Date of Graduation

5-2022

Document Type

Thesis

Degree Name

Master of Science in Chemistry (MS)

Degree Level

Graduate

Department

Chemistry & Biochemistry

Advisor/Mentor

Robert Coridan

Committee Member

Maggie He

Second Committee Member

Julie Stenken

Third Committee Member

Stefan Kilyanek

Keywords

Colloids, Light Concentration, Semiconductors

Abstract

Thin film semiconductors are used as photoconductive absorber layers for the development of broadband terahertz generation. Using a femtosecond laser pulse, the generation of a transient increase in the conductivity occurs by photoexciting conduction band electrons in the semiconductor. These thermalize through the emission of terahertz radiation. The route to terahertz generation is not particularly efficient as significant losses come from the absorption in the substrate that is beneath the photoconductive antenna layer. This work explores the application of hexagonally close-packed monolayers of chemically synthesized nanospheres as a potential light concentration method for ultra-thin films of GaAs and black phosphorus that are relevant to terahertz generation. A nanosphere layer can induce an advantageous scattering texture which can increase the effective path length of light transport through the thin film. The nanosphere layer can also induce a significant absorption increase through optical resonances that are caused by the periodic arrangement of hexagonally close-packed spheres. To aid in the study of these effects, we use finite element simulations of absorption in a model GaAs photoconductive layer since GaAs is the present standard photoconductive absorber layer. These simulations enable us to map the absorption resonances in the material as a function of the photoconductive absorber layer thickness and sphere diameter. We are also able to construct the equivalent materials to characterize the optical absorption increases in real, experimental systems. With the aid of these results, we will show that a simple light concentration strategy is able to generate a significant increase in light absorption. Through the increase in light absorption, an improvement of the light-to-terahertz power conversion efficiency is achieved.

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