Undergraduate Thesis Or Project
 

Application of Coupled Oscillator Model to Terahertz and Optical Control of Plasmon Induced Opacity in Coupled Metamaterials

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https://ir.library.oregonstate.edu/concern/undergraduate_thesis_or_projects/qj72pg21x

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  • Terahertz (THz) frequencies of the electromagnetic spectrum have been underutilized when compared to neighboring microwave and infrared frequencies, largely due to the difficulties controlling and detecting these fields. In a step toward gaining control over THz frequencies, my advisor Dr. Yun-Shik Lee’s group experimentally demonstrated optical-pulse THz-control over a plasmonic metamaterial. To complement the experiments, I theoretically model the system using a coupled linear oscillator model to describe plasmonic oscillations of the metamaterial. The parameters of this oscillator model are fitted to the THz-Time domain spectroscopy results from the experimental counterpart without the presence of an optical pulse. Enhanced control of the THz response was experimentally demonstrated by the group by optically exciting charge carriers in a GaAs substrate, which was found to interfere with the plasmon-induced opacity in a predictable manner. Furthermore, these experiments show that plasmon-induced opacity can be reintroduced in the photoconductive sample by increasing incident THz field strength. These observations are modelled theoretically by introducing time dependence to the damping rates of the coupled linear oscillator model. The relation between the oscillator damping rates and THz field strength is attributed to phonon-assisted intervalley tunneling in large THz fields, which involves a decrease in substrate conductivity as electrons tunnel to side valleys with larger effective mass. A simple description of intervalley scattering is done using a two state population model where only electrons in the lower-energy 𝛤-valley state contribute to substrate conductivity. The final results of my model replicate the THz response of the composite metamaterial sample with and without optical excitation, as well for differing THz field strengths. We observe the oscillation of THz transmission through the sample over sub-picosecond shifts in the optical pulse delay time, which is a consequence of intervalley scattering induced by plasmonic oscillations in the metamaterial. Finally, the size and temporal locations of these modulations in transmission are shown to be intimately related with transport properties of the substrate, such as intervalley scattering rates and mean-free times.
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