Coherent Backscattering from Multiple Scattering Systems - KOPS ...

1 Introduction
There have been many spectacular findings in the field of multiple scattering of light in random
media, from the theoretical work of P. W. Anderson in the 1950s [13] and its application
to electromagnetic waves [14, 28] to the discovery of the coherent backscattering cone some
30 years ago [31, 52, 57] and the resent find of the onset of Anderson localization [5, 6, 48].
This work however focusses on the equally important improvements of the experimental techniques
for the investigation of multiple scattering phenomenons.
Of particular interest were experiments on the coherent backscattering cone, an interference
effect that causes a twofold intensity enhancement in the direction opposite to the incoming
wave. Although it can be observed not only on visible light in the laboratory, but also
in space [24], on microwaves [18], seismic waves [34], sound waves [15], or the de Broglie
waves of electrons in metals [29], the laboratory experiments with visible light have two major
advantages over other systems: There are no rivaling effects like interaction between the scattered
particles or binding in deep minima of the random potential, except for absorption, and
the necessary technical effort is comparatively low. Multiple scattering with electromagnetic
waves in the visible range is therefore used as model system to experimentally investigate
multiple scattering of waves [14, 28].
From the shape of the coherent backscattering cone one can read all kinds of information
about the scattering process in the medium. The cone width for example is a measure for the
transport mean free path l ∗ and therefore for the turbidity of the medium. The shape of the
conetip gives evidence of absorption and in some cases even the transition to a localizing state.
The experiments however require highly sensitive setups and refined theoretical descriptions
to precisely depict the angular distribution of the backscattered light and to correctly fit the
scattering parameters.
This thesis reports the work on two experimental setups, one to record the backscattered
radiation over a wide angular range, the other for detailed measurement of the intensity
distribution close to backscattering direction. The theoretical basis is laid in sec. 2, where
the mathematical description of coherent backscattering is developed starting from the wave
equation. Along the way, some insight is also given in phenomena like single scattering at
Mie particles and Anderson localization. Sec. 3 gives technical information about the setups
used for the scattering experiments. These are not only the two backscattering setups this
work focusses on, but also a time of flight setup which is used to record the time-dependent
scattering in transmission. The next section presents the samples used in the experiments
alongside with some additional characterizing methods. Finally, sec. 5 reports the revision
process of the two setups. For the wide angle setup the evaluation procedure was refined,
and an improved theory of coherent backscattering was developed, while the small angle
setup was tested in experiments on strongly scattering titanium dioxide samples, on teflon as
an example of a weakly scattering material, and on water-fluidized beds of glass beads. The
MATLAB [2] codes used for the evaluations can be found in the appendix.