Nanosphere Lithography Approach To Fabrication Of Novel Metal Nanostructures And The Study Of Their Optical Properties

Metamaterials, artificial composite structures with exotic optical properties that are unattainable with naturally occurring materials, have emerged as a new frontier of science involving physics, material science and information science.Motivated by the fabrication and optical properties of novel metal nanostructures, in this thesis, we show that templating against colloidal crystal can provide alternative and efficient routes to prepare metamaterials by parallel angle-resolved nanosphere lithography. The resonances are shown to allow for tunability over a large spectral range down to near-infrared by controlling the feature sizes of the nanotriangles. We exploited a new-type of surface plasmonic crystal based on double layer colloidal crystal and showed the Ag nanoparticle coated with dielectric thin film application in surface enhanced Raman spectroscopy (SERS). The obtained results are described as follows:1. We developed a fabrication technique for large-area single-domain colloidal crystals and fabricated metamaterials using parallel angle-resolved nanosphere lithography. The prepared metamaterials are composed of pairs of partially-overlapped metallic nanotriangles arranged in a 2D hexagonal lattice with their openings oppositely placed in each unit cell.2. We experimentally and theoretically investigated the electric and magnetic resonances of these metamaterials. From its light-polarization dependent transmission features, we demonstrated that each double-triangle can be viewed as an artificial magnetic element analogous to the conventional metal split-ring-resonator (SRR). We show that under normal-incidence conditions, individual double-triangle can exhibit a strong local magnetic resonance, but the collective response of the metamaterial arrays is purely electric because magnetic resonances of the two double-triangles in a unit cell having opposite openings are out of phase. In contrast, for oblique incidences the metamaterial arrays are shown to support a pure magnetic response at the same frequency band because the impinging magnetic field has a component normal to the sample plane. Therefore, switchable electric and magnetic resonances are readily achieved in double-triangle arrays. Moreover, both the electric and magnetic resonances are shown to allow for tunability over a large spectral range down to near-infrared by controlling the feature sizes of the nanotriangles.3. We investigated the SERS effect of metal nanoparticle-based substrate coated with a dielectric thin film. It is found that a 5 A ta-C film coated Ag nanoparticle substrate shows a higher enhancement of Raman signals than the uncoated substrate, which is further supported by our numerical simulations, which shows that an ultrathin dielectric layer can enhance the electromagnetic field of the metal nanoparticle. Serving as protective layer, a 10 A or thicker ta-C layer is efficient to protect the oxygen-free Ag in air and prevent Ag ionizing in aqueous solutions. This is important to maintain the chemical stability and biocompatibility for an Ag-based substrate in SERS.4. We investigated the optical properties of a new-type plasmonic crystals via deposition of metal nanoparticles on the surface of doulbe layer colloidal crystals. Compare with the plasmonic crystals fabricated by depositing metal nanoparticle on monolayer colloidal crystals, the main resonances of this plasmonic crystals based on double layer colloidal crystals have dramaticlly splitted because of scattering effect of multilayer dielectric photonic crystals. The resonant optical properties of these quasi-three-dimensional plasmonic crystals are associated with the layer number of colloidal crystals.5. We developed metallic nanoshell arrays, and investigated its optical transmission properties. The metallic nanoshells were using colloidal crystal as template and deposited meal film twice. Compare with half shells fabricated by once deposition, the transmission of nanoshell arrays deposited by twice have dramatic changes. For the oblique incidence, the dispersion characteristics of nanoshell were very weak, showing that the mode was highly confined. By controlling the diameter of the nanosphere we can easily tune the resonant wavelength of the mode.