Analytical Methods for Quantification of Light-Matter Interactions in Subwavelength Metal Nanostructures
Date
2023-01-16
Authors
Pati, Amrita
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Abstract
The thesis presents analytical techniques to determine mode propagation characteristics
in subwavelength metallic slits and plasmonic slot waveguides. These
metal-insulator-metal geometries have been successfully applied in wide-ranging
applications having demonstrated unprecedented performance in high-speed electrooptic
modulation and rf-to-fiber conversion. But most of the theoretical studies
focused on them rely on numerical methods, which are resource-intensive and lack
physical insights. The proposed models address these challenges by calculating the
properties in terms of other physical parameters, thereby providing the desired intuition.
In both frameworks, analytic expressions for reflection coefficients at the structure’s
interface with surrounding dielectric media were derived. This was achieved
by employing the single-mode matching to continuum technique under the perfect
electric conductor approximation. Dielectric loading was introduced to account for
the finite permittivity of real metals. In the case of the slit, the reflection coefficient
values for different source and waveguide parameters were used in the Fabry-
Pérot transmission model to calculate field enhancement, power, scattering, and
absorption cross-sections. It was shown that the power in the slit was maximum
if the scattering and absorption cross-sections matched at the resonance condition
of a given slit configuration, a manifestation of the maximum power transfer theorem.
This also implied that the power in the slit, unlike the field enhancement,
was maximum for slit widths typically larger than the narrowest slit under consideration
and can be treated as a key parameter to balance mode confinement and
propagation lengths.
The analysis of the plasmonic slot waveguides was based on a geometric optics
approach. The reflection phase values obtained in the first stage were used in the waveguide transverse resonance condition to obtain values of propagation angles
that allowed the existence of modes in the structure. These angular solutions were
then used to compute modal properties such as mode effective index and propagation
lengths. Both the analytical frameworks were significantly faster (at least two
orders of magnitude) than numerical simulations and demonstrated close agreement
to within 3% of the numerical simulation results. These analytical models
present an efficient way to design and optimize subwavelength slit and plasmonic
slot waveguides for different applications and may be extended to analyze other
plasmonic geometries for wider implementation.
Description
Keywords
subwavelength slit, plasmonic slot waveguide, reflection, propagation, analytical model, maximum power transfer, gap plasmon, extreme confinement, plasmonics