277 research outputs found
Photocount statistics in mesoscopic optics
We report the first observation of the impact of mesoscopic fluctuations on
the photocount statistics of coherent light scattered in a random medium.
Poisson photocount distribution of the incident light widens and gains
additional asymmetry upon transmission through a suspension of small dielectric
spheres. The effect is only appreciable when the average number of
photocounts becomes comparable or larger than the effective dimensionless
conductance g of the sample.Comment: Thoroughly revised text and figures, new data set, new figure adde
Multi-speckle diffusing wave spectroscopy with a single mode detection scheme
We present a detection scheme for diffusing wave spectroscopy (DWS) based on
a two cell geometry that allows efficient ensemble averaging. This is achieved
by putting a fast rotating diffuser in the optical path between laser and
sample. We show that the recorded (multi-speckle) correlation echoes provide an
ensemble averaged signal that does not require additional time averaging. We
find the performance of our experimental scheme comparable or even superior to
camera based multi-speckle techniques that rely on direct spatial averaging.
Furthermore, combined with traditional two-cell DWS, the full intensity
autocorrelation function can be measured with a single experimental setup
covering more than 10 decades in correlation time.Comment: Submitted to PR
Density-dependent interactions and structure of charged colloidal dispersions in the weak screening regime
We determine the structure of charge-stabilized colloidal suspensions at low
ionic strength over an extended range of particle volume fractions using a
combination of light and small angle neutron scattering experiments. The
variation of the structure factor with concentration is analyzed within a
one-component model of a colloidal suspension. We show that the observed
structural behavior corresponds to a non-monotonic density dependence of the
colloid effective charge and the mean interparticle interaction energy. Our
findings are corroborated by similar observations from primitive model computer
simulations of salt-free colloidal suspensions.Comment: Revised version, accepted to Phys. Rev. Let
Temperature-sensitive poly(N-Isopropyl-Acrylamide) microgel particles: A light scattering study
We present a light scattering study of aqueous suspensions of microgel particles consisting of poly(N-Isopropyl-Acrylamide) cross-linked gels. The solvent quality for the particles depends on temperature and thus allows tuning of the particle size. The particle synthesis parameters are chosen such that the resulting high surface charge of the particles prevents aggregation even in the maximally collapsed state. We present results on static and dynamic light scattering (SLS/DLS) for a highly diluted sample and for diffuse optical transmission on a more concentrated system. In the maximally collapsed state the scattering properties are well described by Mie theory for homogenous hard spheres. Upon swelling we find that a radially inhomogeneous density profile develop
Monitoring spatially heterogeneous dynamics in a drying colloidal thin film
We report on a new type of experiment that enables us to monitor spatially
and temporally heterogeneous dynamic properties in complex fluids. Our approach
is based on the analysis of near-field speckles produced by light diffusely
reflected from the superficial volume of a strongly scattering medium. By
periodic modulation of an incident speckle beam we obtain pixel-wise ensemble
averages of the structure function coefficient, a measure of the dynamic
activity. To illustrate the application of our approach we follow the different
stages in the drying process of a colloidal thin film. We show that we can
access ensemble averaged dynamic properties on length scales as small as ten
micrometers over the full field of view.Comment: To appear in Soft Material
Noise in laser speckle correlation and imaging techniques
We study the noise of the intensity variance and of the intensity correlation
and structure functions measured in light scattering from a random medium in
the case when these quantities are obtained by averaging over a finite number N
of pixels of a digital camera. We show that the noise scales as 1/N in all
cases and that it is sensitive to correlations of signals corresponding to
adjacent pixels as well as to the effective time averaging (due to the finite
sampling time) and spatial averaging (due to the finite pixel size). Our
results provide a guide to estimation of noise level in such applications as
the multi-speckle dynamic light scattering, time-resolved correlation
spectroscopy, speckle visibility spectroscopy, laser speckle imaging etc.Comment: submitted 14 May 201
Photon path length distribution in random media from spectral speckle intensity correlations
We show that the spectral speckle intensity correlation (SSIC) technique can be profitably exploited to recover the path length distribution of photons scattered in a random turbid medium. We applied SSIC to the study of Teflon slabs of different thicknesses and were able to recover, via the use of the photon diffusion approximation theory, the characteristic transport mean free path ℓ∗ and absorption length s a of the medium. These results were compared and validated by means of complementary measurements performed on the same samples with standard pulsed laser time of flight technique
Multispeckle diffusing-wave spectroscopy: a tool to study slow relaxation and time-dependent dynamics
A multispeckle technique for efficiently measuring correctly
ensemble-averaged intensity autocorrelation functions of scattered light from
non-ergodic and/or non-stationary systems is described.
The method employs a CCD camera as a multispeckle light detector and a
computer-based correlator, and permits the simultaneous calculation of up to
500 correlation functions, where each correlation function is started at a
different time.
The correlation functions are calculated in real time and are referenced to a
unique starting time.
The multispeckle nature of the CCD camera detector means that a true ensemble
average is calculated; no time averaging is necessary.
The technique thus provides a "snapshot" of the dynamics, making it
particularly useful for non-stationary systems where the dynamics are changing
with time.
Delay times spanning the range from 1 ms to 1000 s are readily achieved with
this method.
The technique is demonstrated in the multiple scattering limit where
diffusing-wave spectroscopy theory applies.
The technique can also be combined with a recently-developed two-cell
technique that can measure faster decay times.
The combined technique can measure delay times from 10 ns to 1000 s.
The method is peculiarly well suited for studying aging processes in soft
glassy materials, which exhibit both short and long relaxation times,
non-ergodic dynamics, and slowly-evolving transient behavior.Comment: 11 pages 13 figures Accepted in Review of Scientific Instrument (june
02
Nonuniversal correlations in multiple scattering
We show that intensity of a wave created by a source embedded inside a
three-dimensional disordered medium exhibits a non-universal space-time
correlation which depends explicitly on the short-distance properties of
disorder, source size, and dynamics of disorder in the immediate neighborhood
of the source. This correlation has an infinite spatial range and is
long-ranged in time. We suggest that a technique of "diffuse microscopy" might
be developed employing spatially-selective sensitivity of the considered
correlation to the disorder properties.Comment: 15 pages, 3 postscript figures, accepted to Phys. Rev.
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