Optical properties and collective modes of plasmonic meta-surfaces
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Plasmonics is an important branch of optics and photonics, focusing on the electromagnetic response of metals or other materials with free carriers. This field has recently experienced a significant expansion due to its importance for applications. Plasmonics has shown great promises in green energies, biosensing, nanolasers, and imaging. The main advantage of plasmonics stems from the existence of unique excitations, referred to as plasmons, representing collective response of the free carriers to the electromagnetic field. While plasmons, both in the bulk and on the surface of the metals, have been known for decades, the recent advances in nano fabrication and material sciences at nano scale have enabled versatile engineering of these modes. Focus of my dissertation is surface plasmons whose properties can be tailored by judiciously nano-patterning metal films and surfaces. Such patterned structures, referred to as metasurfaces, are the main tool to control and boost the light-matter interaction. Appropriately designed metasurfaces provide many-fold electromagnetic energy enhancement on the surface which can be used to amplify numerous surface effects such as SEIRA and nonlinear optical phenomena, facilitate spectroscopy, and enhance absorption of light. In this thesis, I report approaches to shape and engineer the confinement, mode profile, and lifetime of the surface modes. I also investigate how the dielectric environment affects the properties of the modes. The effect of the geometry and topology of the nano patterns on the optical response of metasurfaces is also studied. Finally I study how manipulating symmetries of metasurfaces can be used to tailor polarization state of light and lifetime of the modes using an ultrathin metasurface, instead of bulky traditional optical elements. %The symmetry manipulation results in the plasmonic analogue of Electromagnetically Induced Transparency, a well-known phenomenon in atomic physics. The work summarized in this thesis has brought marked advances in understanding the physics behind the collective surface waves in nano-structured metasurfaces. It paves new avenues for engineering structures with desirable properties. The immediate application of my findings is the compactification of optical elements, and envisioning next-generation plasmonic-based on-chip devices.