Photoexcited carrier relaxation dynamics in pentacene probed by ultrafast optical spectroscopy: Influence of morphology on relaxation processes

We present a comparative study of ultrafast photoexcited state relaxation in pentacene single crystals and in pure and C₆₀-doped pentacene films using optical pump-probe spectroscopy. The photoinduced absorption spectra in pentacene crystals is consistent with a dominant singlet-triplet fission decay channel for above-gap excitation. This decay channel is suppressed in thin films and even further suppressed by electron trapping in C₆₀-doped films. Thus we show that suppression of triplet state production, which is necessary for free carrier formation and thus photovoltaic and photodiode performance, is controllable via sample morphology.     

Analysis of HRTEM images for carbon nanostructure quantification

Image processing algorithms have been developed to extract fringe length, tortuosity and separation from high resolution transmission electron microscopy images. To validate the separation algorithm, a comparison is made between the image-based fringe separation and that obtained by analysis of X-ray diffraction data for a progressively heat-treated carbon black. Agreement is favorable. To illustrate the utility of the analysis parameters for a range of carbon nanostructures, analysis is applied to a series of pyrolytically prepared carbon soots – qualitatively described as containing amorphous, graphitic or fullerenic nanostructure. For all processing, the intermediate image, in the form of a skeletonized binary image of the original high resolution transmission electron micrograph, is shown and found to accurately reflect the nanostructural organization within the carbon as visually observed. Statistical results for each analysis parameter, extracted from the binary images, are presented in the form of histograms and quantitatively distinguish the different carbon nanostructures.     

Pressure induced self-oxidation of Fe(OH)2

M?ssbauer spectroscopy, x-ray diffraction, and electrical resistance [R(P,T)] studies in Fe(OH)(2) to 40 GPa revealed an unforeseen process by which a gradual Fe2+ oxidation takes place, starting at approximately 8 GPa reaching 70% Fe3+ abundance at 40 GPa. The nonreversible process Fe2+-->Fe3++e(-) occurs with no structural transition. The "ejected" electrons form a deep band within the high-pressure electronic manifold becoming weakly localized at P>50 GPa. This process is attributed to an effective ionization potential created by the pressure induced orientationally deformed (OH) dipoles and the unusual small binding energy of the valence electron in Fe2+(OH)(2).     

Wavelength-versatile subpicosecond pulsed lasers using Raman gain in figure-of-eight fiber geometries

A family of compact pulsed fiber lasers is described that employs broadband, wavelength-flexible Raman scattering in passively mode-locked figure-of-eight fiber laser geometries. Specifically, sources at 1.57, 1.33, and 1.41 microm with respective soliton durations of 440, 500, and 860 fs are reported. Operation is possible at other wavelengths with a suitable pump source, gain fiber, and components.     

"Fingered" patterns in electron droplets in nonuniform magnetic fields

A semiclassical analysis of a two-dimensional electron droplet in a high, nonuniform magnetic field predicts that the droplet will form "fingered" patterns upon increasing the number of electrons. We construct explicit examples of these patterns using methods first developed for the flow of two-dimensional viscous fluids. We complement our analytical results with Monte Carlo simulations of the droplet wave function, and find that at the point where the semiclassical analysis predicts a cusp on the interface, the droplet fissions-a type of "quantum breakup" phenomenon.     

Non-linear applications of microstructured optical fibres

The enhancement of different non-linear processes in microstructured optical fibres can be achieved through manipulation of the dispersion characteristics of the fibre. This is demonstrated by extending the region of short wavelength operation of high power supercontinuum generation through four wave mixing in a cascaded fibre geometry where the dispersion of each fibre decreased on propagation. The technique is further refined in a demonstration utilizing long lengths of dispersion decreasing tapered microstructured fibres, where the supercontinuum extends to around 300 nm with spectral power densities in excess of 2 mW/nm in the uv. These long length tapers can also be utilized for adiabatic soliton pulse compression in new spectral regions, allowing the compression of 655 fs pulses to 45 fs.     

Preparation and characterization of biodegradable magnetic carriers by single emulsion-solvent evaporation

This paper describes a single emulsion-solvent evaporation protocol to prepare PEGylated biodegradable/biocompatible magnetic carriers by utilizing hydrophobic magnetite and a mixture of poly(D,L lactide-co-glycolide) (PLGA) and poly(lactic acid-block-polyethylene glycol) (PLA-PEG) (26:1 by mass) polymers. We characterized the magnetic microspheres in terms of morphology, composite microstructure, size and size distribution, and magnetic properties. Results show that the preparation produces magnetic microspheres with a good spherical morphology, small size (mean diameter of 1.2–1.5 μm) by means of large size distributions, and magnetizations up to 20–30 emu/g of microspheres.     

High-performance fiber-optic temperature sensor using low-coherence interferometry

A temperature-sensor system based on low-coherence interferometry with a fiber Mach-Zehnder interferometer as a phase modulator was implemented. A measurement range of 20 to 800 degrees C with a resolution of 0.025 degrees C (corresponding to 0.0004 fringe) was achieved with a 1-mm-long fiber Fabry-Perot interferometer as the sensing element.