**Abstract**

The unique ability of plasmonic structures to concentrate and manipulate photonic signals in deep sub-wavelength domain provides new efficient pathways to generate, guide, modulate and detect light. Due to collective oscillations exhibited by the conducting electrons of metallic nanoparticles, their local fields can be greatly enhanced at the localized surface plasmon resonance (LSPR). Hence, they offer a versatile platform, where localized surface plasmons can be tuned over a broad range of wavelengths by controlling their shape, size and material properties. It has been realized that plasmonic excitations can strengthen nonlinear optical effects in three ways. First, the coupling between the incident beam of light and surface plasmons results in a strong local confinement of the electromagnetic fields, which in turn enhances the optical response. Second, the sensitivity of plasmonic excitations toward the dielectric properties of the metal and the surrounding medium forms the basis for label-free plasmonic sensors. 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. This chapter aims to discuss all the aforementioned interactions of plasmons and their excitonic hybrids in detail and also represent a glimpse of their experimental realizations.

**Keywords:** plasmon, exciton, nonlinear interactions, resonant interactions, nonresonant interactions
