In this thesis, we mainly utilize silver to construct the meta-atoms. Although suffering from easily oxidized, its excellent lossless property throughout the visible range makes it suitable for study. When preserved properly, silver-made metasurfaces can stay stable. To explore other potential materials for plasmonics, a cost-effective approach to fabricating gold and silver alloy is developed. Metalens and metaholograms are demonstrated as an example. Such alloy combines the stability of gold and the lossless property of silver. This low-temperature annealing approach can also apply to gold and palladium alloy when optimized. In Chapter 2, approaches to study the subwavelength scaled meta-atoms are presented. We initially rely on numerical electromagnetics to obtain the reflectance and phase shift of a meta-atom. The meta-atoms are thus fabricated according to the design in simulation, and validate the optical responses with measurement. Because polarized light is applied in simulation, the measurement environment is reproduced by adding polarizers and analyzers in illumination and detection paths. This concept seems straightforward and easy, however, the chiral property of a structure and optical responses of elements can lead to unphysical results if not carefully considered, as presented in the last section. With many verified simulated spectra, we are confident in the observations in the simulations. In Chapter 3, applications of metasurfaces in wavefront manipulation are demonstrated. With the increasing demands for flat optics make the proof-of-concept demonstrations step forward in mass production in the industry. Fabricating dimension-sensitive meta-atoms becomes challenging as a result of many complicated factors during fabrication. It takes much time to optimize the fabrication process to produce perfect structures as simulated. We found that perfect structures do not always guarantee the best, which relieves the restrictions in fabrication. In Chapter 4, the cause of the spectrum from structural colors is investigated. With the understanding of our measurement setup and numerical simulations, we are able to analyze these polarization-dependent structures exhibiting different spectra. Those commonly used geometries of meta-atoms are sensitive to the polarized and exhibit polarization-induced chirality, leading to very diverse responses. We propose to utilize this property for optical encryption, which can easily support a quaternary coding system. At the end of this Chapter, the primary results of an ongoing project are presented, where we combine these polarization-sensitive meta-atoms and artificial intelligence for structural designs in encryption applications. In Chapter 5, strong near-field enhancement of plasmonics is applied to increase the yield of luminescence. Rare-earth ion doped upconversion nanoparticles (NaYF4: Yb3+, Er3+) serve as matters on a platform of double resonance antennas. We utilize the optical properties of the plasmonic antennas to tailor the channels, leading to selectively enhance of the frequency upconversion process. This light and matter interaction can be applied in luminescence. These theory-supported studies are simulated and demonstrated experimentally, displaying their abilities and facilities to harness the visible light. We believe this thesis can shed some light and offers inspiration in the field of flat optics.