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

Dog as an Outgroup to Human and Mouse

Copyright 2007 Gerton Lunter. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Photoassociative Frequency Shift in a Quantum Degenerate Gas

We observe a light-induced frequency shift in single-photon photoassociative spectra of magnetically trapped, quantum degenerate 7Li. The shift is a manifestation of the coupling between the threshold continuum scattering states and discrete bound levels in the excited-state molecular potential induced by the photoassociation laser. The frequency shift is observed to be linear in the laser intensity with a measured proportionality constant that is in good agreement with theoretical predictions. The frequency shift has important implications for a scheme to alter the interactions between atoms in a Bose-Einstein condensate using photoassociation resonances.Comment: 3 figure

Fluorescence Near-Field Microscopy of DNA at Sub-10 nm Resolution

We demonstrate apertureless near-field microscopy of single molecules at sub-10 nm resolution. With a novel phase filter, near-field images of single organic fluorophores were obtained with ~sixfold improvement in the signal-to-noise ratio. The improvement allowed pairs of molecules separated by ~15 nm to be reliably and repeatedly resolved, thus demonstrating the first true Rayleigh resolution test for near-field images of single molecules. The potential of this technique for biological applications was demonstrated with an experiment that measured the helical rise of A-form DNA

Interaction of voluntary activity and functional electrical stimulation in the upper extremity as a method for short-term alteration of corticospinal excitability and force control

Repetitive movement training (RMT) is a well-established method for rehabilitating functional movement. However, many stroke survivors are not able to participate in RMT for the necessary duration to produce results due to rapid muscle fatigue or inability to perform the desired movement at all. Often, functional electrical stimulation (FES) is applied passively, as a rehabilitative therapy, to stroke subjects who are unable to participate in RMT. The effects of voluntary contraction and FES are not well understood for the upper extremity following a stroke. This experiment was designed to elucidate the mechanisms of functional and neurophysiological changes associated with combining FES and voluntary movement vs. the effects of each intervention alone in healthy subjects, with a within-subjects single day design. Eleven right-handed, neurologically healthy subjects participated in a series of three experimental sessions. The testing conditions were voluntary movement alone (VOL), functional electrical stimulation alone (FES), and voluntary movement supplemented by functional electrical stimulation (VOL+FES). Subjects were evaluated for changes in maximum force and force control before and after each session. Corticospinal excitability was evaluated using transcranial magnetic stimulation (TMS) at five time points throughout each session. There were no significant changes pre-post or between conditions for the maximum force or the force control. FES alone was found to immediately and significantly reduce corticospinal excitability; that reduction continued through the post measurement. Both VOL and VOL+FES increased corticospinal excitability pre-post, although not significantly. At the post measurement, both VOL and VOL+FES were significantly larger than FES, although not different from each other. These results indicate that adding voluntary movement to functional electrical stimulation may serve to increase corticospinal excitability while allowing the subject to participate in meaningful rehabilitative movements

Evolution of a collapsing and exploding Bose-Einstein condensate in different trap symmetries

Based on the time-dependent Gross-Pitaevskii equation we study the evolution of a collapsing and exploding Bose-Einstein condensate in different trap symmetries to see the effect of confinement on collapse and subsequent explosion, which can be verified in future experiments. We make prediction for the evolution of the shape of the condensate and the number of atoms in it for different trap symmetries (cigar to pancake) as well as in the presence of an optical lattice potential. We also make prediction for the jet formation in different cases when the collapse is suddenly terminated by changing the scattering length to zero via a Feshbach resonance.Comment: 8 pages, 11 ps figures, Physical Review

Tip-enhanced fluorescence microscopy of high-density samples

Journal ArticleHigh-density samples of fluorescent quantum dots (QDs) were imaged using an apertureless near-field optical microscopy technique. QD fluorescence was modulated by oscillating a silicon atomic force microscope tip above an illuminated sample and a lock-in amplifier was used to suppress background from the excitation laser. Spatial resolution near 10 nm and a peak signal-to-noise ratio (SNR) of ~60 were achieved. Individual QDs within high-density ensembles were still easily resolved (SNR>5) at a density of 14 QDs/µm2. These results have favorable implications for the eventual nanoscale imaging of viable biological systems, such as cellular membranes