In the development of optical components, achieving full control of electromagnetic (EM) waves across all frequencies has long been a significant goal with wide applications in fields like communication, defense, and energy. One major challenge is how to control EM waves efficiently in a compact form. Conventional optics use accumulated propagating phases, requiring large spaces (>1000 times wavelength), and suffer from frequency-dependent performance due to material dispersion. Recently, metasurfaces have been proposed as an alternative solution to overcome these limitations. These ultra-thin, engineered surfaces allow tailored control of wave transmission and reflection, making them ideal for compact and efficient optical devices.This dissertation introduces several key advancements in metasurface design and application. First, we present a high-efficiency, broadband metasurface beam splitter/combiner, achieving near-perfect diffraction uniformity (>97%) and efficiency (>90%) across a wide wavelength range from 1525 to 1575 nm. This device, optimized using particle swarm optimization, significantly enhances metasurface performance in terms of uniformity, bandwidth, and efficiency, paving the way for applications in high-power laser systems, quantum photonics, and depth sensing. Additionally, we demonstrate a metasurface-enabled all-optical helicity-dependent magnetic switching platform, capable of ultrafast, room-temperature switching in ferromagnetic thin films. This platform combines an all-dielectric metasurface that generates circularly polarized light with a multilayer ferromagnetic thin film, enabling deterministic control of magnetization. This technology holds great promise for the future of high-density memory storage and ultrafast information processing. In the final chapter, we demonstrated the first experimental observation of a subwavelength phase singularity in a chiral medium. This groundbreaking discovery highlights the versatility of metasurfaces and subwavelength-scale nanostructures, opening up new possibilities for high-sensitivity, chip-scale photonic devices. Notably, the phase singularity we observed exhibits remarkable robustness against fabrication imperfections, offering a more flexible platform for the design and manufacturing of photonic devices. This unprecedented combination of subwavelength light confinement and robust phase control holds great promise for both quantum and classical applications, paving the way for innovative devices that remain stable under external stimuli and fabrication imperfections.