ABSTRACT
The aim of this chapter is a profound discussion of the interplay
between gain media and plasmon elements in metamaterials for
visible light. The exciton-plasmon dynamics arising by specific cou-
pling configurations has been the core of scientific discussions and
experimental studies to explain extraordinary physical processes. In
fact, the coupling plasmon-gain has been proposed as a challenging
solution to tackle and solve the unavoidable issue of optical losses
inmetal-based nanostructures with plasmonic resonances at optical
frequencies. The fascinating ability of metal nanostructures to
localize light at scales much shorter than visible wavelengths is
accompanied by enormous ohmic losses, with direct consequences
as the remarkable augment of the extinction cross section of the
material. This implies that extraordinary physical properties related
to light localization effects at the nanoscale cannot be harnessed to
design optical materials because of the strong radiation damping.
The idea to bring gain molecules in close proximity to metallo-
dielectric nanostructures is based on coherent effects of excitation
energy transfer between resonant bands of the two materials.
Radiation-less transitions from exciton levels of chromophores to
plasmon states of metal nanoparticles (NPs) are responsible of
the modification of the energy of their quasi-static field, that in
turn modify the complex dielectric function of metastructures. To
increase the efficiency of the resonant energy transfer, several
coupling parameters have to be accounted both chemically and
physically [Fang et al. (2009)]. This would enable promising new
applications of these materials in fields such as materials science
[Imahori and Fukuzumi (2001)], biophysics [Powell et al. (1997)],
molecular electronics, and fluorescence-spectral engineering based
on surface-enhancement effects [Lakowicz (2001)]. Compensation
of the strong losses caused by metal absorption would permit
us to operate at optical frequencies, opening the possibility to
investigate phenomena such as perfect lenses [Pendry (2000)],
cloaking [Schurig et al. (2006); Cai et al. (2008); Veltri (2009)], and
others not yet conceived. Recent experimental works performed on
plasmonic structures with gain units dissolved in solution [Noginov
et al. (2006); De Luca et al. (2011, 2013); Strangi et al. (2011)]
showed that the presence of fluorescent molecules in a mixture
may modify the scattering intensity as a function of the gain
owing to the enhancement of the quality factor of surface plasmon
resonances (SPRs). It is well known that relevant modifications
of the fluorescence of dye molecules placed in close proximity
to metal NPs are due to mutual interactions with NPs surface
plasmons, including resonant energy transfer (RET) [Gersten and
Nitzan (1981); Das and Puri (2002); Weitz et al. (1983)]. A
localized surface plasmon represents a collective oscillation of
electron charges in metallic NPs, whose resonance frequency is
sensitive to dielectric changes of the environment, as well as to
the size and shape of the NP. A phenomenon relevant to localized
surface plasmons is a surface plasmon polariton (SPP), that is, a
surface electromagnetic wave propagating parallel to the interface
between two media possessing permittivities with opposite signs,
such as a metal and a dielectric. In both cases, oscillations are
excited by light, exhibiting enhanced near-field amplitude at the
resonance wavelength. Localized surface plasmons have been found
on rough surfaces [Ritchie (1973); Moskovits (1985)], in engineered
nanostructures [Quinten et al. (1998); Averitt et al. (1999);
Brongersma et al. (2000); Mock et al. (2002)], as well as in clusters
and aggregates of NPs [Kreibig and Vollmer (1995); Su et al. (2003);
Quinten (1999)]. In 1989, Sudarkin and Demkovich suggested to
increase the propagation length of an SPP by utilizing the population
inversion created in the dielectric medium adjacent to the metallic
film. Recently, gain-assisted propagation of SPPs at the interface
between a metal and a dielectric with optical gain has been
analyzed theoretically [Nezhad et al. (2004); Avrutsky (2004)]. The
enhancement of two orders of magnitude of the SPP at the interface
between the silver film and the dielectric medium with optical gain
(laser dye) has been demonstrated by Seidel et al. (2005), and
described by Noginov for Ag aggregates in solution [Noginov et al.
(2006)]. Furthermore, a relevant phenomenon of surface plasmon
amplification by stimulated emission of radiation (SPASER), based
on Fo¨rster-like energy transfer from excitedmolecules to resonating
metallic nanostructures introduced by Stockman et al. in 2003
[Bergman and Stockman (2003); Stockman (2008)], has been
theoretically analyzed by Zheludev et al. (2008) and experimentally
demonstrated by Noginov et al. (2009). At the same time, a theoret-
ical self-consistent calculation on gain-assisted metamaterials was
proposed in 2009 by Fang et al. [Noginov et al. (2006)], showing
that 2D dispersive metamaterial losses can be compensated (Im(ε)
= 0), whereas both positive and negative values of Re(ε) can be obtained. Nevertheless, most of these studies have been performed
in “gain-assisted” systems, where striking plasmonic properties
have been evidenced, but the unavoidable problem of absorptive
losses still requires many technical and scientific problems to
be solved. The research strategies reported in this chapter deal
with cross-disciplinary approaches that involve design and tailoring
of electromagnetic properties, materials preparation, advanced
experimental studies, and theoretical modeling. In particular, in
Section 6.2 theory and modeling of the plasmon-gain interplay
in metastructures is discussed by means of the semiclassical
approach, where different systems are modeled as function of the
gain. Materials functionalization and experimental investigations
are reported in Section 6.3, time-resolved and transient absorption
spectroscopy, spectrophotometry and spectroscopic ellipsometry
are only a few of the experimental techniques utilized to study how
plasmon-gain dynamics can be directed to mitigate optical losses
across scales.