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Optical Nanoantennas for Sensors, Microscopy and Spectroscopy

Creative Commons 'BY-NC-ND' version 4.0 license

Metallic nanosctructures possess electromagnetic properties useful for various applications, such as surface enhanced Raman spectroscopy (SERS), harmonic generation and solar cells. Specifically, because of their ability in confining electromagnetic fields to nanoscopic dimensions, they provide an exquisite platform for conducting studies on molecules placed in their vicinity. In this dissertation, I introduce and study various plasmonic nanostructures to achieve electric and magnetic field enhancement for spectroscopy and microscopy applications.

First, I investigate feasibility of CMOS-compatible optical structures to develop novel integrated spectroscopy systems. I show that local field enhancement is achievable utilizing dimers of plasmonic nanospheres that can be assembled from colloidal solutions on top of a CMOS-compatible optical waveguide. The resonant dimer nanoantennas are excited by modes guided in the integrated silicon nitride waveguide. Specifically, I investigate how the field enhancement depends on dimer location, orientation, distance and excited waveguide mode. However, the field enhancement achievable with using oligomers is limited due to inherent losses of plasmonic particles. Thus, I study a novel structure called two-scale, in which Rayleigh anomaly caused by a 1D set of periodic nanorods is utilized. A thorough study of this structure is carried out by using an effective analytical-numerical model which is also compared to full-wave simulation results. Experimental results comparing enhancements in surface enhanced Raman scattering measurements with and without nanorods demonstrate the effectiveness of a Rayleigh anomaly in boosting the field enhancement.

The other side of the dissertation is dedicated to magnetic field enhancement. I propose various metallic and dielectric nanostructures for local magnetic field enhancement at optical frequencies. I show that dielectric structures can be a good alternative for their plasmonic counterpart due to their low loss. The idea behind each structure is supported by results from full-wave simulations. More importantly, I utilize azimuthally polarized beam as a way of boosting local magnetic field and isolating it from electric field. The magnetic field enhancement of these structures can be utilized in studying magnetic dipole transitions, magnetic imaging and enhanced spectroscopy applications.

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