This dissertation focuses on exploring novel light steering and spectral engineering functionalities by flat optics, which includes photonic architectures of flat plasmonic metasurfaces, nanoparticle arrays, and thin film coatings. We have designed, fabricated and characterized different plasmonic metasurfaces (trapezoid-shaped antennas, nanorings/nanowires, etc.), nanoparticle arrays (single-layered or multilayered nanocubes and nanodisks),...