Electron-Hole Asymmetry in Single-Walled Carbon Nanotubes Probed by Direct Observation of Transverse Quasi-Dark Excitons

We studied the asymmetry between valence and conduction bands in single-walled carbon nanotubes (SWNTs) through the direct observation of spin-singlet transverse dark excitons using polarized photoluminescence excitation spectroscopy. The intrinsic electron-hole (e-h) asymmetry lifts the degeneracy of the transverse exciton wavefunctions at two equivalent K and K' valleys in momentum space, which gives finite oscillator strength to transverse dark exciton states. Chirality-dependent spectral weight transfer to transverse dark states was clearly observed, indicating that the degree of the e-h asymmetry depends on the specific nanotube structure. Based on comparison between theoretical and experimental results, we evaluated the band asymmetry parameters in graphene and various carbon nanotube structures.Comment: 11 pages, 4 figure

Quantitative analysis of directional spontaneous emission spectra from light sources in photonic crystals

We have performed angle-resolved measurements of spontaneous-emission spectra from laser dyes and quantum dots in opal and inverse opal photonic crystals. Pronounced directional dependencies of the emission spectra are observed: angular ranges of strongly reduced emission adjoin with angular ranges of enhanced emission. It appears that emission from embedded light sources is affected both by the periodicity and by the structural imperfections of the crystals: the photons are Bragg diffracted by lattice planes and scattered by unavoidable structural disorder. Using a model comprising diffuse light transport and photonic band structure, we quantitatively explain the directional emission spectra. This provides detailed understanding of the transport of spontaneously emitted light in real photonic crystals, which is essential in the interpretation of quantum-optics in photonic band-gap crystals and for applications wherein directional emission and total emission power are controlled.Comment: 10 pages, 10 figures, corrected pdf, inserted new referenc

Time-gated transillumination of biological tissues and tissuelike phantoms

The applicability and limits of time-resolved transillumination to determine the internal details of biological tissues are investigated by phantom experiments. By means of line scans across a sharp edge, the spatial resolution (Ax) and its dependence on the time-gate width (At) can be determined. Additionally, measurements of completely absorbing bead pairs embedded in a turbid medium demonstrate the physical resolution in a more realistic case. The benefit of time resolution is especially high for a turbid medium with a comparatively small reduced scattering coefficient of approximately pL,' = 0.12 mm-1. Investigations with partially absorbing beads and filled plastic tubes demonstrate the high sensitivity of time-resolving techniques with respect to spatial variations in scattering or absorption coefficients that are due to the embedded disturber. In particular, it is shown that time gating is sensitive to variations in scattering coefficients. Key words: Time-resolved transillumination, turbid media, light scattering, streak camera

Photonic engineering of hybrid metal-organic chromophores

We experimentally demonstrate control of the absorption and emission properties of individual emitters by photonic antennas in suspension. The method results in a new class of water-soluble chromophores with unprecedented photophysical properties, such as short lifetime, low quantum yield but high brightness

Quantitative photoluminescence of broad band absorbing melanins: A procedure to correct for inner filter and re-absorption effects

We report methods for correcting the photoluminescence emission and excitation spectra of highly absorbing samples for re-absorption and inner filter effects. We derive the general form of the correction, and investigate various methods for determining the parameters. Additionally, the correction methods are tested with highly absorbing fluorescein and melanin (broadband absorption) solutions; the expected linear relationships between absorption and emission are recovered upon application of the correction, indicating that the methods are valid. These procedures allow accurate quantitative analysis of the emission of low quantum yield samples (such as melanin) at concentrations where absorption is significant.Comment: 20 pages, 13 figure

Properties of the mechanosensitive channel MscS pore revealed by tryptophan scanning mutagenesis

Funding This work was supported by a Wellcome Trust Programme grant [092552/A/10/Z awarded to I.R.B., S.M., J. H. Naismith (University of St Andrews, St Andrews, U.K.), and S. J. Conway (University of Oxford, Oxford, U.K.)] (T.R. and M.D.E.), by a BBSRC grant (A.R.) [BB/H017917/1 awarded to I.R.B., J. H. Naismith, and O. Schiemann (University of St Andrews)], by a Leverhulme Emeritus Fellowship (EM-2012-060\2), and by a CEMI grant to I.R.B. from the California Institute of Technology. The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007-2013 FP7/2007-2011) under Grant PITN-GA-2011-289384 (FP7-PEOPLE-2011-ITN NICHE) (H.G.) (awarded to S.M.).Peer reviewedPublisher PD

Size-Dependence of the Wavefunction of Self-Assembled Quantum Dots

The radiative and non-radiative decay rates of InAs quantum dots are measured by controlling the local density of optical states near an interface. From time-resolved measurements we extract the oscillator strength and the quantum efficiency and their dependence on emission energy. From our results and a theoretical model we determine the striking dependence of the overlap of the electron and hole wavefunctions on the quantum dot size. We conclude that the optical quality is best for large quantum dots, which is important in order to optimally tailor quantum dot emitters for, e.g., quantum electrodynamics experiments.Comment: 5 pages, 3 figure

Multimodal wide-field two-photon excitation imaging: characterization of the technique for in vivo applications

We report fast, non-scanning, wide-field two-photon fluorescence excitation with spectral and lifetime detection for in vivo biomedical applications. We determined the optical characteristics of the technique, developed a Gaussian flat-field correction method to reduce artifacts resulting from non-uniform excitation such that contrast is enhanced, and showed that it can be used for ex vivo and in vivo cellular-level imaging. Two applications were demonstrated: (i) ex vivo measurements of beta-amyloid plaques in retinas of transgenic mice, and (ii) in vivo imaging of sulfonated gallium(III) corroles injected into tumors. We demonstrate that wide-field two photon fluorescence excitation with flat-field correction provides more penetration depth as well as better contrast and axial resolution than the corresponding one-photon wide field excitation for the same dye. Importantly, when this technique is used together with spectral and fluorescence lifetime detection modules, it offers improved discrimination between fluorescence from molecules of interest and autofluorescence, with higher sensitivity and specificity for in vivo applications

Microscopic theory of surface-enhanced Raman scattering in noble-metal nanoparticles

We present a microscopic model for surface-enhanced Raman scattering (SERS) from molecules adsorbed on small noble-metal nanoparticles. In the absence of direct overlap of molecular orbitals and electronic states in the metal, the main enhancement source is the strong electric field of the surface plasmon resonance in a nanoparticle acting on a molecule near the surface. In small particles, the electromagnetic enhancement is strongly modified by quantum-size effects. We show that, in nanometer-sized particles, SERS magnitude is determined by a competition between several quantum-size effects such as the Landau damping of surface plasmon resonance and reduced screening near the nanoparticle surface. Using time-dependent local density approximation, we calculate spatial distribution of local fields near the surface and enhancement factor for different nanoparticles sizes.Comment: 8 pages, 6 figures. Considerably extended final versio