Date of Award
It is known that carbon-nanotube (CNT) forests, nanopillar arrays, and other high aspect-ratio conducting and semi-conducting nanostructures can have extremely low reflectance in a wide range of wavelengths, and that reflectance begins to rise at long wavelengths. The mechanism for this behavior is poorly understood. It has been shown that the reflectance of CNT forest varies with the morphology of the forest, which indicates that the interface of such a material may play a primary role in its reflectance and absorptance.
Simulations of the reflection of light from arrays of conducting nanorods using commercial finite-difference-time-domain software predict a similar increase in reflectance at long wavelengths, and the reflectance spectra have characteristics that hint at the presence of interference phenomena. In order to understand the role of interference in the properties of a broadband absorber with a nonuniform conducting interface, the reflection of light from a two dimensional square array of conducting patches is modeled with scalar waves using the Huygens-Fresnel principle. This model qualitatively explains the characteristic increase of reflectance with an increase in wavelength, and the observation that a decrease in CNT forest density increases the onset wavelength of reflectance.
However, a true theoretical understanding must be based on Maxwell’s equations which describe electromagnetic radiation precisely. As a first step, the reflection of light from a one dimensional periodic conducting interface is calculated based on electromagnetic scattering theory. Contrary to the prediction of the scalar wave model, this solution predicts a decrease in reflectance with an increase in wavelength, indicating that a nonuniform conducting interface may have a decreased role in reflectance at long wavelengths. Additionally, this solution fails to explain the extremely low reflectance of short wavelength light observed from many conducting and semi-conducting nanostructures. Further analysis and an extension of this scattering theory to a two dimensional interface is necessary to fully understand the properties of broadband absorbing nanostructures.
Lange, Christian, "The Reflection of Light from Periodic Conducting Interfaces" (2021). Undergraduate Honors Capstone Projects. 697.
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Departmental Honors Advisor
Capstone Committee Member