Investigation of photoinduced effects in plasmonic nanocavities

Light matter interaction can be boosted by several orders of magnitude by tailoring the photonic environment, thus enabling a wide range of applications. One particular example are plasmonic nanostructures that support localized surface plasmon polariton (LSPP) leading to enhancement and confinement of incident electromagnetic fields. These novel properties provide access to a large number of optically driven effects within matter placed in the vicinity of such nanostructures. In this thesis, we investigate the effects of optical excitation on gold nanostructures in the presence of Raman active molecules. This is accomplished by fabricating nanoparticle on mirror (NPoM) plasmonic nanostructures where a gold nanoparticle is placed at a fixed distance from a smooth gold film using molecular monolayers. As the nanoparticle and mirror are placed only a few nanometers apart, optical excitation leads to formation of hybridized LSPP modes due to coupling between the plasmons within the nanoparticle and the mirror. This plasmonic coupling also results in confinement and subsequent enhancement of the incident light within the gap and is extremely sensitive to structural parameters of the NPoM. The creation of a homogeneous gap between the nanoparticle and mirror is quite challenging due the roughness of the mirror. Moreover, due to the large number of NPoM nanostructures present on the sample it is quite difficult to locate a specific NPoM. To bypass these difficulties atomically smooth and patterned substrates were developed using template stripping combined with conventional photolithography techniques. Next, in order to characterize the optical response of the NPoMs, a custom optical setup was built. The setup was primarily designed to investigate the plasmonic properties of the NPoM using elastic scattering spectroscopy along with surface enhanced Raman scattering (SERS) from the molecules within the gap. The combination of these two techniques was later used to investigate the effect of laser power on the NPoMs. It was discovered that the scattering spectra of the NPoMs changed irreversibly after laser exposure even with a few ÎŒW/ÎŒm2 incident intensities. Moreover, the plasmonic response of the NPoMs fabricated with well organized molecular monolayer changed much faster than the NPoMs prepared with disorganized ones. This phenomenon was also investigated in NPoMs prepared using a dielectric gap layer, by changing the gap conductivity, and by changing the nanoparticle shape. The experimental results combined with simulations suggest a decrease in gap height due to the reorientation of the molecular monolayer under optical excitation.