ABSTRACT In this dissertation, we develop new composite optical materials by the assembly of silica nanoparticles (NPs) into micron-scaled geometries and embedding them into selected polymer media to achieve tunable and synergistic optical properties for potential applications, including photovoltaics, photobioreactors, or solar-integrated smart windows. The size and the refractive index of NPs, and the geometric arrangements are explored to optimize the targeted optical properties, including forward scattering and the reflectance of light. We also explore different types of the polymer matrix to allow for matching refractive index or introducing stimuli-responsive light scattering. Firstly, we develop a novel design of semi-transparent composite films for delivering equal and optimally efficient “doses” of sunlight to photosynthetic cells, algae, in a photobioreactor system. The efficient re-distribution of solar flux can allow the algae to receive spatially diluted solar flux and thus avoid photodamages from the direct sunlight. We develop colloidal NPs are assembled into microspheres via water-based emulsion evaporation method which is easy to set up and suitable for large-scale fabrication. Backed by numerical calculations, the overall shape/size of the microsphere and the effective refractive index of the composites are optimized, the synthetic scatterers that recapitulate the salient forward-scattering behavior of the Tridacnid clam system are presented. These hierarchically structured beads also generate various structural colors due to the periodic packing of NPs, which allows back-reflection of photosynthetically inefficient green or yellow lights depending on the sizes of NPs. Our method is simple yet scalable, cost-effective, and environmentally benign, which inspires new designs for small-footprint biofuel applications. Secondly, we develop new strategies to tune the optical transmittance via mechano-responsive polymers. Stretchable smart window film consisting of an elastomer with micro-scale wrinkles and silica (SiO2) NPs can provide reversible and switchable transparency-opacity. We design and fabricate mechanically responsive films composed of silica NPs and elastomers. By incorporating deformable and controllable wrinkling patterns into the particle-embedded polymer systems with wrinkles and NPs on opposite side of the films, we achieve the desired transparency at a low mechanical strain (< 20%) without wasting energy. The wavelength, amplitude of wrinkles as well as their local ordering can be fine-tuned by the pre-strain vs. applied strain, particle size, oxygen plasma treatment intensity and time, thus, changing the total amount of light refracted from the curved surfaces. We note that since the nano-/micron-sized voids are created at the interface between NPs and polymer matrix upon stretching, they act as light scatters to “reveal” the embedded structural colors, resulting in drastic transparency-opacity switching. Besides, we fabricate the particle-embedded PDMS composite films with wrinkles on the same side of NPs and demonstrate their mechano-responsive optical properties. We confirm that the formation of ordered wrinkling patterns is impeded by the presence of particles on the surface and the homogeneity of wrinkle formation is increased by higher pre-stretch strain and longer plasma exposure time. Thus, we tune the optical transmittance of films by varying the pre-strains and the size of NPs separately. Our material designs offer new insights on how to fine-tune optical properties with minimal energy consumption for applications including on-demand smart windows and strain sensors.