**3. Types of plasmon-exciton interaction**

Based on the spectral overlap between the plasmonic modes and optical exciton of the organic molecules their mutual interaction mechanism is broadly classified into nonresonant and resonant interactions.

#### **3.1 Nonresonant interaction**

These types of interactions are further classified into refractive index-dependent interaction and plasmon enhanced fluorescence.

### *3.1.1 Refractive index dependent*

The type of interaction between plasmonic-organic hybrids where the electronic absorption band of the organic molecule and the plasmonic resonances are far away from each other and the refractive index of the organic molecule is nearly wavelength independent (**Figure 2A**). The adsorption of organic molecules merely increases the refractive index or the dielectric constant of the nanoenvironment surrounding metal nanocrystals. Therefore, as the induced polarization charges increase, a more pronounce screening of the Columbic restoring effect leading to a red shift in the resonance band of the plasmonic nanostructures [7, 29]. A prime example of such a plasmon molecule interaction is refractive index-based sensors where, the adsorption of such organic molecules enhances the refractive index of the environment around the plasmonic nanostructures [30].

#### *3.1.2 Plasmon enhanced fluorescence*

The organic molecules which demonstrate both absorption and fluorescence emission, tend to drastically alter their fluorescence rate when hybridized with metal

**Figure 2.**

*Schematic diagram illustrating three types of plasmon-molecule interactions. (A) Plasmon shift due to adsorption; (B) resonance coupling between metal nanocrystals and adsorbed molecules with strong absorption and (C) fluorescence enhancement when fluorophores are adjacent to metal nanocrystals [29].*

nanostructures [29, 31]. Primarily, the enhancement in the fluorescence of the organic molecule may be due to an increase in the excitation rate of the electrons from ground state to the excited state which happens due to the strengthening of the local electric field in the vicinity of the metal nanosurfaces. On the contrary fluorescence enhancement may also be attributed to the increase in electronic radiative emission from the excited states to the ground state owing to aggregation of photonic states around the metal nanostructures [7, 32]. In both these phenomena, the distance between the fluorophore and the plasmonic structure plays a key role. Novotny et al. demonstrated that organic fluorophores when conjugated with metal nanoparticles displaying intense local field enhancements then the radiative decay rate dominates the nonradiative decay rate and fluorescence enhancement takes place which further reduces the average lifetime of the organic molecule [33]. Moreover, due to the coupling with the plasmon resonance, the emission direction as well as the spectral shape of fluorophores can also be modified [34–36]. All of these open up new approaches for manipulating light at the nanoscale.

#### **3.2 Resonant interactions between plasmons and excitons**

Resonant coupling takes place if the absorption band of the organic molecules and the LSPR frequency of the metal nanoparticles overlap with each other. Plasmons when resonantly excited, amplify the local electric multiple times in magnitude. In such a case the coupling between the plasmonic resonances and the degenerate energy levels of the organic molecule leads to hybridized molecular states [7, 29, 31]. Based on the perturbations in the wave functions of the plasmons and, the interaction between them can be classified into weak, strong and extreme coupling [1, 37, 38]. If the wave

*Types of Nonlinear Interactions between Plasmonic-Excitonic Hybrids DOI: http://dx.doi.org/10.5772/intechopen.105833*

functions remain unaltered it is called weak coupling, origination of new dispersion relations due to the formation of hybrid states is referred to as strong coupling regime whereas, in the extreme coupling the resonance exchange energy oscillates between the upper and lower energy levels leading to a split in the absorption band [1, 39].

Pockrand et al. and Glass et al. in 1980 first demonstrated the resonance coupling effect between plasmons and excitons both theoretically and experimentally [40, 41]. Later, it was realized that the ultraviolet and visible light could switch the resonant coupling between the dye molecules and excitons this led to efficient switching with power density �6.0 mW/cm<sup>2</sup> and switching power 0.72 nW/device [42]. Since then, efforts to decipher interaction mechanisms between strongly coupled plasmons and excitons have gained a lot of interest amongst researchers. Wiederrecht et al. first reported literature on coherent coupling between a J-aggregated molecular dye and noble metallic nanospheres (**Figure 3**) [43].

They demonstrated that the interaction between plasmons and excitons is strongly dependent on the properties of the metal nanostructure. The J-aggregated dye when hybridized with Ag nanocrystals lead to an increased absorption whereas the absorption decreased when integrated with Au nanocrystals [43]. Further, Fofang et al. investigated wavelength-dependent coupling between the J-aggregates of dye 2,2<sup>0</sup> dimethyl-8-phenyl-5,6,5<sup>0</sup> ,6<sup>0</sup> -dibenzothiacarbocyanine chloride. Their study claims that the plasmonic resonances can be tuned over a wide spectral range when strongly coupled with a metal nanostructure. In fact, the coupling energy diagram of 2,2 dimethyl-8-phenyl-5,6,5,6-dibenzothiacarbocyanine chloride and Au nanoshells hybrids depicted both asymmetric energy splitting and an anticrossing behavior [1]. The anticrossing behavior amongst resonantly coupled hybrids of Au nanorods and various other organic dyes have also been reported [44–47]. One of the major applications demonstrated by resonantly coupled plasmons and organic dyes is ultrasensitive detection of analytes. Where, resonant binding amongst Ag nanoparticles, camphor and cytochrome P450cam protein (CYP101) demonstrated a plasmon shift of up to 104 nm [48, 49]. Therefore, a complete understanding of the resonance coupling effects between plasmonic nanostructures and excitonic

molecules is of utmost importance for realizing active photonic devices such as optical switches, lasers and energy transfer-based sensors [50].

Since, these interactions between plasmons and excitons cannot be explained by the dielectric function therefore, it was realized that energy transfer plays a vital role in delineating the characteristics of the hybrid states.

### *3.2.1 Exciton-plasmon resonant energy transfer mechanisms*

An optically excited excitonic molecule placed in the vicinity of a plasmonic nanostructure may result in an energy transfer *via* radiative or nonradiative channel.

Owing to the physical and chemical conditions the probabilistic resonant interactions between the plasmonic-organic hybrids can be due to Dexter energy transfer (DET), exciton plasmon resonance energy transfer (EPRET), Foster resonance energy transfer (FRET), nanometal surface energy transfer (NSET), metal enhanced fluorescence (MEF), enhancement of absorption cross section (lightening rod effect), enhanced photostability, and the increase of excitation rate [51].

The origin of metal enhanced fluorescence is understood in terms of the increase in optical density of states of the emitter when placed near a metal nanosurface, due to the local confinement of the incident electric field [52, 53]. This in accordance with Purcell effect which leads to an increase in the radiative decay rate of the organic molecule due to a decrease in the volume of the cavity [54].

Further, MEF is extremely sensitive to the orientation of the exciton and nanoparticle (**Figure 4**).

Both experimental and theoretical studies infer that it is the orientation of the exciton and plasmonic dipoles in MEF due to which the net luminance of the hybrid quenches or strengthens.

A radiation less energy transfer from excited state donor to ground state acceptor via long range dipole-dipole interactions is termed as FRET [55, 56].

The efficiency of this energy transfer critically depends on various factors such as the distance of separation between the acceptor and donor molecule, their spectral overlap and their emission quantum yield (QY) [57, 58]. **Figure 5** illustrates the Jablonski Diagram of this mechanism.

FRET plays an important role in the determination of sub-microscopic separations amongst interactive molecules [59, 60]. The dipole-dipole interaction between the

#### **Figure 4.**

*(a) Parallel (tangential) and (b) perpendicular (radial) orientation of chromophore dipole moment to the surface of spherical nanoparticles leading to the suppression or enhancement of the radiative decay rate of the exciton, respectively [51].*

*Types of Nonlinear Interactions between Plasmonic-Excitonic Hybrids DOI: http://dx.doi.org/10.5772/intechopen.105833*

**Figure 5.** *Jablonski diagram illustrating FRET [59].*

**Figure 6.** *Comparison of energy transfer efficiency between FRET and NSET [19].*

excited donor molecule (D) and the ground state acceptor molecule (A) results in a nonradiative energy exchange between them in this occurrence. Because the energy transfer efficiency is inversely related to the sixth power of the distance between the donor and acceptor molecules, the length scale of nanoscopic FRET is limited to 8 nm beyond which it is too weak to be used [60]. Further, a long-range dipole-surface contact mechanism based on NSET has recently been developed, with energy transfer range twice that of FRET (**Figure 6**) [19, 29]. In metal nanoparticles, the rate of energy transfer from the oscillating dipole to the continuum of electron-hole pair excitations is inversely proportional to the fourth power of the donor to acceptor distance [61, 62].

In the linear regime, both FRET and NSET have been extensively recognized as an efficient tool for the determination of the distance between the sub-microscopic particles and in predicting the dynamics of a coupled hybrid [29, 63, 64].

EPRET, on the other hand, is a nonradiative dipole-dipole resonant interaction discovered by Forster. The various parameters on which EPRET critically depends are inter-particle distance between the donor and the acceptor, spectral overlap between the excitonic emission band and their plasmonic absorption band, relative orientation of the dipole moments of the organic molecule with respect to the plasmonic modes, strength of transition dipole moment, the morphology of plasmonic nanostructures and concentration and molar extinction coefficient of the plasmonic and organic molecules [51, 65]. A significant contrast between FRET and EPRET is in terms of relative orientation amongst dipoles of the donor and acceptor pair. FRET is forbidden when the dipoles are perpendicular to each other while it is maximum in case of parallel dipoles whereas, for EPRET, the probability of energy transfer is minimum when the relative orientation of the dipoles of the donor acceptor pair is perpendicular to each other, but it is never zero [51].

When the plasmon and exciton are placed very close to each other (5–10 nm) it is found that DET dominates the interaction. This type of energy transfer occurs due to hoping of electrons between the overlapping wave functions of the donor and acceptor molecule [66]. Photo-stability occurs when the short-lived excited states reduce the potential of photo-bleaching and other interactions that would otherwise destroy the chromophores' fluorescent nature, resulting in an increase in their photo-stability [51]. Furthermore, because more excited molecules are now pushed down to the ground state and ready to absorb and participate in the emission process, this impact raises the molecules excitation rate, resulting in an increase in the overall emission rate [67]. Another effect which is in closely related is the lightning rod effect and it takes place when the absorption band of the chromophore overlaps with the plasmon band of the nanoparticle. In this phenomenon, the plasmon band of the nanoparticle acts as a receiver nanoantenna and confines the electromagnetic field, and this significantly strengthens the excitation rate of the chromophore and thus enhances the total emission rate [51].
