Study of the radiative properties of aligned carbon nanotubes and silver nanorods
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Arrays of nanotubes/rods made of appropriate materials can yield unique radiative properties, such as large absorption and optical anisotropy, with broad applications from high-efficiency emitters and absorbers for energy conversion to the polarization conversion via anisotropic responses. The objective of this dissertation is to investigate the radiative properties of arrays formed by aligned carbon nanotubes (CNTs) and silver nanorods (AgNRs). The CNT arrays used in the present study consist of multi-walled CNTs synthesized vertically on silicon substrates using thermal chemical vapor deposition. Their close-to-unity absorptance is demonstrated by measuring the directional-hemispherical reflectance in the visible and near-infrared spectral ranges using an integrating sphere. The bidirectional reflectance distribution function and angle-resolved reflectance were measured at the 635-nm wavelength. The results demonstrate that high-absorptance CNT arrays may be diffusely or specularly reflecting and have important applications in radiometry. Theoretical modeling based on the effective medium theory (EMT) and reflectivity of an anisotropic medium are developed to explain the high absorption and polarization dependence. The effective optical constants of the CNT array for both ordinary and extraordinary polarizations are quantitatively determined by fitting the angle-resolved reflectance. The AgNR arrays used in the present study were fabricated using oblique angle deposition, which results in inclined Ag nanorods that can be modeled as an effective homogenous and optically anisotropic thin film. The spectral and directional radiative properties of AgNRs grown on different substrates, including a glass slab with a silver film, and compact disc gratings, were characterized at the 635-nm and 977-nm wavelengths for different polarizations. The results are analyzed based on the EMT, rigorous coupled-wave analysis, and anisotropic thin-film optics. The results of this dissertation help gain a better understanding of radiative properties of anisotropic nanostructures for potential applications in high-efficiency energy conversion, radiometric devices, and optical systems.