**1. Introduction**

The basic requirement for the realization of ultrafast photonic switches, optical limiters and modulators is substantial third-order optical nonlinearity of materials at low light powers [1–3]. However, most of the natural materials possess insignificant nonlinearity in the low light regime [4]. Therefore, design and fabrication of nanoengineered hybrid materials with tunable absorption/emission spectra and considerable third-order optical nonlinearity are a topic of global research [3, 5–7].

It has been realized that plasmonic oscillations can enhance nonlinear optical (NLO) effects majorly in three ways.

First, the coupling between the incident beam and surface plasmons results in a strong local confinement of the electromagnetic fields which in turn enhances the

optical response [8, 9]. This phenomenon forms the basis of surface-enhanced Raman scattering (SERS), where plasmonic excitations arising from metal nanosurfaces are used to boost otherwise weak Raman process by several orders of magnitude [10, 11].

Second, the sensitivity of plasmonic excitations towards the dielectric properties of the metal and the surrounding medium forms the basis for label-free plasmonic sensors. Even the slightest alterations in the refractive index of the environment surrounding the metal surface leads to considerable modifications in the resonance of the plasmonic nanostructures [12, 13]. In nonlinear optical phenomena, this extraordinary sensitivity may be effectively used to control photon-photon interaction. Where, the control beam may be used to modify the dielectric properties of the medium, which in turn would change the plasmonic resonances of the propagating signal beam [14].

Finally, the excitation and relaxation dynamics of plasmonic nanostructures responds to a timescale of femtoseconds regime, thus allowing ultrafast processing of the incident optical signals [15, 16]. This property of plasmonic nanostructures may be conveniently exploited to attain an ultrahigh switching contrast in *All Optical Switching* applications.

Thus, the confinement of surface plasmons in the nanoscale regime not only provides a flexible means of tailoring the optical properties of plasmonic nanostructures but also allows hybridization of metal nanostructures with other molecules such as semiconductors, organic molecules, inorganic molecules [17, 18]. Exclusively, plasmonic-organic hybrids have gathered a lot of attention due to their flexible and versatile interaction mechanisms which can be further fine-tuned to achieve the desired photonic characteristics. Plasmon coupled organic molecules have led to substantial progress in high-throughput DNA detection [19, 20], bio-imaging [21], drug delivery [22], photovoltaic [23] and light-emitting diodes [24], surface-enhanced Raman spectroscopy [11], nanoscale lasers [25], ultrasensitive chemical and biological sensors [26].
