Thermal radiative properties of disordered media and deep metal gratings

Abstract

Controlling thermal radiation through surface roughness, volumetric inhomogeneity and periodic micro/nanostructures has enormous applications in energy harvesting and saving, thermal management, photonics and optical sensing. The present dissertation investigates the spectral radiative properties of various materials and micro/nanostructures by performing three research tasks. (1) Radiative properties of dense ceramic Al2O3, AlN, and Si3N4 plates are studied from the visible to the mid-infrared region at room temperature, by measuring the directional-hemispherical and specular transmittance and reflectance. The spectral scattering and absorption coefficient of Al2O3 and AlN are retrieved from the measured radiative properties. The dielectric functions are modeled which indicate a distinctive polycrystalline structure of the Si3N4 plates compared with previous reports. (2) A diffusive dual-layer solar reflector is fabricated and characterized. Its record-high solar reflectance and high mid-infrared emittance suggests the potential use for passive daytime radiative cooling. The directional-hemispherical and bidirectional radiative properties of the structure were measured and compared with a Monte Carlo ray-tracing program to elucidate the underlining mechanism. (3) The excitation of magnetic polaritons is experimentally demonstrated in several microfabricated grating samples in the near-to-mid infrared region. The influence of incidence angle, plane of incidence, polarization, and the trench depth on the spectral reflectance is investigated. Based on the rigorous coupled wave analysis and an LC-circuit model, the MP dispersion for off-plane layout is studied and demonstrated for the first time. The results obtained in this thesis are expected to impact a broad range of practical applications such as radiometric devices, passive radiative cooling and energy conversion devices.