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.