Tinkering with Light at the Nanoscale using Plasmonic Metasurfaces and Antennas: From Fano to Function
Abstract: Surface plasmons are charge density oscillations that can couple strongly to light and be excited in, for instance, thin metal films and metal nanoparticles. The plasmonic excitation squeezes the light down to nanometric length scales, far smaller than the wavelength of the light. This localization of light can be utilized in several surface-enhanced spectroscopies, for photothermal therapy, in optical trapping methodologies and in refractometric sensing schemes. This thesis focuses on various excitation schemes and spectroscopic measurements of surface plasmons and their sensitivity to the dielectric surrounding the metal. Plasmonic excitations in metal films and nanoparticles have several common features, although only the former has successfully been commercialized as a refractometric biosensing platform. In a direct comparison of the two, both platforms performed equally well, from a sensitivity point-of-view. However, there are two significant advantages of nanoparticle plasmonic sensing schemes: The much relaxed excitation conditions and the miniscule size of the nanoparticle sensors. In a combination of these features, hundreds of individual nanoparticles were simultaneously interrogated in order to approach the few to single molecule detection limit. The data were obtained using a hyperspectral imaging methodology in combination with an enzymatic precipitation reaction that enhanced the plasmonic response from individual adsorbed molecules. The results demonstrated a sensitivity in the single molecule range, but a number of inhomogeneous broadening effects prevented counting the exact number of molecules per particle. In a different line of research, plasmonic nanoparticles placed in a large two dimensional array with small interparticle spacing and supported with a glass substrate were interrogated. The nanoplasmonic layer then act as a metamaterial that can support strongly asymmetric resonances, dispersive modes and even complete light absorption. These effects are due to a so-called Fano interference between the plasmon excitation and the reflection from the dielectric boundary. Complete absorption enhances the optical near-fields, which can be utilized in, for instance, surface enhanced spectroscopy techniques. However, minimizing the reflection has another interesting feature: A rapid phase jump of the reflected light. The phase is shown to vary about one order of magnitude faster than the reflected intensity and, therefore, also provides around one order of magnitude higher sensitivity to molecular adsorption. Altogether, the results presented in this thesis provides a basis for several interesting sensing schemes, as well as insight into some fundamentally intriguing phenomena regarding absorption, nanoscale coherence and light localization.
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