Study of Raman Antenna Effects of Anisotropic Plasmonic Metal Nanostructures

Abstract

The interactions of 0D and 1D nanostructures with a 2D plasmonic film have seldom been studied in literature. The cavity or the "hot-spot" that forms between the nanostructure and the film are interesting structures, as the field is highly confined and enhanced in these regions. Thus, placing Raman active dyes in these locations will lead to huge enhancement of the Raman signal obtained from them. The structures will also have antenna effects, which will lead to the radiation properties, such as directionality and polarization states, being changed according to the geometry of the structures. As the Raman antenna effects of such structures have not been probed in great detail in literature, this project aims to do so for two anisotropic nanostructures, gold nanorods and silver nanowires. The first chapter gives an introduction about the concepts that are necessary to understand the effects observed in the thesis. This includes an introduction to plasmons and various related concepts, antenna effects, and molecular emission processes. The second chapter mainly deals with the synthesis of symmetric and asymmetric metal nanoparticles, which includes Au nanospheres, Au nanotriangles and Au nanorods. Such structures are important for the experimental studies that have been done later in the thesis. The third chapter details the experimental studies done on Au nanorods and Ag nanowires placed over gold films, which act as plasmonic mirrors. The studies consist of observation of enhancement, input polarization dependence of the SERS signal, and in case of Ag nanowires, remote excitation experiments wherein one end of the wire is excited and the signal is collected and analyzed at the distal end. The fourth chapter investigates the simulation aspects of the Ag nanowire-over-gold film system. Simulations were done to corroborate the experimental results obtained in chapter 3. Simulations have also been done to gain insights about the system that can not be obtained experimentally. Thus simulations have been used both as a tool to aid experiments as well as to guide experiments in further directions.