metals) by the external light sources or embedded emitters. In the language of
quantum mechanics, this process can be explained as the excitation of plasmons
(quanta of plasma oscillations). The collective dynamics of plasmons is driven by the
long-range correlations caused by Coulomb forces, and can be drastically modified
by engineering the boundaries of the spatial region filled by electron plasma. In
particular, bulk plasmons confined in thin metal films can couple with photons to
create surface plasmon-polariton (SPP) waves [22]. On the other hand, illuminationinduced collective response of plasmons confined in nanoparticles is manifested in
the excitation of quantized localized surface plasmon (LSP) modes with different
angular momenta [23].
Although being quantum in nature, many plasmonic effects can be well described
in the frame of the classical electromagnetic theory by using a semi-classical DrudeLorentz-Sommerfeld model to define the frequency-dependent permittivity of metals.
As a result, many fundamental principles and engineering solutions established in
the electromagnetic modeling of radio-frequency (RF) antennas, microwave transmission lines, circuit elements, etc. can be applied to study and design plasmonic
nanostructures. These include the concepts of antenna resistance, directivity and
efficiency [24] and the principle of impedance matching [25, 26] just to name a few.
Overall, the striking analogies with RF and microwave engineering–which focus on
reversible interfacing of the propagating radiation with the fields localized inside
sub-wavelength-size components–cemented the use of the design framework that
treats plasmonic nanostructures as nanoscale analogs of RF antennas [17, 24, 27–
29] and waveguides [2, 26, 30–32]. Alternative yet closely-related formulations are
based on the treatment of plasmonic components as lumped circuit elements [33, 34]
and resonators for plasmons [35–37].
The concept of an optical antenna is a very straightforward and visual analogy to
describe a plasmonic nanoparticle, which can serve as both electromagnetic transmitter and receiver capable of converting the incoming radiation into localized surface
plasmon modes, subsequently re-radiating it into the far-field, or modifying emission properties of embedded molecules. The oscillating dipole moments in individual
particles induced by external sources can in turn be treated as secondary sources of
electromagnetic radiation, i.e., dipole antennas (Fig. 12.1a). Dimer-gap nanoantenna
configuration, which provides larger radiation efficiency as well as higher spectral
tunability than an individual nanoparticle, can be constructed by analogy with a
classical radio-frequency radiator, the Hertzian dipole (Fig. 12.1b) [27, 38]. The
RF technology analogies so deeply penetrated the field of plasmonics research, that
the terms ‘noble-metal nanoparticle(s)’ and ‘optical nanoantenna(s)’ became almost
interchangeable. Furthermore, since the collective response of multi-particle struc-