Analytical Methods for Quantification of Light-Matter Interactions in Subwavelength Metal Nanostructures




Pati, Amrita

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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.



subwavelength slit, plasmonic slot waveguide, reflection, propagation, analytical model, maximum power transfer, gap plasmon, extreme confinement, plasmonics