Large Intensity Fluctuations Research Articles

Study on the optical bistability in the laser oscillation of Yb doped vanadate crystals

We have studied the effect of optical bistability exhibited in the laser oscillation of Yb:LuVO4, Yb:YVO4, and Yb:GdVO4, the three Yb doped vanadate crystals. The bistability range can be in excess of 1 W in terms of the pump power absorbed by the crystal. At the high-power-end side of the bistability region, i.e., the upper threshold for laser oscillation, discontinuous growing in the output power takes place, along with large fluctuations in the laser intensity occurring on sub-millisecond time scale. It is found that the bistability effect is affected significantly by the output coupling of the resonator, the crystal thickness, and the thermal effect existing in the crystal.

Numerical Simulation of Hurricane Bonnie (1998). Part III: Energetics

Abstract Despite considerable research on tropical cyclones (TCs), few studies have been performed to examine inner-core energy conversions because of the lack of high-resolution data. In this study, the TC energetic characteristics in relation to intensity and structural changes under different sheared environments are investigated using a 5-day cloud-resolving simulation of Hurricane Bonnie (1998). Results show that in the presence of intense vertical shear Bonnie undergoes high-frequency fluctuations in intensity and energy conversions (at a time scale of 3 h) during the partial eyewall stage. The fluctuations are closely related to the life cycle of individual convective elements that propagate cyclonically around the downshear portion of the eyewall. The energy conversions are shown to be maximized in the vicinity of the radius of maximum wind (RMW), thus affecting strongly TC intensity. On average, about 2% of latent energy can be converted to kinetic energy to increase TC intensity. After the vertical shear subsides below a threshold, intensity fluctuations become small as convective elements reorganize into an axisymmetric eyewall in which energy conversions are more evenly distributed. Fourier decomposition is conducted to separate the wavenumber-0, -1, and -2 components of inner-core energetics. Whereas wavenumber-1 perturbations dominate the partial eyewall stage, the propagation of wavenumber-2 perturbations is shown to be closely related to individual convective elements during both the partial eyewall and axisymmetric stages. The wavenumber-2 perturbations can be traced as they move around the eyewall in the form of vortex–Rossby waves, and they play a role in determining the large intensity fluctuations during the partial eyewall stage and the formation of an outer eyewall to replace the partial inner eyewall at the later stage.

Non-blinking semiconductor nanocrystals

The photoluminescence from a variety of individual molecules and nanometre-sized crystallites is defined by large intensity fluctuations, known as 'blinking', whereby their photoluminescence turns 'on' and 'off' intermittently, even under continuous photoexcitation. For semiconductor nanocrystals, it was originally proposed that these 'off' periods corresponded to a nanocrystal with an extra charge. A charged nanocrystal could have its photoluminescence temporarily quenched owing to the high efficiency of non-radiative (for example, Auger) recombination processes between the extra charge and a subsequently excited electron-hole pair; photoluminescence would resume only after the nanocrystal becomes neutralized again. Despite over a decade of research, completely non-blinking nanocrystals have not been synthesized and an understanding of the blinking phenomenon remains elusive. Here we report ternary core/shell CdZnSe/ZnSe semiconductor nanocrystals that individually exhibit continuous, non-blinking photoluminescence. Unexpectedly, these nanocrystals strongly photoluminesce despite being charged, as indicated by a multi-peaked photoluminescence spectral shape and short lifetime. To model the unusual photoluminescence properties of the CdZnSe/ZnSe nanocrystals, we softened the abrupt confinement potential of a typical core/shell nanocrystal, suggesting that the structure is a radially graded alloy of CdZnSe into ZnSe. As photoluminescence blinking severely limits the usefulness of nanocrystals in applications requiring a continuous output of single photons, these non-blinking nanocrystals may enable substantial advances in fields ranging from single-molecule biological labelling to low-threshold lasers.

The effects of pulsed laser injection seeding and triggering on the temporal behavior and magnitude of laser-induced incandescence from soot

The effect of sub-nanosecond fluence fluctuations and triggering on time-resolved laser-induced incandescence (LII) from soot has been studied using an injection-seeded pulsed Nd:YAG laser that produces a smooth laser temporal profile. Without injection seeding, this multi-mode laser generates pulses with large intensity fluctuations with sub-nanosecond rise times. The experimental results described here demonstrate that at fluences below 0.6 J/cm2 LII signals are insensitive to fluence fluctuations on nanosecond time scales. At fluences above 0.6 J/cm2 fluctuations in the laser profile cause the rising edge of the LII profile to move around in time relative to the center of the laser pulse causing a broader average profile that shifts to earlier times. Such fluctuations also lead to a decrease in the average LII temporal profile by up to 12% at a fluence of 3.5 J/cm2. A timing jitter on the trigger of the data acquisition, such as that produced by triggering on the laser Q-switch synchronization pulse, has a negligible effect on the shape and temporal maximum of the LII signal. Additional jitter, however, considerably reduces the peak of the LII temporal profiles at fluences as low as 0.15 J/cm2. Neither fast fluence fluctuations nor trigger jitter have a significant effect on gated LII signals, such as those used to infer soot volume fraction.

The Solar Energetic Particle Event of 14 December 2006

The solar energetic particle event on 14 December 2006 was observed by several near-Earth spacecraft including the Advanced Composition Explorer (ACE), STEREO A and B, SOHO and Wind. An interesting feature of this event is a series of unusual fluctuations in the particle intensity that occurred during the first few hours. These fluctuations were observed inside a magnetic cloud that originated in a solar event on 13 December and show both similarities and variations at the different spacecraft. Interestingly, the most striking difference is between observations at the two closely-separated STEREO spacecraft. In particular, large fluctuations in the proton intensity were seen by the High Energy Telescope (HET) on STEREO A, and to a lesser extent at Wind and ACE, but not by the STEREO B HET. We conclude that the differences in intensity-time profiles were caused by anisotropies in the particle distribution and the different viewing directions of the individual particle telescopes. The intensity/anisotropy variations suggest that flux tubes with different particle propagation conditions existed within this magnetic cloud despite the absence of local magnetic field signatures associated with these regions. The intensity fluctuations are similar to those occasionally seen in impulsive particle events. There were also spacecraft-to-spacecraft differences during the onset of the particle event. An initial rapid onset of energetic (> 40 MeV) protons was observed by the STEREO A and B spacecraft outside the magnetic cloud, but not by spacecraft such as SOHO that were already inside the magnetic cloud at this time. The latter spacecraft observed a slower, lower intensity increase. Evidently, energetic proton propagation from the solar event to the vicinity of Earth was inhibited within the magnetic cloud compared to outside.

What is the "contrast" in contrast adaptation?

Event Abstract Back to Event What is the "contrast" in contrast adaptation? During natural vision, retinal ganglion cells encode enormous variations of light intensity. In response, they adapt to the luminance distribution?s mean and width, the latter usually quantified by contrast (C = standard deviation / mean). This contrast adaptation compresses the large dynamic range of stimuli into a narrow range of firing rates. Here we describe ganglion cell responses via a linear-nonlinear model and ask, how should the nonlinearity adapt to different stimulus ensembles to efficiently fill the cells "response bandwidth" To study this, we passed each stimulus through a fixed linear filter followed by rectification and a nonlinearity, the output of which is taken to model an OFF ganglion cell?s firing rate. We then sought the nonlinearity that maximized information transfer subject to a constraint on the peak firing rate. The predicted nonlinearity maps the distribution of filter outputs to a uniform distribution of firing rates. An added mean rate constraint gives a truncated exponential rate distribution. First, we considered a Gaussian stimulus ensemble. For this ensemble, the model predicts a sigmoidal nonlinearity with a gain (slope at inflection point) proportional to 1/C. To test this, we made patch-clamp recordings of guinea pig retinal ganglion cells subject to full-field white noise stimulation at several contrasts. We found very close agreement between the predicted and measured nonlinearities at each contrast level. Natural light fluctuations are strongly non-Gaussian: they have temporal correlations and a skewed distribution. To study how this affects the optimal nonlinearity, we sorted a natural time series (van Hateren, 1997) into segments of fixed contrast. For all segment lengths we found that the distribution of contrasts was wide and skewed (peak C ~ 0.5 and 10% of segments with C > 1.5). In addition, for any given contrast, the light intensity distribution was also highly skewed. As above, we filtered and rectified segments of a given contrast; the resulting optimal gain was proportional to 1/C^0.5. This behavior differs from the Gaussian results in that it predicts less adaptation to increasing contrast. This is because skewed stimuli feature large but rare intensity fluctuations that strongly affect C, but excessive adaptation to these outliers would render the cell insensitive to the much more frequent small intensities. These considerations led us to ask whether a measure of contrast which is robust to outliers would be more natural for skewed intensity distributions. One possibility is the q-width, namely the range around the mean containing q% of intensity values. For Gaussian stimuli the q-width is proportional to the standard deviation. We found that the optimal gain predicted from the natural time series went approximately as the reciprocal of the 60-width. Thus, using this measure, the predicted gains have the same functional form for both naturally skewed and Gaussian distributions. Experiments in preparation will measure contrast adaptation in response to naturalistic stimuli, allowing us to test the prediction that contrast gain control is insensitive to outliers. Conference: Computational and systems neuroscience 2009, Salt Lake City, UT, United States, 26 Feb - 3 Mar, 2009. Presentation Type: Poster Presentation Topic: Poster Presentations Citation: (2009). What is the "contrast" in contrast adaptation?. Front. Syst. Neurosci. Conference Abstract: Computational and systems neuroscience 2009. doi: 10.3389/conf.neuro.06.2009.03.144 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 02 Feb 2009; Published Online: 02 Feb 2009. Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Google Google Scholar PubMed Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Intensity Correlations and Mesoscopic Fluctuations of Diffusing Photons in Cold Atoms

We study the angular correlation function of speckle patterns that result from multiple scattering of photons by cold atomic clouds. We show that this correlation function becomes larger than the value given by Rayleigh law for classical scatterers. These large intensity fluctuations constitute a new mesoscopic effect specific to atom-photon interactions, that could not be observed in other systems such as weakly disordered metals. We provide a complete description of this behavior and expressions that allow for a quantitative comparison with experiments.

Determination of agarose gel pore size: Absorbance measurements vis a vis other techniques

The absorbance measurements in the wavelength range 700 nm to 800 nm were used to probe the agarose gel topology evolution and extract the pore size of the trapped solvent. By following the changes in absorbance and pore size, the gelation process could be clearly divided into three stages - induction stage, gelation stage and pseudo-equilibrium stage. The gelation mechanism is explained as a nucleation and growth process. Following the kinetics of gelation using dynamic light scattering is complicated by multiple scattering (for high concentrations) and large fluctuations in intensity and relaxation time. Comparatively, scanning the absorption spectrum is fast and the method is suitable for a wide range of concentrations and setting temperatures. Pore size determination using absorbance is a fast and non-invasive method when compared to the DNA electrophoresis measurements, which extend over several hours and use probe diffusion.

Fluctuations of surface-enhanced Raman spectra of CO adsorbed on gold substrates

Surface-enhanced Raman scattering (SERS) of CO adsorbed on roughened gold surfaces has been studied using a Raman microscope with high spatial resolution. Sequentially recorded SERS spectra showed variations of Raman lines in their number, intensity, frequency, and halfwidth. Sometimes, in addition to the band for CO adsorbed at top sites, Raman peaks for bridge-bonded CO appear, showing large intensity fluctuations. Obviously, SERS fluctuations represent a more general property of SERS and point to local restructuring of substrates that changes temporarily the character of adsorption sites and/or their contribution to SERS. The single molecule ‘blinking’ effect is also discussed.

SCALING OF RETURN TIMES FOR A HIGH-RESOLUTION RAINFALL TIME SERIES

Rainfall is a highly intermittent field over a wide range of time and space scales. We study a high resolution rainfall time series exhibiting large intensity fluctuations and localized events. We consider the return times of a given intensity, and show that the time series composed of these return times is itself also very intermittent, obeying to a hyperbolic probability density, entailing that the mean return time diverges. This is an unexpected property since mean return times are often introduced in meteorology, especially for the study of risk associated to extreme events. It suggests that the intermittency of first return times of extreme events should be taken into account when making statistical predictions.

In-plane and in-depth nonuniformities in defect distribution in GaN and InGaN epilayers

The in-plane and in-depth characteristics of the GaN and InGaN epilayers grown by the metalorganic chemical vapour deposition (MOCVD) on three different substrates (sapphire, SiC and bulk GaN) are evaluated. Relatively large intensity fluctuations of “edge” GaN and InGaN emissions are observed and are related to the details of the micro-structure of the GaN and InGaN films studied. The experiments indicate a nonuniform defect distribution in all types of the MOCVD films studied. In particular, the decoration of structural defects with impurities, an increased defect accumulation at the interfaces and a surprisingly small influence of the micro-structure on the in-plane homogeneity of the yellow band cathodoluminescence emission are observed.

Image metrics in the statistical analysis of DNA microarray data.

DNA microarrays represent an important new method for determining the complete expression profile of a cell. In "spotted" microarrays, slides carrying spots of target DNA are hybridized to fluorescently labeled cDNA from experimental and control cells and the arrays are imaged at two or more wavelengths. In this paper, we perform statistical analysis on images of microarrays and show that quantitating the amount of fluorescent DNA bound to microarrays is subject to considerable uncertainty because of large and small-scale intensity fluctuations within spots, nonadditive background, and fabrication artifacts. Pixel-by-pixel analysis of individual spots can be used to estimate these sources of error and establish the precision and accuracy with which gene expression ratios are determined. Simple weighting schemes based on these estimates are effective in improving significantly the quality of microarray data as it accumulates in a multiexperiment database. We propose that error estimates from image-based metrics should be one component in an explicitly probabilistic scheme for the analysis of DNA microarray data.

Coherence properties of a continuous atom laser

We investigate the coherence properties of an atomic beam evaporatively cooled in a magnetic guide, assuming thermal equilibrium in the quantum degenerate regime. The gas experiences two-dimensional, transverse Bose-Einstein condensation rather than a full three-dimensional condensation because of the very elongated geometry of the magnetic guide. First order and second order correlation functions of the atomic field are used to characterize the coherence properties of the gas along the axis of the guide. The coherence length of the gas is found to be much larger than the thermal de Broglie wavelength in the strongly quantum degenerate regime. Large intensity fluctuations present in the ideal Bose gas model are found to be strongly reduced by repulsive atomic interactions; this conclusion is obtained with a one-dimensional classical field approximation valid when the temperature of the gas is much higher than its chemical potential, k B T » |μ|.

Single Molecules as Probes of Lipid Membrane Microenvironments

The single-molecule dynamics of the membrane probe diIC18 dispersed in well-controlled lipid environments were studied. Lipid monolayers and bilayers of DPPC, transferred onto a solid substrate using the Langmuir−Blodgett technique, were utilized to systematically control the environment around single diIC18 molecules. The single-molecule emission trajectories revealed large intensity fluctuations that were strongly coupled to the lipid environment surrounding the probe molecule. For example, as the surface pressure of a DPPC monolayer was increased from the liquid-expanded/liquid-condensed (π = 5 mN/m) region to the solid condensed (π = 30 mN/m) region, the characteristic fluorescence fluctuation times increased from approximately 440 ms to over 1 s. For bilayer films, we found characteristic fluctuation times on the order of 2 s, regardless of which side of the bilayer the probe molecule resided. The monolayer and bilayer results are most consistent with a mechanism for intensity fluctuations driven by ...

Enhanced Intensity Fluctuations of Multiply Scattered Light in Nonlinear Absorbing Media

The properties of light transmission and interference phenomena in high dense media are investigated by means of numerical simulations in terms of nonlinear optical effect. Large intensity fluctuations are observed for nonlinear absorbing media, and do not agree with the fact that a decrease in absorption efficiency results in smaller intensity fluctuations for linear absorption media. From a variation in the speckle contrast it is found that the high-order path-crossing contributes to such intensity fluctuations. Moreover, the probability density of the total transmission changes from Gaussian statistics to log-normal ones for the small total transmission and to negative exponential ones for the large transmission, both of which appear in the strong localization regime. This indicates that the localization of optical waves might be controlled by the nonlinearity of the media.

Suppression of classic and quantum radiation pressure noise by electro-optic feedback

We present theoretical results that demonstrate a new technique that can be used to improve the sensitivity of thermal noise measurements: intracavity intensity stabilization. It is demonstrated that electro-optic feedback can be used to reduce intracavity intensity fluctuations, and the consequent radiation pressure fluctuations, by a factor of 2 below the quantum-noise limit. We show that this reduction is achievable in the presence of large classic intensity fluctuations in the incident laser beam. The benefits of this scheme are a consequence of the sub-Poissonian intensity statistics of the field inside a feedback loop and the quantum nondemolition nature of radiation pressure noise as a readout system for the intracavity intensity fluctuations.

Near-Field Spectroscopy of a Gated Electron Gas: A Direct Evidence for Electron Localization

The near-field photoluminescence of a gated two-dimensional electron gas is measured. We use the negatively charged exciton, formed by binding an electron to a photoexcited electron-hole pair, as an indicator for the local presence of charge. Large spatial fluctuations in the luminescence intensity of the negatively charged exciton are observed. These fluctuations are shown to be due to electrons localized in the random potential of the remote ionized donors. We use these fluctuations to image the electron and the donor distribution in the plane.

Electronic quenching of the A 1Σ + state of NaH

We have measured the rates for quenching of the v = 4 and v = 5 vibrational levels of the A 1Σ + state of sodium hydride using helium, argon, and nitrogen as the quenching gas. The measured rates are compared with earlier results for state-to-state quenching rates. The measured rates for quenching by helium and argon are similar, whereas that for quenching by nitrogen is an order of magnitude higher. In addition, at high nitrogen gas pressures the fluorescence was observed to undergo large scale intensity fluctuations. It is suggested that this may be due to a chemical rather than a physical mechanism.

Artifacts in functional magnetic resonance imaging from gaseous oxygen.

Unexpectedly large fluctuations in signal intensity were identified in the functional MRI (FMRI) of normal subjects breathing pure oxygen intermittently. To test the hypothesis that the signal changes were due to fluctuating concentrations of gaseous (paramagnetic) oxygen in the magnetic field, echo planar gradient echo images were acquired of a phantom contiguous to an oxygen mask through which pure oxygen was administered intermittently via plastic tubing. As a control, room air was administered intermittently or oxygen continuously in the same experimental protocol. Signal intensity changes of up to 60% temporally correlated with the administration of oxygen were produced in the phantom. In functional images prepared from the echo planar images, the signal intensity changes resulted in artifacts especially at interfaces in the phantom. The intermittent administration of pure oxygen during acquisition of data for FMRI may produce signal intensity changes that stimulate or obscure function.

Intensity correlations in electronic-wave propagation in a disordered medium: The influence of spin-orbit scattering.

We obtain explicit expressions for the correlation functions of transmission and reflection coefficients of coherent electronic waves propagating through a disordered quasi-one-dimensional medium with purely elastic diffusive scattering in the presence of spin-orbit interactions. We find in the metallic regime both large local intensity fluctuations and long-range correlations which ultimately lead to universal conductance fluctuations. We show that the main effect of spin-orbit scattering is to suppress both local and long-range intensity fluctuations by a universal symmetry factor 4. We use a scattering approach based on random transfer matrices.