Department of Physics, University of Fribourg, Switzerland
Scattering and propagation of light in mesoscopic random media
Sandor Balog
In this thesis we present an experimental study of mesoscopic correlations in the optical speckle pattern. The thesis
addresses two main topics: I) Speckle correlations in the narrow-beam limit and II) Photon count statistics in the presence
of long -range speckle correlations.
Most of the previous work on optical speckle has been carried out either deep in the diffusive regime or in the single
scattering approximation. Little attention has been paid to the transient regime, where the (effective) size of the scattering
volume is comparable to the transport mean free path. In this very interesting regime the nature and origin of speckle
correlations displays a pronounced dependence on optical parameters and the size of the scattering volume. In this thesis
we study the dependence of speckle fluctuations and correlations on the effective scattering volume of a disordered slab.
The effective scattering volume can be determined experimentally by illuminating the sample with a strongly focused laser
beam of variable diameter w. For a liquid suspension of strongly scattering colloidal particles we demonstrate that the
temporal fluctuations due to long-range correlations C2 can be modeled quantitatively in this "narrow-beam limit". Our
experimental results provide further evidence that the effective scattering volume scales with w3. We further show that in
the multiple scattering regime an additional mechanism exists that can induce fluctuations of the total transmission
coefficient. We present a series of experiments that indicate that particle number fluctuations are at the origin of this
phenomenon. So far the influence of number fluctuations has been reported only in single scattering systems. Surprisingly
in the multiple scattering regime we find a very similar dependence of the amplitude and relaxation time scaling on the size
of the illuminating beam. Qualitatively these findings give support to the physical picture of an effective scattering volume
determined by the diameter of the incident beam.
The second main topic of the current thesis is the study of the photon count statistics in the presence of long-range speckle
correlations. Based on the semi-classical theory of Mandel we develop a formalism that treats the fluctuations of discrete
photon numbers instead of studying only the classical intensity fluctuations. Although the quantized nature of light is widel y
recognized, almost all studies of multiple light scattering did not address quantum mechanical effects and concepts.
However, the features of quantum fluctuations can be dominating at low photon numbers. Further their dependence on the
scattering parameters is usually different compared to classical fluctuations. By extending the description of multiple light
scattering using quantum mechanical concepts we have developed a refined tool to describe the process of light
propagation in the mesoscopic regime. In this thesis we characterize, for the first time, the impact of coherent mesoscopic
correlations on the photon count statistics of coherent light scattered in disordered media. We show that two main regimes
can be distinguished in the full spectrum of the photon counts of the total transmitted signal. At high average photon
numbers classical fluctuations dominate the statistical properties of the photon count distribution. On the other hand it can
be easily shown that at very small average photon numbers quantum fluctuations dominate, and the shape of the
probability distribution is insensitive to multiple scattering and mesoscopic correlations. We show that the transition
between the two regimes is determined by the dimensionless conductance g of the sample under study. The quantitative
characterization of the photon count distribution presented in this thesis should provide an excellent basis for future studies
using, for example, non-classical light sources such as amplitude squeezed light, where quantitative theoretical predictions
are already available.
Jury:
Director of the thesis: Prof. Dr. Frank Scheffold, University of Fribourg
Expert: Prof. Dr. Peter Schurtenberger, University of Fribourg
Expert: Prof. Dr. Sergey E. Skipetrov, CNRS – Grenoble
President of the jury: Prof. Dr. Jean-Claude Dousse, University of Fribourg