Doctor of Philosophy

dissertationThis work studies the optical interactions between single emitters, mainly quantum dots (QD) and a sharp tip. The fluorescence intensity, quantum yield and angular emission of a single emitter can be strongly modifi ed by near- field coupling with the sharp tip. Gold, silicon, and carbon nanotube (CNT) tips are employed in order to understand the physical mechanisms which are responsible for the various near- field eff ects. Each of these materials carries diff erent properties, which modify the optical properties of QDs in unique ways. In order to maximize the amount of information accessible by our near- field scanning microscope (NSOM), a novel near-f ield tomography technique is implemented. This technique facilitates the revelation of a number of interesting three-dimensional near- field features and is instrumental in the study of the di fferent near- field mechanisms. The flexibility in the data acquisition (DAC) technique allows us to study the influence of fluorescence intermittency (blinking) in QDs on the near- field coupling with the probes. The fluorescence emission from states with high quantum yield is more sensitive to quenching due to energy transfer, while in the low-yield states, near- field signal enhancement is more pronounced. The emission fluctuations of the QDs are progressively suppressed upon approach of a gold tip due to strong near- field coupling of gold tips to the QDs. Moreover, the angular emission of QDs in proximity to gold tips is very sensitive to the exact tip-QD position but does not depend on the intrinsic quantum yield of the QD. Energy transfer dominates the interactions of single CNTs with the QDs. Precision measurements of the energy transfer exhibit unique features as a result of the one-dimensional nature of CNTs. In particular, the energy transfer efficiency saturates at ~96% for all CNTs tried, even though the CNTs are expected to have a distribution of chiralities

Energy transfer from an individual quantum dot to a carbon nanotube

A detailed understanding of energy transduction is crucial for achieving precise control of energy flow in complex, integrated systems. In this context, carbon nanotubes (CNTs) are intriguing model systems due to their rich, chirality-dependent electronic and optical properties. Here, we study the quenching of fluorescence from isolated quantum dots (QDs) upon approach of individual CNTs attached to atomic force microscope probes. Precision measurements of many different CNT/QD pairs reveal behavior consistent with resonant energy transfer between QD and CNT excitons via a Fohrster-like dipole-dipole coupling. The data reveal large variations in energy transfer length scales even though peak efficiencies are narrowly distributed around 96%. This saturation of efficiency is maintained even when energy transfer must compete with elevated intrinsic non-radiative relaxation rates during QD aging. These observations suggest that excitons can be created at different locations along the CNT length, thereby resulting in self-limiting behavior.Comment: 8 pages, 8 figures, with supplementary informatio

Surface plasmon delocalization in silver nanoparticle aggregates revealed by subdiffraction supercontinuum hot spots

The plasmonic resonances of nanostructured silver films produce exceptional surface enhancement, enabling reproducible single-molecule Raman scattering measurements. Supporting a broad range of plasmonic resonances, these disordered systems are difficult to investigate with conventional far-field spectroscopy. Here, we use nonlinear excitation spectroscopy and polarization anisotropy of single optical hot spots of supercontinuum generation to track the transformation of these plasmon modes as the mesoscopic structure is tuned from a film of discrete nanoparticles to a semicontinuous layer of aggregated particles. We demonstrate how hot spot formation from diffractively-coupled nanoparticles with broad spectral resonances transitions to that from spatially delocalized surface plasmon excitations, exhibiting multiple excitation resonances as narrow as 13 meV. Photon-localization microscopy reveals that the delocalized plasmons are capable of focusing multiple narrow radiation bands over a broadband range to the same spatial region within 6 nm, underscoring the existence of novel plasmonic nanoresonators embedded in highly disordered systems

Indirect Exciton Formation due to Inhibited Carrier Thermalization in Single CdSe/CdS Nanocrystals

Temperature-dependent single-particle spectroscopy is used to study interfacial energy transfer in model light-harvesting CdSe/CdS core–shell tetrapod nanocrystals. Using alternating excitation energies, we identify two thermalized exciton states in single nanoparticles that are attributed to a strain-induced interfacial barrier. At cryogenic temperatures, emission from both states exemplifies the effects of intraparticle disorder and enables their simultaneous characterization, revealing that the two states are distinct in regards to emission polarization, spectral diffusion, and blinking