Using a sharp metal tip to control the polarization and direction of emission from a quantum dot.

Optical antennas can be used to manipulate the direction and polarization of radiation from an emitter. Usually, these metallic nanostructures utilize localized plasmon resonances to generate highly directional and strongly polarized emission, which is determined predominantly by the antenna geometry alone, and is thus not easily tuned. Here we show experimentally that the emission polarization can be manipulated using a simple, nonresonant scanning probe consisting of the sharp metallic tip of an atomic force microscope; finite element simulations reveal that the emission simultaneously becomes highly directional. Together, the measurements and simulations demonstrate that interference between light emitted directly into the far field with that elastically scattered from the tip apex in the near field is responsible for this control over polarization and directionality. Due to the relatively weak emitter-tip coupling, the tip must be positioned very precisely near the emitter, but this weak coupling also leads to highly tunable emission properties with a similar degree of polarization and directionality compared to resonant antennas.

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.

Structure of DNA coils in dilute and semidilute solutions.

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.

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.

Using the near-field coupling of a sharp tip to tune fluorescence-emission fluctuations during quantum-dot blinking.

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.

Enhancing long-range exciton guiding in molecular nanowires by H-aggregation lifetime engineering.

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.

Energy transfer from an individual quantum dot to a carbon nanotube.

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.

Marginal nature of DNA solutions.

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.

Three-dimensional mapping of near-field interactions via single-photon tomography.

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.