Excitonic transitions in organic semiconductors are associated with large oscillator strength that limits the excited-state lifetime and can in turn impede long-range exciton migration. We present perylene-based emissive H-aggregate nanowires where the lowest energy state is only weakly coupled to the ground state, thus dramatically enhancing lifetime. Exciton migration occurs by thermally activated hopping, leading to luminescence quenching on topological wire defects. An atomic force microscope tip can introduce local topological quenchers by distorting the H-aggregate structure, demonstrating long-range exciton migration at room temperature and offering a potential route to writing fluorescent "nanobarcodes" and excitonic circuits.

Precision measurements of resonant energy transfer from isolated quantum dots (QDs) to individual carbon nanotubes (CNTs) exhibit unique features due to the one-dimensional nature of CNTs. In particular, excitons can be created at varying distances from the QD at different locations along the CNT length. This leads to large variations in energy transfer length scales for different QDs and a novel saturation of the energy transfer efficiency at ∼96%, seemingly independent of CNT chirality.

We demonstrate a near-field tomography method for investigating the coupling between a nanoscopic probe and a fluorescent sample. By correlating the arrival of single fluorescence photons with the lateral and vertical position of an oscillating tip, a complete three-dimensional analysis of the near-field coupling is achieved. The technique is used to reveal a number of interesting three-dimensional near-field features and to improve image contrast in tip-enhanced fluorescence microscopy.

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.

We apply scanning fluorescence correlation spectroscopy to study the structure of individual DNA coils in dilute and semidilute solutions. In dilute solutions, over two decades in length, from 0.6 to 46 μm, DNA behave as ideal chains, in agreement with theoretical predictions and in disagreement with prior experiments. In semidilute solutions, up to very high densities, the structures of individual DNA coils are independent of concentration, unlike flexible coils that shrink with increasing density. Our experimental findings are consistent with the marginal solution theory of semiflexible polymers.

We demonstrate that the cycling between internal states of quantum dots during fluorescence blinking can be used to tune the near-field coupling with a sharp tip. In particular, the fluorescence emission from states with high quantum yield is quenched due to energy transfer, while that from low-yield states is elevated due to field enhancement. Thus, as a quantum dot blinks, its emission fluctuations are progressively suppressed upon approach of a tip.

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.

We adapt a scanning fluorescence correlation spectroscopy technique to measure the structure factor of complex fluid systems and present the first measurements of the structure of semidilute solutions of long DNA polymers. The measured structure factors exhibit screening effects which, as expected for semidilute polymer solutions, grow stronger with increasing DNA concentration c. The measured concentration dependence of the screening length xi proportional to c{0.53+/-0.02} is unusual, but can be understood within the framework of a marginal solutions theory for semiflexible polymers.