Enhancement of light via surface plasmon coupling in the visible

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  • March 20, 2019
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  • Ray, Emily A.
    • Affiliation: College of Arts and Sciences, Department of Physics and Astronomy
Abstract
  • The incidence of light with momentum components outside the light cone on the surface of a negative permittivity material results in the excitation of a surface plasmon polariton and the enhancement of the incident signal when there is momentum and energy conservation. This process has an impact across many fields including imaging, optical computing, signaling, and photovoltaic devices, among others. I examine the role and tunability of light-surface plasmon interactions in several applications. I demonstrate a tuned metamaterial grating system that allows the signal from evanescent waves to be detected in the far field in the visible regime. I fabricate a metamaterial that is tuned to support surface plasmons that couple to visible light across a wide range of wavelengths. I characterize the plasmonic response through a simple technique wherein a the reflection from a subwavelength grating on a metamaterial indicates surface plasmon coupling when its intensity dips. With this I demonstrate that the reflection trends match well with simulation, indicating that coupling of light to surface plasmons occurs at the expected crossing points. The strength of coupling (denoted by the drop in reflection) however, is less than expected. Transmission measurements reveal a depolarizing effect that accounts for the decrease in evanescent light enhancement by the surface plasmons and is due to the surface roughness at the interfaces between the metal and dielectric. I also use a tuned metamaterial perforated with a subwavelength array of circular apertures to exhibit extraordinary transmission in the visible. I compare the transmission of the metamaterial to that of a thin film of Ag with equivalent thickness that has fewer plasmon modes and a resonance position in the UV to find that for 400 nm, both thin films exhibit a transmission minimum at 650 nm. Both film spectra have plasmon-aided extraordinary transmission peaks where there is momentum and energy conservation between the evanescent waves produced by the diffraction grating and the surface plasmons in the metamaterial at 570 nm and 700 nm. Here, more light is transmitted through the holes than is incident on them. Furthermore, I see that the surface plasmon generation by the holes themselves is negligible compared to those generated by the surface plasmon. I then explore the mechanism of increased external quantum efficiency with plasmonic structures in organic bulk heterojunction solar cells. I build an inverted bulk heterojunction solar cell with a Ag back cathode patterned with a diffraction grating to separate the possible mechanisms of enhanced current production. I-V curves from the patterned cell signify a total efficiency 3 times larger than a flat reference cell and the incident photon to electron conversion efficiency exhibits peaks where there is an increase in interaction path length of the incident light in the active layer due to scattering and none at the surface plasmon resonance position leading to the conclusion that the increase in performance is due to scattering and not plasmon generation.
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  • ... in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Physics and Astronomy.
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  • López Noriega, René
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