Fabrication and applications of AuAg alloy metasurfaces and hybrid metal dielectric nanoantennas

The most prevalent materials used in the field of plasmonics are Au and Ag. However, for the past few years, the plasmonic community has also been looking for alternative materials that have lower losses than Au and higher stability than Ag. This thesis is an attempt to provide some nanotechnological solutions in that direction. In this thesis, I have experimentally demonstrated plasmonic anostructures and metasurfaces with other alternative materials like AuAg alloys and hybrid metal-dielectric nanoantennas: Al disk stacked on top of Si cylinder. The development of new processes for the incorporation of novel materials for the fabrication of nanostructures requires rigorous optimization procedures which have been detailed in the greater part of the thesis. In the first part of the thesis, I describe the development of a novel low-temperature alloying process for fabricating AuAg alloyed nanostructures. With this process, nanostructures of varying stoichiometries were fabricated by annealing a bilayer of Au and Ag, the relative thickness of the bilayer determined the composition. The process made possible to perfectly retain the shape of the nanostructures after annealing, withoutthe use of insulating material like SiO2 as a protective cover. Further, it only required a single annealing step after the completion of fabrication by conventional lithography and enabled the fabrication on the same substrate of alloyed nanostructures with different stoichiometries, unlocking an additional degree of freedomfor nanostructure design. I extended this versatile and cost-effective technique successfully to the fabrication of complex 4-rod Fano resonant nanostructures and to binary plasmonic metahologram and metalenses. The metasurfaces were built from similar shape nanostructures with the different phase responses provided by a variation of composition (Au0.2Ag0.8 and Au0.8Ag0.2 ) rather than the conventionally used variation in geometrical parameters. This technique paves the way for fabricating plasmonic metasurfaces with meta-atoms varying in both geometry and composition. In the second part of the thesis I investigate the effect of introducing dielectric to metal nanostructures. In order to do this analysis, I use a hybrid metal-dielectric nanoantenna with stacked geometry comprising of a metal disk on a Si cylinder. I perform a systematic study showing the evolution of the multipoles along with the spectra for this hybridmetal-dielectric nanoantenna as its dimensions are varied one by one. I broaden the analysis to demonstrate the "magnetic light" at energies above 1 eV by varying the height of the Ag on the Si cylinder and below 1 eV by introducing insulating spacing between them. The appearance of the anapole state is also explored along with some exceptionally narrow spectral features by varying the radius of the Ag disk. This geometry has also been fabricated using Al as the metal which also acted as an etching mask, simplifying the fabrication process and Si as the dielectric, together with a SiO2 spacer. These nanostructures have been used for bulk refractive index sensing with glucose solution showing a sensitivity of 208 nm/RIU, which has been further improved to 245 nm/RIU by under etching the SiO2 spacer. I believe the nanotechnological processes developed in this thesis will facilitate the fabrication of metasurfaces with these alternative materials resulting in new and superior performances.