Exciton-plasmon interactions in hybrid metal-semiconductor nanostructures
This thesis reports experimental study of surface plasmon excitations--localized surface plasmons (SPs) and propagating surface plasmon polaritons (SPPs)--and their interactions with dipole emitters CdSe/ZnS (core/shell) nanocrystals. This study will contribute to potential applications of SP-enhanced fluorescent sensors and fast SPP-waveguided electronics.
Our angle-dependent, polarization-related extinction spectra show that SPs in 2D nanodisk arrays are not only related to the intrinsic properties of individual nanoparticles, but also dependent on the dipole-dipole interactions among them. SP resonance peaks are red-shifted with increasing incidence angle. As the nanodisk center-to-center distance decreases within sub-wavelengths, coupling to waveguide modes and diffracted evanescent wave modifies the transmission. The out-of-disk-plane dipole surface plasmon resonance is used to couple to nanocrystals and to test the conventional assumption that dipole emission rates are homogeneous in time-resolved photoluminescence (PL) measurements of ensemble samples. Our new finding is that the spontaneous emission rate of dipole emitters deposited on a 2D gold nanodisk array depends on the detection angle and polarization. At the band-edge emission wavelength of nanocrystals, the out-of-incidence-plane, s-polarized PL measurements are detection angle-independent, and the in-plane-of-incidence, p-polarized PL measurements show an additional decay caused by SP-enhanced emission.
In planar gold films we perform reflectivity measurements in the Kretschmann-Raether (KR) configuration and determine the frequency- and momentum-dependent SPP resonance. In hybrid samples of planar gold films and semiconductor nanocrystals, the coupling between the dipole emitters and SPPs can generate SPP emission through an inverted KR hemisphere prism. For the first time we observed a decay rate increase of SPP emission as a function of nanocrystals emission wavelength in gold films with silica separation layers, as compared to free-space dipole emission detected in the front of the metal surface. Simulations based on the theory of Ford and Weber show that this increase is primarily due to energy transfer of perpendicular dipoles into lossy surface waves.
Our results of polarization-selective and angle-dependent SP-enhanced emission can be used to optimize and tune the performance of light sources or fluorescent sensors. The study of SPP emission will lead to efficient energy transfer in fast plasmonic device applications.
Keywords: Au, CdSe/ZnS nanocrystals, SP, SPP emission, nanodisk arrays, time-resolved single photon counting.