The effect of geometry and surface morphology on the optical properties of metal-dielectric systems
Hasegawa, Keisuke, 1977-
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Hasegawa, Keisuke, 1977-
We analyze the effect of geometry and surface morphology on the optical properties of metal-dielectric systems. Using both analytical and numerical modeling, we study how surface curvature affects the propagation of surface plasmon polaritons (SPPs) along a metal-dielectric interface. We provide an intuitive explanation for how the curvature causes the phase front to distort, causing the SPPs to radiate their energy away from the metal-dielectric interface. We quantify the propagation efficiency as functions of the radius of curvature, and show that it depends nonmonotonically on the bend radius. We also show how the surface morphology influences the transmittance and the reflectance of light from disordered metal-dielectric nanocomposite films. The films consist of semicontinuous silver films of various surface coverage that are chemically deposited onto glass substrates. They exhibit a large and broadband reflection asymmetry in the visible spectral range. In order to investigate how the surface morphology affects the asymmetry, we anneal the samples at various temperatures to induce changes in the morphology, and observe changes in the reflection spectra. Our study indicates that the surface roughness and the metal surface coverage are the key geometric parameters affecting the reflection spectra, and reveals that the large asymmetry is due to the different surface roughness light encounters when incident from different side of the film. Additionally, we analyze how thin metal and dielectric layers affect the optical properties of metal-dielectric systems. Using the concept of dispersion engineering, we show that a metal-dielectric-metal microsphere--a metal sphere coated with a thin dielectric shell, followed by a metal shell--support a band of surface plasmon resonances (SPRs) with nearly identical frequencies. A large number of modes belonging to this band can be excited simultaneously by a plane wave, and hence enhancing the absorption cross-section. We also find that the enhanced absorption is accompanied by a plasmon assisted transparency due to an avoided crossing of dominant SPR bands. We demonstrate numerically that both the enhanced absorption and the plasmon assisted transparency are tunable over the entire visible range. We also present an experimental study of light scattering from silica spheres coated with thin semicontinuous silver shells, and attempt to describe their optical response using a modified scaling theory. This dissertation includes previously published co-authored materials.