1,153 research outputs found
Spatial imaging of modifications to fluorescence lifetime and intensity by individual Ag nanoparticles
Highly ordered periodic arrays of silver nanoparticles have been fabricated
which exhibit surface plasmon resonances in the visible spectrum. We
demonstrate the ability of these structures to alter the fluorescence
properties of vicinal dye molecules by providing an additional radiative decay
channel. Using fluorescence lifetime imaging microscopy (FLIM), we have created
high resolution spatial maps of the molecular lifetime components; these show
an order of magnitude increase in decay rate from a localized volume around the
nanoparticles, resulting in a commensurate enhancement in the fluorescence
emission intensity.Comment: 3 pages, 2 figures, submitted Applied Physics Letter
Plasmonic engineering of metal nanoparticles for enhanced fluorescence and Raman scattering
We have investigated the effects of tuning the localized surface plasmon
resonances (LSPRs) of silver nanoparticles on the fluorescence intensity,
lifetime, and Raman signal from nearby fluorophores. The presence of a metallic
structure can alter the optical properties of a molecule by increasing the
excitation field, and by modifying radiative and non-radiative decay
mechanisms. By careful choice of experimental parameters we have been able to
decouple these effects. We observe a four-fold increase in fluorescence
enhancement and an almost 30-fold increase in decay rate from arrays of Ag
nanoparticles, when the LSPR is tuned to the emission wavelength of a locally
situated fluorophore. This is consistent with a greatly increased efficiency
for energy transfer from fluorescence to surface plasmons. Additionally,
surface enhanced Raman scattering (SERS) measurements show a maximum
enhancement occurs when both the incident laser light and the Raman signal are
near resonance with the plasmon energy. Spatial mapping of the SERS signal from
a nanoparticle array reveals highly localized differences in the excitation
field resulting from small differences in the LSPR energy.Comment: Submitted to Plasmonics (Springer
Optical characteristics of single wavelength-tunable InAs/InGaAsP/InP(100) quantum dots emitting at 1.55 um
We have studied the emission properties of individual InAs quantum dots (QDs)
grown in an InGaAsP matrix on InP(100) by metal-organic vapor-phase epitaxy.
Low-temperature microphotoluminescence spectroscopy shows emission from single
QDs around 1550 nm with characteristic exciton-biexciton behavior, and a
biexciton antibinding energy of more than 2 meV. Temperature-dependent
measurements reveal negligible optical-phonon induced broadening of the exciton
line up to 50 K, and emission from the exciton state clearly persists above 70
K. Furthermore, we find no measurable polarized fine structure splitting of the
exciton state within the experimental precision. These results are encouraging
for the development of a controllable photon source for fiber-based quantum
information and cryptography systems.Comment: 3 pages, 4 figures, submitted AP
Maternal glucose and fatty acid kinetics and infant birth weight in obese women with type 2 diabetes
The objectives of this study were 1) to describe maternal glucose and lipid kinetics and 2) to examine the relationships with infant birth weight in obese women with pregestational type 2 diabetes during late pregnancy. Using stable isotope tracer methodology and mass spectrometry, maternal glucose and lipid kinetic rates during the basal condition were compared in three groups: lean women without diabetes (Lean, n = 25), obese women without diabetes (OB, n = 26), and obese women with pregestational type 2 diabetes (OB+DM, n = 28; total n = 79). Glucose and lipid kinetics during hyperinsulinemia were also measured in a subset of participants (n = 56). Relationships between maternal glucose and lipid kinetics during both conditions and infant birth weight were examined. Maternal endogenous glucose production (EGP) rate was higher in OB+DM than OB and Lean during hyperinsulinemia. Maternal insulin value at 50% palmitate R(a) suppression (IC50) for palmitate suppression with insulinemia was higher in OB+DM than OB and Lean. Maternal EGP per unit insulin and plasma free fatty acid concentration during hyperinsulinemia most strongly predicted infant birth weight. Our findings suggest maternal fatty acid and glucose kinetics are altered during late pregnancy and might suggest a mechanism for higher birth weight in obese women with pregestational diabetes
Charged exciton emission at 1.3 m from single InAs quantum dots grown by metalorganic chemical vapor deposition
We have studied the emission properties of self-organized InAs quantum dots
(QDs) grown in an InGaAs quantum well by metalorganic chemical vapor
deposition. Low-temperature photoluminescence spectroscopy shows emission from
single QDs around 1300 nm; we clearly observe the formation of neutral and
charged exciton and biexciton states, and we obtain a biexciton binding energy
of 3.1 meV. The dots exhibit an s-p shell splitting of approximately 100 meV,
indicating strong confinement.Comment: 3 pages, 3 figures, submitted AP
Fine structure and magneto-optics of exciton, trion, and charged biexciton states in single InAs quantum dots emitting at 1.3 um
We present a detailed investigation into the optical characteristics of
individual InAs quantum dots (QDs) grown by metalorganic chemical vapor
deposition, with low temperature emission in the telecoms window around 1300
nm. Using micro-photoluminescence (PL) spectroscopy we have identified neutral,
positively charged, and negatively charged exciton and biexciton states.
Temperature-dependent measurements reveal dot-charging effects due to
differences in carrier diffusivity. We observe a pronounced linearly polarized
splitting of the neutral exciton and biexciton lines (~250 ueV) resulting from
asymmetry in the QD structure. This asymmetry also causes a mixing of the
excited trion states which is manifested in the fine structure and polarization
of the charged biexciton emission; from this data we obtain values for the
ratio between the anisotropic and isotropic electron-hole exchange energies of
(Delta1)/(Delta0)= 0.2--0.5. Magneto-PL spectroscopy has been used to
investigate the diamagnetic response and Zeeman splitting of the various
exciton complexes. We find a significant variation in g-factor between the
exciton, the positive biexciton, and the negative biexciton; this is also
attributed to anisotropy effects and the difference in lateral extent of the
electron and hole wavefunctions.Comment: 7 pages, 6 figures, submitted to Phys. Rev.
Investor behavior and the benefits of direct stock ownership
Whereas prior research emphasizes the monitoring benefits of concentrated stock ownership, we examine a variety of potential benefits of dispersed ownership for public companies. Using a field experiment to rule out reverse causality, we examine whether a small investment in a company’s stock leads investors to purchase more of the company’s products and adopt other views and preferences that benefit the company. We find little evidence consistent with our hypotheses using frequentist statistics, and Bayesian parameter estimation shows substantial downward belief revision for more optimistic ex ante expectations of the treatment effects for the
average investor. However, we do find that the effects of ownership on product purchase behavior and on regulatory preferences are intuitively stronger for certain subgroups of investors—namely, for investors who are most likely to consume the types of products offered by the company and for investors who are most likely to be politically active, respectively. The results contribute to our understanding of the benefits of dispersed stock ownership and are informative to public company managers and directors
Measuring diet in primary school children aged 8-11 years: validation of the Child and Diet Evaluation Tool (CADET) with an emphasis on fruit and vegetable intake.
Background/Objectives:The Child And Diet Evaluation Tool (CADET) is a 24-h food diary that measures the nutrition intake of children aged 3-7 years, with a focus on fruit and vegetable consumption. Until now CADET has not been used to measure nutrient intake of children aged 8-11 years. To ensure that newly assigned portion sizes for this older age group were valid, participants were asked to complete the CADET diary (the school and home food diary) concurrently with a 1-day weighed record. Subjects/Methods:A total of 67 children with a mean age of 9.3 years (s.d.: ± 1.4, 51% girls) participated in the study. Total fruit and vegetable intake in grams and other nutrients were extracted to compare the mean intakes from the CADET diary and Weighed record using t-tests and Pearson's r correlations. Bland-Altman analysis was also conducted to assess the agreement between the two methods. Results: Correlations comparing the CADET diary to the weighed record were high for fruit, vegetables and combined fruit and vegetables (r=0.7). The results from the Bland-Altman plots revealed a mean difference of 54 g (95% confidence interval: -88, 152) for combined fruit and vegetables intake. CADET is the only tool recommended by the National Obesity Observatory that has been validated in a UK population and provides nutrient level data on children's diets. Conclusions:The results from this study conclude that CADET can provide high-quality nutrient data suitable for evaluating intervention studies now for children aged 3-11 years with a focus on fruit and vegetable intake
Longitudinal photocurrent spectroscopy of a single GaAs/AlGaAs v-groove quantum wire
Modulation-doped GaAs v-groove quantum wires (QWRs) have been fabricated with
novel electrical contacts made to two-dimensional electron-gas (2DEG)
reservoirs. Here, we present longitudinal photocurrent (photoconductivity/PC)
spectroscopy measurements of a single QWR. We clearly observe conductance in
the ground-state one-dimensional subbands; in addition, a highly
temperature-dependent response is seen from other structures within the
v-groove. The latter phenomenon is attributed to the effects of structural
topography and localization on carrier relaxation. The results of
power-dependent PC measurements suggest that the QWR behaves as a series of
weakly interacting localized states, at low temperatures
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