Eyal Shafran

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.

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.

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.

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.

Snapshot High-resolution Hyper-spectral Imager based on an Ultra-thin Diffractive Filter

By introducing an ultra-thin diffractive filter, a compact snapshot hyper-spectral imager is built and characterized. It is enabled by a fast computational algorithm and offers high-resolution, high-throughput real-time imaging and also 3D imaging.

Carbon Nanotubes as Optoelectronic Energy Transducers

The transduction of energy from one type to another is frequently a limiting factor in the efficiency of optoelectronic devices. Such transduction processes are often necessary for energy to propagate across the interface between materials. As an example, next generation solar cells may require several energy transduction steps: conversion of electromagnetic radiation to internal energy of one component, to internal energy of another component, and finally to electric power. In this scenario, each sequential transduction process must be more rapid than the internal relaxation time, or the energy will be dissipated as radiation or heat. Recently, we demonstrated that when a carbon nanotube (CNT) is brought into close proximity with a fluorophore, the fluorophore’s emission is nearly completely quenched [1, 2]. This indicates that the fluorophore’s internal energy is transduced into electronic modes within the CNT. Nanotubes are particularly intriguing candidates for studying nanoscale energy transduction because on the one hand, they can support ballistic charge transport, and on the other, the detailed electronic properties vary from tube to tube according to its chirality. Furthermore, they have been synthesized as dense vertically-aligned networks, facilitating their incorporation into complex optoelectronic materials, and finally they have been used as nanoscale components in molecular electronics.

Nanoscale fluorescence microscopy using carbon nanotubes

We demonstrate the first reported use of single-walled carbon nanotubes as nano-optical probes in apertureless near-field fluorescence microscopy. We show that, in contrast to silicon probes, carbon nanotubes always cause strong fluorescence quenching when used to image dye-doped polystyrene spheres and Cd-Se quantum dots. For quantum dots, the carbon nanotubes induce very strong near-field contrast with a spatial resolution of 20 nm. Images of dye-doped spheres exhibit crescent-shaped artifacts caused by distortions in the surface water layer found in ambient conditions.