Graphene plasmonics for surface-enhanced infrared spectroscopy

Abstract

The work, presented in this thesis, focuses on studying graphene as a signal enhancing material for spectroscopic applications. Among many outstanding characteristics of graphene, it also exhibits attractive plasmonic properties. Tunability of the resonance within THz to Mid-IR range and high field confinement factor makes it a great candidate for the surface enhanced infrared spectroscopy application. This thesis presents the results of computational and experimental investigation of graphene-based optical resonators. The numerical study was focused on the optimization of two-dimensional graphene geometries, looking to achieve the highest enhancement factor. The experimental part of the work included the fabrication process optimization and characterization of produced graphene structures.

Numerical simulations of plasmonic resonance of structured graphene at far infrared range was performed using Finite-Difference Time-Domain method. Simulated results demonstrated the possibility to achieve the enhancement factor of approximately 10^5 for near-field coupled structures spaced as close as 10 nm. Two-dimensional periodicity of studied geometries demonstrated switchable resonance modes, accessible via polarization of the incident light. Numerical studies also revealed a substantial degradation of the enhancement factor related to the quality of graphene.

The experimental work consisted of the optimization of graphene patterning process, fabrication of the active plasmonic device and its characterization using FTIR microscopy and scanning probe imaging. A novel approach for graphene patterning was utilized. Substituting the conventional lithography, focused ion beam was used to selectively remove graphene, producing high-resolution patterns. Surface profile imaging of milled structures demonstrated an excellent performance and accuracy for 30 keV neon beam. FTIR measurements did not produce the reliable results. Observed spectral variations are not certain to be caused by plasmonic excitations. The uncertainty of infrared absorption measurements may be linked to the overall design of the device and the fabrication method chosen. Further discussion is given in the thesis.