8
M. I. Stockman
Thus, we have arrived at the basic understanding of the qualitative features of
nanoplasmonics. Consider a plasmonic nanosystem whose size R satisfies a condition
l nl ∪ R ∪ l s . This nanosystem is excited by an external field in resonance. In this
case, the local optical field in the vicinity of such a nanosystem is enhanced by a
factor ∼Q, which does not depend on R. The spatial extension of the local field
scales with the size of the nanosystem ∝ R. This is because R ∪ l s , and l s is the
smallest electromagnetic length; thus there is no length in the system that R can
be comparable to. When the external field changes, the local field relaxes with the
relaxation time Q/ω that does not depend on R; the lifetimes of the SP are in the
femtosecond range.
In many cases of fundamental and applied significance, the size of a nanosystem
can be comparable to or even greater than l s but still subwavelength, λ R l s .
In such a case, the coupling to far-field radiation and radiative losses may greatly
increase as we will discuss below in Sects. 1.2.2 and 1.2.3. Another important subfield of nanoplasmonics that is related to extended systems is the surface plasmon
polaritons—see, e.g., a collective monograph [36]. We consider some polaritonic
phenomena relevant to coherent control below in Sect. 1.4.5.
1.2.2 Nanoantennas
Consider a molecule situated in the near-field of a metal plasmonic nanosystem. Such
a molecule interacts not with the external field but with the local optical field E(r)
at its location r. The interactions Hamiltonian of such a molecule with the optical
field is H = −E(r)d, where d is the dipole operator of this molecule. Note that a
modal expansion of the quantized local field operator is given below in this chapter
by Eq. (1.64).
Consequently, the enhanced local fields cause enhancement of radiative and nonradiative processes in which such a molecule participate. In particular, the rates
of both the excitation and emission are enhanced proportionality to the local field
intensity, i.e., by a factor of ∼Q 2 . This effect is often referred to as nanoantenna
effect [37–64] in analogy with the common radio-frequency antennas. For the recent
review of the concept and applications of optical nanoantennas see Ref. [65]. Currently, the term nanoantenna or optical antenna is used so widely that it has actually
became synonymous with the entire field of nanoplasmonics: any enhancement in
nanoplasmonic systems is called a nanoantenna effect.
General remarks about the terms “nanoantenna” or “optical antenna” are due. The
term “antenna” has originated in the conventional radio-frequency technology where
it is used in application to receivers for devices that convert the wave energy of farfield radio waves into local (near-field) electric power used to drive the input circuitry.
For transmitters, antennas perform the inverse transformation: from the local field
electric power to that of the emitted radio waves. Due to the general properties of
time reversal symmetry there is no principal difference between the receiving and
transmitting antennas: any receiving antenna can work as a transmitting one and
M. I. Stockman
Thus, we have arrived at the basic understanding of the qualitative features of
nanoplasmonics. Consider a plasmonic nanosystem whose size R satisfies a condition
l nl ∪ R ∪ l s . This nanosystem is excited by an external field in resonance. In this
case, the local optical field in the vicinity of such a nanosystem is enhanced by a
factor ∼Q, which does not depend on R. The spatial extension of the local field
scales with the size of the nanosystem ∝ R. This is because R ∪ l s , and l s is the
smallest electromagnetic length; thus there is no length in the system that R can
be comparable to. When the external field changes, the local field relaxes with the
relaxation time Q/ω that does not depend on R; the lifetimes of the SP are in the
femtosecond range.
In many cases of fundamental and applied significance, the size of a nanosystem
can be comparable to or even greater than l s but still subwavelength, λ R l s .
In such a case, the coupling to far-field radiation and radiative losses may greatly
increase as we will discuss below in Sects. 1.2.2 and 1.2.3. Another important subfield of nanoplasmonics that is related to extended systems is the surface plasmon
polaritons—see, e.g., a collective monograph [36]. We consider some polaritonic
phenomena relevant to coherent control below in Sect. 1.4.5.
1.2.2 Nanoantennas
Consider a molecule situated in the near-field of a metal plasmonic nanosystem. Such
a molecule interacts not with the external field but with the local optical field E(r)
at its location r. The interactions Hamiltonian of such a molecule with the optical
field is H = −E(r)d, where d is the dipole operator of this molecule. Note that a
modal expansion of the quantized local field operator is given below in this chapter
by Eq. (1.64).
Consequently, the enhanced local fields cause enhancement of radiative and nonradiative processes in which such a molecule participate. In particular, the rates
of both the excitation and emission are enhanced proportionality to the local field
intensity, i.e., by a factor of ∼Q 2 . This effect is often referred to as nanoantenna
effect [37–64] in analogy with the common radio-frequency antennas. For the recent
review of the concept and applications of optical nanoantennas see Ref. [65]. Currently, the term nanoantenna or optical antenna is used so widely that it has actually
became synonymous with the entire field of nanoplasmonics: any enhancement in
nanoplasmonic systems is called a nanoantenna effect.
General remarks about the terms “nanoantenna” or “optical antenna” are due. The
term “antenna” has originated in the conventional radio-frequency technology where
it is used in application to receivers for devices that convert the wave energy of farfield radio waves into local (near-field) electric power used to drive the input circuitry.
For transmitters, antennas perform the inverse transformation: from the local field
electric power to that of the emitted radio waves. Due to the general properties of
time reversal symmetry there is no principal difference between the receiving and
transmitting antennas: any receiving antenna can work as a transmitting one and