Measurements Of Intensity Fluctuations Research Articles

Microwell-enhanced optical rapid antibiotic susceptibility testing of single bacteria

Bacteria that are resistant to antibiotics present an increasing burden on healthcare. To address this emerging crisis, novel rapid antibiotic susceptibility testing (AST) methods are eagerly needed. Here, we present an optical AST technique that can determine the bacterial viability within 1h down to a resolution of single bacteria. The method is based on measuring intensity fluctuations of a reflected laser focused on a bacterium in reflective microwells. Using numerical simulations, we show that both refraction and absorption of light by the bacterium contribute to the observed signal. By administering antibiotics that kill the bacteria, we show that the variance of the detected fluctuations vanishes within 1 h, indicating the potential of this technique for rapid sensing of bacterial antibiotic susceptibility. We envisage the use of this method for massively parallelizable AST tests and fast detection of drug-resistant pathogens.

LIMFAST. II. Line Intensity Mapping as a Probe of High-redshift Galaxy Formation

The epoch of reionization (EoR) offers a unique window into the dawn of galaxy formation, through which high-redshift galaxies can be studied by observations of both themselves and their impact on the intergalactic medium. Line intensity mapping (LIM) promises to explore cosmic reionization and its driving sources by measuring intensity fluctuations of emission lines tracing the cosmic gas in varying phases. Using LIMFAST, a novel seminumerical tool designed to self-consistently simulate LIM signals of multiple EoR probes, we investigate how building blocks of galaxy formation and evolution theory, such as feedback-regulated star formation and chemical enrichment, might be studied with multitracer LIM during the EoR. On galaxy scales, we show that the star formation law and the feedback associated with star formation can be indicated by both the shape and redshift evolution of LIM power spectra. For a baseline model of metal production that traces star formation, we find that lines highly sensitive to metallicity are generally better probes of galaxy formation models. On larger scales, we demonstrate that inferring ionized bubble sizes from cross-correlations between tracers of ionized and neutral gas requires a detailed understanding of the astrophysics that shape the line luminosity–halo mass relation. Despite various modeling and observational challenges, wide-area, multitracer LIM surveys will provide important high-redshift tests for the fundamentals of galaxy formation theory, especially the interplay between star formation and feedback by accessing statistically the entire low-mass population of galaxies as ideal laboratories, complementary to upcoming surveys of individual sources by new-generation telescopes.

Double-pulse speckle contrast correlations with near Fourier transform limited free-electron laser light using hard X-ray split-and-delay

The ability to deliver two coherent X-ray pulses with precise time-delays ranging from a few femtoseconds to nanoseconds enables critical capabilities of probing ultra-fast phenomena in condensed matter systems at X-ray free electron laser (FEL) sources. Recent progress made in the hard X-ray split-and-delay optics developments now brings a very promising prospect for resolving atomic-scale motions that were not accessible by previous time-resolved techniques. Here, we report on characterizing the spatial and temporal coherence properties of the hard X-ray FEL beam after propagating through split-and-delay optics. Speckle contrast analysis of small-angle scattering measurements from nanoparticles reveals well-preserved transverse coherence of the beam. Measuring intensity fluctuations from successive X-ray pulses also reveals that only single or double temporal modes remain in the transmitted beam, corresponding to nearly Fourier transform limited pulses.

Fluorescence lifetime correlation spectroscopy: Basics and applications

This chapter presents a concise introduction into the method of Fluorescence Lifetime Correlation Spectroscopy (FLCS). This is an extension of Fluorescence Correlation Spectroscopy (FCS) that analyses fluorescence intensity fluctuations from small detection volumes in samples of ultra-low concentration. FCS has been widely used for investigating diffusion, conformational changes, molecular binding/unbinding equilibria, or chemical reaction kinetics, at single molecule sensitivity. In FCS, this is done by calculating intensity correlation curves for the measured intensity fluctuations. FLCS extends this idea by calculating fluorescence-lifetime specific intensity correlation curves. Thus, FLCS is the method of choice for all studies where a parameter of interest (conformational state, spatial position, molecular environmental condition) is connected with a change in the fluorescence lifetime. After presenting the theoretical and experimental basis of FLCS, the chapter gives an overview of its various applications.

Simple statistical channel model for weak temperature-induced turbulence in underwater wireless optical communication systems.

In this Letter, we use laser beam intensity fluctuation measurements to model and describe the statistical properties of weak temperature-induced turbulence in underwater wireless optical communication (UWOC) channels. UWOC channels with temperature gradients are modeled by the generalized gamma distribution (GGD) with an excellent goodness of fit to the measured data under all channel conditions. Meanwhile, thermally uniform channels are perfectly described by the simple gamma distribution which is a special case of GGD. To the best of our knowledge, this is the first model that comprehensively describes both thermally uniform and gradient-based UWOC channels.

A single molecule as a high-fidelity photon gun for producing intensity-squeezed light

A two-level atom cannot emit more than one photon at a time. As early as the 1980s, this quantum feature was identified as a gateway to "single-photon sources", where a regular excitation sequence would create a stream of light particles with photon number fluctuations below the shot noise. Such an intensity squeezed beam of light would be desirable for a range of applications such as quantum imaging, sensing, enhanced precision measurements and information processing. However, experimental realizations of these sources have been hindered by large losses caused by low photon collection efficiencies and photophysical shortcomings. By using a planar metallo-dielectric antenna applied to an organic molecule, we demonstrate the most regular stream of single photons reported to date. Measured intensity fluctuations reveal 2.2 dB squeezing limited by our detection efficiency, equivalent to 6.2 dB intensity squeezing right after the antenna.

Application of Fluorescence Correlation Spectroscopy to Study Dynamics of Proteins Involved in Neuronal Synapse-to-Nucleus Signaling

Activity-dependent gene transcription is required for the long-term stabilization of synaptic plasticity, a process that is key to learning and memory. We are studying the transcription factor, NFAT, which has been shown to be actuated by postsynaptic signaling. Ca2+ influx through voltage-gated L-type calcium channels plays an exclusive role in initiating NFAT signaling via the action of the Ca2+/calmodulin- phosphatase calcineurin (CaN). CaN dephosphorylates multiple residues on NFAT, resulting in the exposure of its nuclear localization sequence and subsequent translocation to the nucleus. A required component of this cascade is the A-kinase anchoring protein (AKAP) 79/150, which serves to recruit CaN to the channel. New evidence from our group indicates that NFAT is also tethered to this complex via a transient association with AKAP.This scenario suggests a model wherein NFAT signaling is instigated by local synaptic activity and the actuated transcription factor is then conveyed to the nucleus over long intracellular distances via diffusion or active transport. However, the molecular mechanisms of this process have yet to be elucidated. A potentially powerful approach to investigate the dynamics of the signaling molecules in this pathway is fluorescence correlation spectroscopy (FCS). This technique yields quantitative information on protein mobility in living cells by measuring intensity fluctuations produced by fluorescent (or fluorescently tagged) molecules diffusing through a confined detection volume.Here we present FCS measurements characterizing NFAT dynamics in response to induced synaptic activity in cultured hippocampal neurons, utilizing models that account for dendritic morphology. Additionally, we have found that there is a prominent component in the autocorrelation curve due to GFP photophysics (triplet state transitions and dynamic quenching) in FCS data from these cells, which requires further modifications to the standard fitting models.

Ultraviolet luminosity density of the universe during the epoch of reionization.

The spatial fluctuations of the extragalactic background light trace the total emission from all stars and galaxies in the Universe. A multiwavelength study can be used to measure the integrated emission from first galaxies during reionization when the Universe was about 500 million years old. Here we report arcmin-scale spatial fluctuations in one of the deepest sky surveys with the Hubble Space Telescope in five wavebands between 0.6 and 1.6 μm. We model-fit the angular power spectra of intensity fluctuation measurements to find the ultraviolet luminosity density of galaxies at redshifts greater than 8 to be . This level of integrated light emission allows for a significant surface density of fainter primeval galaxies that are below the point-source detection level in current surveys.

100% correlation of chaotic thermal light

We report an experimental study on the intensity fluctuation-fluctuation correlations of chaotic-thermal light. This experiment introduced a detection protocol, which distinguishes positive and negative intensity fluctuations and lead to an intensity fluctuation-fluctuation correlation with 100% contrast from jointly measured photodetection events. This type of correlation provides a solution to achieve high visibility in thermal light ghost imaging. This study also explored other potential applications in which the higher-order coherence is necessary to be distinguished from the coexisting lower-order effects. In addition to its obvious importance in practical applications, this experiment brings us a broader vision in understanding the physical relationship between locally measured intensity fluctuations and the distant fluctuation-fluctuation correlations of chaotic-thermal light.

Spectral Observations of Filament Activation

AbstractStudies of solar filament (prominence) activation and eruption are often based on measurements of intensity fluctuations in various solar emission bands and rarely on Doppler velocity measurements. The goal of this paper is to analyze the process of quiescent filament activation, using spectral data, and its associated events in solar UV band. Motions have been examined in a small southern fragment of a quiescent, extended filament in the northern hemisphere prior to and during its activation on June 14 2012. A part of the fragment disappeared after the filament activation.

Noninvasive vasculature detection using laser speckle imaging in avian embryos through intact egg in early incubation stage

Monitoring the vital signs of a developing embryo is very useful in avian breeding programs, especially during early days of incubation, so that dead or unfertilized eggs can be timely removed from incubator and new eggs can be placed in. A noninvasive system for detecting the vital signs of avian embryo through intact egg in early stage of incubation has been developed using laser speckle imaging (LSI). The system was based on the measurement of intensity fluctuations of speckle caused by the embryo’s blood flow in the intact egg under laser light illumination. This system was found to be feasible in imaging the vasculature in the egg as well as confirming its fertilization or survival from the second day to fifth day of incubation while other reported noninvasive methods cannot detect vital signs of the embryo until the sixth day of incubation.

Correlation singularities in partially coherent electromagnetic beams

We demonstrate that coherence vortices, singularities of the correlation function, generally occur in partially coherent electromagnetic beams. In successive cross sections of Gaussian Schell-model beams, their locus is found to be a closed string. These coherence singularities have implications for both interference experiments and correlation of intensity fluctuation measurements performed with such beams.

The effect of phase scintillations on the accuracy of phase screen simulation using deterministic screens derived from GPS and ALTAIR measurements

Radio receivers capable of high‐rate sampling such as GPS scintillation monitors and the ALTAIR VHF/UHF tracking radar can measure ionospheric phase fluctuations suitable for scintillation modeling using phase screen techniques. For modeling purposes, the phase variations caused by the refractive effects of electron density irregularities encountered along the propagation path are desired. The phase fluctuations measured by ground‐based receivers, however, also include the unwanted effects of diffraction (phase scintillations). In this paper, we investigate the effect of phase scintillations on the accuracy of phase screen simulation when using the phase measured on the ground as a proxy for the ionospheric screen. Using stochastic and deterministic (measured) phase screens, we quantitatively assess the accuracy of this approach by cross‐correlating the predicted and measured intensity fluctuations. We find that the intensity cross‐correlation is less than unity even in the weak scatter limit, due to the presence of weak phase scintillations. This correlation decreases rapidly with increasing irregularity strength once rapid transitions in the phase (strong phase scintillations) develop. We demonstrate that, when using the measured phase on the ground as a proxy for the ionospheric screen, both the temporal structure of simulated fluctuations and their statistics deviate increasingly from those of the measurements as the turbulence strength increases, especially when strong phase scintillations are present. We also demonstrate that back‐propagating the complex signal up to ionospheric altitudes prior to the forward propagation calculation yields improved results, but some errors still remain as a consequence of neglecting amplitude fluctuations which develop inside the random medium.

A novel technique to measure intensity fluctuations in EUV images and to detect coronal sound waves nearby active regions

Context. In the past years, evidence for the existence of outward-moving (Doppler blue-shifted) plasma and slow-mode magnetoacoustic propagating waves in various magnetic field structures (loops in particular) in the solar corona has been found in ultraviolet images and spectra. Yet their origin and possible connection to and importance for the mass and energy supply to the corona and solar wind is still unclear. There has been increasing interest in this problem thanks to the high-resolution observations available from the extreme ultraviolet (EUV) imagers on the Solar TErrestrial RElationships Observatory (STEREO) and the EUV spectrometer on the Hinode mission. Aims. Flows and waves exist in the corona, and their signatures appear in EUV imaging observations but are extremely difficult to analyse quantitatively because of their weak intensity. Hence, such information is currently available mostly from spectroscopic observations that are restricted in their spatial and temporal coverage. To understand the nature and origin of these fluctuations, imaging observations are essential. Here, we present measurements of the speed of intensity fluctuations observed along apparently open field lines with the Extreme UltraViolet Imagers (EUVI) onboard the STEREO mission. One aim of our paper is to demonstrate that we can make reliable kinematic measurements from these EUV images, thereby complementing and extending the spectroscopic measurements and opening up the full corona for such an analysis. Another aim is to examine the assumptions that lead to flow versus wave interpretation for these fluctuations. Methods. We have developed a novel image-processing method by fusing well established techniques for the kinematic analysis of coronal mass ejections (CME) with standard wavelet analysis. The power of our method lies with its ability to recover weak intensity fluctuations along individual magnetic structures at any orientation, anywhere within the full solar disk, and using standard synoptic observing sequences (cadence <3 min) without the need for special observation plans. Results. Using information from both EUVI imagers, we obtained wave phase speeds with values on the order of 60−90 km s −1 , compatible with those obtained by other previous measurements. Moreover, we studied the periodicity of the observed fluctuations and established a predominance of a 16-min period, and other periods that seem to be multiples of an underlying 8-min period. Conclusions. The validation of our analysis technique opens up new possibilities for the study of coronal flows and waves, by extending it to the full disk and to a larger number of coronal structures than has been possible previously. It opens up a new scientific capability for the EUV observations from the recently launched Solar Dynamics Observatory. Here we clearly establish the ubiquitous existence of sound waves which continuously propagate along apparently open magnetic field lines.

A method to improve the visibility of ghost images obtained by thermal light

A method, which is based on the correlation measurement of intensity fluctuations in time–space-average domain, is invented to improve the visibility of ghost images obtained by thermal light. We demonstrate that the visibility of ghost images can be significantly enhanced using this method. A corresponding evaluation standard of the visibility of images and effects influencing the visibility of ghost images, such as the weight factor γ and speckle frame numbers, are also discussed.

Can laser speckle flowmetry be made a quantitative tool?

The ultimate objective of laser speckle flowmetry (and a host of specific implementations such as laser speckle contrast analysis, LASCA or LSCA; laser speckle spatial contrast analysis, LSSCA; laser speckle temporal contrast analysis, LSTCA; etc.) is to infer flow velocity from the observed speckle contrast. Despite numerous demonstrations over the past 25 years of such a qualitative relationship, no convincing quantitative relationship has been proven. One reason is a persistent mathematical error that has been propagated by a host of workers; another is a misconception about the proper autocorrelation function for ordered flow. Still another hindrance has been uncertainty in the specific relationship between decorrelation time and local flow velocity. Herein we attempt to dispel some of these errors and misconceptions with the intent of turning laser speckle flowmetry into a quantitative tool. Specifically we review the underlying theory, explore the impact of various analytic models for relating measured intensity fluctuations to scatterer motion, and address some of the practical issues associated with the measurement and subsequent data processing.

Statistical properties of macroscopic laser fields after coherent interaction with an atomic vapour

We present a series of experiments measuring intensity fluctuations and correlations of laser beams after propagation through an atomic vapour at room temperature in situations where coherent effects play a major role. In particular, we investigate these properties in the condition of electromagnetically induced transparency (EIT) in three-level lambda systems and in degenerate two-level systems. In a similar way, we also study the Hanle-EIT effect and electromagnetically induced absorption (EIA) in degenerate two-level systems. These results show the great influence of coherent interaction with the atomic system over the noise properties of the macroscopic laser fields, and indicate new sensitive ways of probing an atomic sample.

Introduction: Renaissance of Scintillometry

In the 1970s impressive progress was made in research on turbulence and the use of scintillometers. Muschinski and Lenschow (2002) summarised the outcome of the workshop on ‘Future Directions for Research on Meterand Submeter-Scale Atmospheric Turbulence’ held at Boulder, Colorado, U.S.A. where the development in this field was discussed. One of their comments noted that, after a blooming period in the 1970s and consolidation in the 1980s, funding and interest in scintillometry faded rapidly in the early 1990s. This special BLM issue is a result of a renewed interest, a renaissance, in scintillometry. Of work done in the last 40 years, some of the most important contributions include the work of Tatarskii (1961), the textbook by Monin and Yaglom (1975), the collection of scintillation papers brought together by Andreas (1990) and reviews by e.g., Hill (1997). Here are some main lines. A scintillometer is an instrument that consists of a transmitter and a receiver. The receiver measures intensity fluctuations in the radiation emitted by the transmitter caused by refractive scattering of turbulent eddies in the scintillometer path. For laser sources, or small aperture scintillometers (SAS), the observed intensity fluctuations are a measure of the structure parameter of the refractive index, C2 n, and the inner scale of turbulence, 0. For large aperture scintillometers (LAS), they are a measure of C2 n only. At optical wavelengths the contribution of temperature fluctuations dominates, i.e., the structure parameter of temperature C2 T can be deduced from the C2 n measurement. For radio wavelengths (>1 mm) on the other hand, water vapour fluctuations contribute most to the scintillometer signal, i.e. the structure parameter of moisture C2 q can be deduced from the C 2 n measurement. Surface fluxes of sensible heat, latent heat and momentum can be determined from the obtained C2 T , C 2 n and 0 respectively by applying Monin–Obukhov similarity theory (MOST). A number of length scales play a role in scintillometry: • The inner and outer length scales of turbulence. The first, 0, is proportional to the Kolmogorov length (ν3/ )1/4, where ν is the molecular kinematic viscosity and the dissipation rate of turbulent kinetic energy (TKE). The outer length scale is proportional to the height above the surface, z, in the surface layer; • the wavelength of the light source of the scintillometer, λ; • the aperture, D, of the scintillometer. Note that the transmitter and receiver have the same aperture in the scintillometers used in the studies presented here;

Measurements of intensity fluctuations in a laser with a saturable absorber

Fluctuations of light intensity in a laser with an intracavity nonlinear absorber are studied experimentally. Measurements are made close to the tricritical point by varying the operating parameters of the laser. Experimental results are in agreement with the theoretical predictions based on the nonlinear oscillator model of the laser in which both the third- and fifth-order nonlinearities in the field amplitude are retained.

Polymer Nanoparticle Characterization in Aqueous Suspensions

This article evaluates several techniques for measuring nanoparticle size in suspension. The direct techniques measure the nanoparticle size by electron microscopy. However, with this technique, the particles must be observed dry, except in the case of environmental microscopy, and their distribution is calculated by performing image analysis. Other indirect methods, based on measurements of intensity fluctuations allow size measurement but these methods are dependent on sample preparation (concentration, polydispersity, etc.). Two types of materials are characterized and discussed herein: model polystyrene latex particles used for size standardization in several devices and polydisperse wheat protein nanoparticles prepared in our laboratory. Our results show that no size characterization method is totally universal and absolute. Coupling at least two different methods allows for more rigorous size characterization.