250 research outputs found
Efficient Radiative Pumping of Polaritons in a Strongly Coupled Microcavity by a Fluorescent Molecular Dye
KGaA, Weinheim.The optical properties of a series of strongly coupled microcavities containing the fluorescent molecular dye BODIPY-Br (bromine-substituted boron-dipyrromethene) dispersed into a transparent dielectric matrix are explored, with each cavity having a different exciton-photon detuning. Using temperature dependent emission, time-resolved spectroscopy, white-light reflectivity, and measurements of fluorescence quantum yield, the population of polaritons is explored along the lower polariton branch. It is found that both the cavity fluorescence quantum efficiency and the distribution of polariton states along the lower polariton branch is a function of exciton-photon detuning. Importantly, it is shown that in the most negatively detuned cavities, the emission quantum efficiency approaches that of a control (noncavity) film. A simple fitting model is developed, which is based upon direct radiative pumping of polariton states along the lower polariton branch and used it to obtain an excellent agreement with measured photoluminescence as a function of temperature and exciton-photon detuning, and qualitative agreement with the measured photoluminescence quantum efficiency. The radiative pumping mechanism indicates that to facilitate the formation of a nonequilibrium polariton condensate in strongly-coupled microcavities containing dispersed molecular dyes, it is important to utilize materials having high fluorescent quantum efficiency and fast radiative rates
Templates for Convex Cone Problems with Applications to Sparse Signal Recovery
This paper develops a general framework for solving a variety of convex cone
problems that frequently arise in signal processing, machine learning,
statistics, and other fields. The approach works as follows: first, determine a
conic formulation of the problem; second, determine its dual; third, apply
smoothing; and fourth, solve using an optimal first-order method. A merit of
this approach is its flexibility: for example, all compressed sensing problems
can be solved via this approach. These include models with objective
functionals such as the total-variation norm, ||Wx||_1 where W is arbitrary, or
a combination thereof. In addition, the paper also introduces a number of
technical contributions such as a novel continuation scheme, a novel approach
for controlling the step size, and some new results showing that the smooth and
unsmoothed problems are sometimes formally equivalent. Combined with our
framework, these lead to novel, stable and computationally efficient
algorithms. For instance, our general implementation is competitive with
state-of-the-art methods for solving intensively studied problems such as the
LASSO. Further, numerical experiments show that one can solve the Dantzig
selector problem, for which no efficient large-scale solvers exist, in a few
hundred iterations. Finally, the paper is accompanied with a software release.
This software is not a single, monolithic solver; rather, it is a suite of
programs and routines designed to serve as building blocks for constructing
complete algorithms.Comment: The TFOCS software is available at http://tfocs.stanford.edu This
version has updated reference
Water-gated organic nanowire transistors
We gated both p-type, and n-type, organic nanowire (NW) films with an aqueous electric double layer (EDL) in thin-film transistor (TFT) architectures. For p-type NWs, we used poly(3-hexylthiophene) (P3HT) NWs grown via two different routes. Both can be gated with water, resulting in TFTs with threshold lower than for conventionally cast P3HT films under the same gating conditions. However, TFT drain currents are lower for NWs than for conventional P3HT films, which agrees with similar observations for ‘dry’ gated TFTs. For n-type NWs, we have grown ‘nanobelts’ of poly(benzimidazobenzophenanthroline) (BBL) by a solvent/non-solvent mixing route with later displacement of the solvent, and dispersion in a non-solvent. Water-gating such films initially failed to give an observable drain current. However, BBL nanobelts can be gated with the aprotic solvent acetonitrile, giving high n-type drain currents, which are further increased by adding salt. Remarkably, after first gating BBL NW films with acetonitrile, they can then be gated by water, giving very high drain currents. This behaviour is transient on a timescale of minutes. We believe this observation is caused by a thin protective acetonitrile film remaining on the nanobelt surface
Intermolecular States in Organic Dye Dispersions: Excimers vs Aggregates
Rapid excited-state quenching in the solid state is a widespread limitation for organic chromophores. Even when molecules are dispersed in neutral host matrices, photoluminescence quantum yields decrease sharply with increased concentration, pointing to efficient intermolecular non-radiative decay pathways that remain poorly understood. Here we study the nature of the intermolecular states formed in dispersions of the prototypical BODIPY dyes. Using temperature-dependent and time-resolved photoluminescence measurements, we describe the processes of energy transfer into excimer states and, in materials with suitable chemical structure, excitonically coupled dimers. These dimer states exhibit remarkable near-unity quantum yield
Can Racial Identity Be Promotive of Academic Efficacy?
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/89953/1/oyserman__harrison__bybee__2001.pd
Produção de leite de vacas mestiças Holandês x Zebu em pastagem de capim-elefante, manejada em sistema rotativo com e sem suplementação durante a época das chuvas
Recovery of soil physical properties by green manure, liming, gypsum and pasture and spontaneous native species¹
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