This presentation will begin with a brief overview of the rapidly evolving field of quantum plasmonics and nanophotonics, setting the stage to discuss our recent contributions. Specifically, we examine how the surface plasmon resonances of metal nanoparticles facilitate enhanced local electromagnetic fields, enabling their application as photonic or plasmonic nano-antennas and optical cavities. By coupling semiconductor nanoparticles with these plasmonic resonances, we discuss the formation of hybrid light–matter quasiparticles known as exciton-polaritons. The talk will explore our theory and modeling efforts covering the nonlinear optical properties of polaritonic states in metal–semiconductor nanostructure arrays, highlighting mechanisms for generating coherent photon states in the up-conversion regime. We will also address down-conversion processes, including difference-frequency generation and terahertz (THz) pulse production, both driven by the nonlinear response of plasmonic arrays. Finally, the presentation will cover our recent work on modeling generation of entangled photon pairs via spontaneous parametric down-conversion (SPDC) within the nanostructured plasmonic arrays.