Hyperspectral Light-trapping within Graded Gratings Using Adiabatically Coupled Plasmonic Waveguide-Resonators

Rainbow-trapping or slowing down broadband electromagnetic radiation over a subwavelength grating, provides new opportunities for light-matter interaction on a nanometer scale. Previous efforts have shown rainbow-trapping is possible on functionally graded structures. Here, a new gradient parameter is proposed for designing rainbow-trapping gratings, which takes advantage of the close correlation between the groove-width and the overlap of the evanescent fields within the grooves. In the suitable range (~150 nm), this width parameter is as important as other known variables such as groove depth and materials composition, but with the added advantage that tailoring groove widths is remarkably more feasible in practice. This is shown to be the case through nanofabrication techniques described in this dissertation. The effect of groove-width on the dispersion relation of the nano-groove is studied, resulting in an analytical solution for the effective index of the groove. Adjusting this effective index through the width-parameter establishes the basis for the graded-index gratings discussed herein, where groove-widths are tapered across the grooves of a grating. Accordingly, using groove-width as a design parameter gives rise to rainbow-trapping in linear, as well as other geometries such as bullâ s eye type structures. Furthermore, the extension of graded gratings to other geometries such as cylindrical and spiky spherical nanoparticles is studied through transformation optics. The case of spiky nanoparticles with a tapered spike geometry is closely examined through Multiphysics simulations to predict the thermal effects of light-matter interaction in pico- and nanosecond timescales, as well as experiments conducted under continuous wave conditions. Using gold nanoparticles with diameters of (â ź30-100 nm) and various spike aspect-ratios, enables exploiting both the plasmonic resonance of the spherical particle (Mie resonance), as well as the plasmons contributed and guided by the graded geometry of the spikes. The interaction of these two types of resonances results in an unprecedented and high-resolution control of the plasmonic field distribution and temperature profile This deeply sub-wavelength heat control allows the scalable genesis of self-dressing nanoparticles in a thermally triggered precursor of the shell under controllable laser illumination. The result is nanoparticles with controllable partial shell-formation, where the tips of gold spikes are left exposed. This is in contrast to the blanket coverage of the particle that can lead to mode suppression, by blocking lightâ s access to the spikes which are responsible for light-coupling to surface plasmons. The derived analytical formula obtained through treating each nano-groove as a plasmonic waveguide resonator is compared with simulations, and near-field optical measurements in the visible and infrared regions. These results closely agree with numerical simulations, fabrication, and characterization results.

Keywords: graded gratings
Light-trapping
plasmonic gratings
Plasmonics
Subwavelength gratings
super-resolution
Advisor: Kherani, Nazir P