In this talk, I will introduce two recent theoretical works in my group related to nanophotonics. First, we derive a complete and rigorous theoretical framework from first principles (i.e., Maxwell’s equations), with all needed parameters directly computable from the electromagnetic (EM) responses of involved plasmonic resonators, to understand the rich coupling-induced physics in complex plasmonic open systems. Our theory is quantitatively justified by both full-wave simulations and optical experiments, on both simple-particle systems and strongly coupled complex ones. In addition, we can utilize the theory to freely tailor the line-shapes of complex plasmonic systems, which offers a power tool for fast designing functional optical devices facing diversified application requests. Second, we establish a unified framework to understand two distinct spin-orbit-coupling (SOC) – induced effects discovered in beam scatterings at optical interfaces (i.e., the vortex generation and photonic spin Hall effect) on the same foot, and further predict that an intriguing phase transition between them can happen under certain conditions. We show that for an incident beam striking at an optical interface, whereas some wave components inside the beam can gain Berry phases generating an optical vortex, the remaining wave components gain Berry phases contributing a spin-Hall shift, and thus the competitions between these two effects lead to many fascinating effects. Intriguingly, the strengths of these two terms can be efficiently tuned by varying the incident angle and width of the beam, dictated by the topology changes of different k-cones inside the beam. We finally describe more implications and applications of our discovery.