Role of magnetic resonance and wave interference in tailoring the radiative properties of micro/nanostructures.

摘要

The spectral and directional control of radiative properties by utilizing engineered micro/nanostructures has enormous applications in photonics, microelectronics, and energy conversion systems. The present dissertation aims at: (1) design and analysis of micro/nanostructures based on wave interference and magnetic resonance effects to achieve tunable coherent thermal emission or enhanced optical transmission; (2) microfabrication of the designed structures including multilayered thin films and patterned structures; and (3) development of a high-temperature emissometer to experimentally demonstrate coherent thermal emission from fabricated samples using spectrometric techniques at temperatures from 300 to 800 K.;An asymmetric Fabry-Perot resonant cavity, sandwiched by a thick Au film and an ultra-thin Au film of few nanometers, was studied as a potential coherent emission source based on the wave interference effect. The coherent emission behavior from fabricated samples was successfully demonstrated from room temperature up to 800 K. The reflectance was measured at room temperature using a Fourier-transform infrared spectrometer, and then the emittance can be indirectly obtained from Kirchhoff's law. A high-temperature emissometer was built to measure the thermal emission of fabricated multilayer samples at elevated temperatures, and the temperature effect on the emission peaks was discussed. To facilitate the computation of thermal emission from layered structures with nonuniform temperature distributions, the direct (fluctuational electrodynamics) and indirect (matrix formulation) approaches were analyzed and unified, resulting in a generalized Kirchhoff's law.;A comprehensive investigation was performed to understand and potentially utilize the magnetic resonance effect for tailoring radiative properties in periodic grating microstructures. Contrary to the conventional explanations using the coupled-surface-plasmon-polaritons or Fabry-Perot-cavity resonance effect, the effect of magnetic resonance was identified to be responsible for the resonant transmission/absorption in metallic grating structures. By using capacitor-inductor circuit models and rigorous coupled-wave analysis, good agreement on the resonant conditions was shown, through which the physical mechanism as magnetic resonance was verified. Another finding was to identify phonon-mediated magnetic polaritons (MPs) in microstructures made of polar materials, and to predict extraordinary radiative properties in the infrared region as their counterparts in metallic microstructures. Based on the excitation of MPs, an innovative coherent thermal emitter was designed and extended in particular for thermophotovoltaic (TPV) applications. The spectral selectivity and directional insensitivity associated with MPs offer unique emission spectra favored in TPV systems to improve the conversion efficiency. The unique characteristics of magnetic polaritons were, for the first time, experimentally demonstrated from fabricated microstructured surfaces at room temperature as well as elevated temperatures. Both experimental and theoretical studies suggest that the resonance wavelength can be tuned with strip widths for different applications, and the emittance peak changes little with temperature, indicating that high performance can be achieved without much degradation at elevated temperatures.;The fundamental understanding and experimental results obtained from the dissertation will facilitate the design and applications of micro/nanostructures in energy systems to harvest solar energy as well as recover waster heat.