Coherent Backscattering from Multiple Scattering Systems - KOPS ...

6 Summary
The focus of the work presented in this thesis lay on improvements of the experimental techniques
for the investigation of multiple scattering phenomena, and in particular the coherent
backscattering cone of visible light.
This interference effect had been found to apparently violate the principle of conservation of
energy: The total amount of backscattered energy can not exceed the energy which the light
source inserts into the sample. The intensity enhancement of the cone should therefore be balanced
by a corresponding intensity cutback, which however had never been observed neither
in experiment nor in theory. Inaccuracies in the latter two being the only possible explanation,
we reviewed our backscattering experiments and the evaluation of the backscattering
data, and also tested an improved theory which had been worked out by E. Akkermans and
G. Montambaux.
The two theoreticians extended the theoretical description of the backscattering cone by two
additional terms that contribute to the total backscattered energy in the same order as the
cone itself. They lead to a cutback of the scattered intensity below the level of the incoherent
addition of the backscattered intensity which balances the intensity enhancement of the cone.
In the new theoretical description of the backscattering cone the energy is therefore conserved.
The key to a precise measurement of the shape of the coherent backscattering cone was the
correct calibration of the photodiodes of the wide angle setup. For this a teflon reference
sample is used, the albedo of which differs from the titania albedo by about 10%. In earlier
experiments this difference had been neglected, which led to a misrepresentation of the
backscattering cone, so that conservation of energy seemed to be seriously violated. If however
the albedos of sample and reference are considered in the evaluation, one can observe
an intensity cutback at the wings of the cone, which balances the intensity enhancement in
backscattering direction, and which also agrees with the predictions made by Akkermans and
Montambaux.
A second part of this work concerned the remodeling of the small angle setup for the measurement
of the scattered intensity distribution in a small range around backscattering direction.
One goal was to detect Anderson localization by the rounding of the tip of the coherent
backscattering cone. For this an older CCD camera was replaced by a high resolving model,
which improves the maximum intensity resolution by more than two orders of magnitude and
the maximum angular resolution or respectively the maximum angular range by a factor of
three. However, it turned out that the optical components between sample and camera cause
too much extraneous light, so that the necessary intensity resolution can not be achieved.
Still, the improved setup can be used to measure the transport mean free path of weakly
scattering samples like for example teflon. For teflon one can measure a cone with FWHM ≈
0.03 ◦ , corresponding to a transport mean free path l ∗ = 180 µm and a diffusion coefficient