Doctor of Philosophy (PhD)
The School of Electrical Engineering and Computer Science
In this dissertation we design and analyze nanostructures for subwavelength guiding and enhanced light-matter interactions.
We first investigate three-dimensional plasmonic waveguide-cavity structures, built by side-coupling stub resonators that consist of plasmonic coaxial waveguides of finite length, to a plasmonic coaxial waveguide. These structures are capable of guiding and manipulating light in deep-subwavelength volumes. We show that three-dimensional plasmonic coaxial waveguides offer a platform for practical realization of deep-subwavelength optical waveguides.
We then introduce compact wavelength-scale slit-based structures for coupling free space light into the fundamental mode of plasmonic coaxial waveguides. We consider single-, double-, and triple-slit structures optimized at the optical communication wavelength and find that, when the slits are at resonance, the coupling to the plasmonic coaxial waveguide increases. We also investigate slit-based outcoupling structures for light extraction from the waveguide into free space.
We also numerically design and experimentally test a SERS-active substrate for enhancing the SERS signal of a single layer of graphene in water. The graphene is placed on top of an array of silver-covered nanoholes in a polymer and is covered with water. We report a large enhancement in the SERS signal of the graphene on the patterned plasmonic nanostructure for a 532 nm excitation wavelength. We find that the enhancement is due to the increase in the confinement of electromagnetic fields on the location of graphene that results in enhanced light absorption in graphene at the excitation wavelength. We also find that water droplets increase the density of optical radiative states at the location of graphene, leading to enhanced spontaneous emission rate of graphene.
Finally, we introduce a structure for near total absorption in a graphene monolayer at visible wavelengths. The optical interaction of graphene with local fields is enhanced by means of critical coupling. The graphene monolayer is placed on a grating slab without being covered with other structures, so the quality of graphene remains intact. We investigate the enhanced light-graphene interactions in this structure. We use experimental data for the dielectric permittivity of the materials used in the structure. The structure could find applications in the design of efficient nanoscale photodetectors and modulators.
Mahigir, Amirreza, "Plasmonic Structures for Subwavelength Guiding and Enhanced Light-Matter Interactions" (2018). LSU Doctoral Dissertations. 4534.