15 research outputs found
Heterodyne Near-Field Scattering
We describe an optical technique based on the statistical analysis of the
random intensity distribution due to the interference of the near-field
scattered light with the strong transmitted beam. It is shown that, from the
study of the two-dimensional power spectrum of the intensity, one derives the
scattered intensity as a function of the scattering wave vector. Near-field
conditions are specified and discussed. The substantial advantages over
traditional scattering technique are pointed out, and is indicated that the
technique could be of interest for wave lengths other than visible light.Comment: 3 pages, 2 figure
A schlieren method for ultra-low angle light scattering measurements
We describe a self calibrating optical technique that allows to perform
absolute measurements of scattering cross sections for the light scattered at
extremely small angles. Very good performances are obtained by using a very
simple optical layout similar to that used for the schlieren method, a
technique traditionally used for mapping local refraction index changes. The
scattered intensity distribution is recovered by a statistical analysis of the
random interference of the light scattered in a half-plane of the scattering
wave vectors and the main transmitted beam. High quality data can be obtained
by proper statistical accumulation of scattered intensity frames, and the
static stray light contributions can be eliminated rigorously. The
potentialities of the method are tested in a scattering experiment from non
equilibrium fluctuations during a free diffusion experiment. Contributions of
light scattered from length scales as long as Lambda=1 mm can be accurately
determined.Comment: 7 pages, 3 figure
Probing Transverse Coherence with the Heterodyne Speckle Approach: Overview and Perspectives☆
Abstract The properties of spatial coherence of radiation emitted by relativistic electrons is far from being trivial. Assessing the coherence of high-brilliance X-ray sources (3rd generation synchrotrons or free electron lasers) is of crucial importance for machine diagnostics, as well as in planning experiments exploiting coherent techniques. The Heterodyne Speckles method, firstly described by Alaimo et al. (2009), is a valuable alternative to standard methods (e.g. Young's interferometer) which 1) provides a direct measure of transverse coherence without any a-priori assumption, 2) provides a full 2D map of coherence, 3) is capable of one shot, time-resolved measures, 4) is scalable over a wide range of wavelengths. It relies upon the statistical analysis of radiation scattered by spherical particles randomly distributed and suspended in a fluid. Here we give an overview of this method, from the theoretical framework to the operating conditions to be adopted in order to obtain coherence measurements in several conditions
Thermal Fluctuations in a Layer of Liquid CS 2 Subjected to Temperature Gradients with and without the Influence of Gravity
We report data for nonequilibrium density fluctuations in a layer of liquid CS(2) subjected to temperature gradients on Earth and in a satellite. The structure factor S(q) was measured using a calibrated shadowgraph. Upon removing gravity, S(q) increased dramatically at small wave vector, until the fluctuations generated by thermal noise were limited only by the 3 mm sample thickness. The results agree with theory to within a few percent on Earth and are ∼14% below theory in microgravity, demonstrating that the use of equilibrium Langevin forces is appropriate in this nonequilibrium situation
Equilibrium and nonequilibrium fluctuations at the interface between two fluid phases
We have performed small-angle light-scattering measurements of the static
structure factor of a critical binary mixture undergoing diffusive partial
remixing. An uncommon scattering geometry integrates the structure factor over
the sample thickness, allowing different regions of the concentration profile
to be probed simultaneously. Our experiment shows the existence of interface
capillary waves throughout the macroscopic evolution to an equilibrium
interface, and allows to derive the time evolution of surface tension.
Interfacial properties are shown to attain their equilibrium values quickly
compared to the system's macroscopic equilibration time.Comment: 10 pages, 5 figures, submitted to PR
Kerr effect from fractal aggregates of polystyrene microspheres
We study the slow aggregation of aqueous suspensions of optically isotropic microspheres by means of transient electric birefringence. Aggregating clusters exhibit large Kerr constants, and consequently we provide direct experimental evidence that the clusters do posses form anisotropy. From the birefringence relaxation time we derive information on the average cluster size and on the size distribution, and determine their evolution during the aggregation process. Static and dynamic light scattering data confirm the validity of the transient electric birefringence results