Modeling of porous metal-based nanostructures

多孔金屬納米材料的模擬與研究

Student thesis: Doctoral Thesis

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Author(s)

  • Shiwei SHU

Detail(s)

Awarding Institution
Supervisors/Advisors
Award date14 Feb 2014

Abstract

A porous metal-based nanostructure of various different structural designs is a promising material for use in constructing reflectors, absorbers and thermal emitters with extra properties like superior electrical conductivity, thermal conductivity, magnetic responses, and the mechanical performance of metals. Such a multilayer porous metal-based nanostructure is believed to open up new routes to manufacturing a wide range of multi-functional optoelectronic devices. The first chapter of this thesis describes the conception of the one-dimensional (1D) multilayer nanostructure including a rugate filter and Bragg stack. The calculation methods of Finite Difference Time Domain (FDTD) and Scattering Matrix Method (SMM) are introduced in detail, before a brief discussion of the background to absorbers and thermal emitters. The second chapter sets out how a metal-based rugate filter can be utilized as a reflector. Randomly dispersed metallic nanorod arrays with sinusoidally varying porosity along the film thickness are proposed. It is shown that metallic rugate filters can display strong optical responses in the visible and near-infrared regions for both Transverse Electric (TE) and Transverse Magnetic (TM) polarizations and for a wide range of incident angles. Moreover, the optical features of the metallic rugate filter can be fine-tuned by adjusting its structure. Interestingly, multiple-peak rugate filters can be conveniently achieved by combining different periodicities in one rugate structure. The third chapter goes on to demonstrate how as metal-based rugate filter can also be utilized as a near-perfect absorber in the visible and near-infrared regions. The model presented also builds on nanoporous metal films whose porosity follows a sinewave along the film thickness. By setting the initial phase of porosity at the top surface as zero, near perfect absorption is obtained. Furthermore, the rugate absorbers also show near-perfect absorption for TE and TM polarizations and large incident angles. The fourth chapter builds on the foregoing to propose that lossy Bragg stacks can also be utilized as reflectors and absorbers by controlling their structural and material parameters. Moreover, their reflection and absorption behavior persists for different polarization and wide incident angles, as for metallic rugate filters. A mechanism study is conducted to show that the reflection and absorption properties depend on the structural arrangement and are mainly due to the interference of lossy multilayers. The fifth chapter proposes a selective thermal emitter based on a metallic multilayered structure consisting of a graded antireflection top layer, a middle layer with uniform porosity, and a nonporous substrate layer. The proposed emitters feature an emission edge in the near-IR region where the emissivity drops from over 0.9 to below 0.1 for both the TE and TM polarizations and a wide range of emission angles with the emission edge nearly nonshifted. The sixth chapter focuses on Salisbury screen absorption in the visible and near-IR regions. The absorber proposed here is an unpatterned metal/dielectric/metal triple layer; for example, a 20 nm-thick metal film as the top, a 250 nm-thick dielectric film as the middle, and a 200 nm-thick metal film as the bottom layer. A mechanism study is conducted and shows that the high-efficiency absorption at specific wavelengths is mainly due to the Fabry-Perot (FP) resonances in the dielectric middle layer, which result in trapping of the resonant light and thus enhanced absorption efficiency.

    Research areas

  • Nanostructured materials, Porous materials, Microstructure, Metals