Coherent Backscattering from Multiple Scattering Systems - KOPS ...

5 Experiments
220
200
180
l* [µm]
160
140
120
100
2 4 6 8 10 12 14 16
lens position [mm]
Figure 5.12: Transport mean free path in dependence of the lens position. Between
7 and 10 mm the transport mean free path of teflon has a slight maximum of approximately
180 µm, which can be clearly derived from the trend of the data. (The orange
line was drawn as a guide to the eye.)
with the refractive index of teflon, n = 1.35, and the diffusion coefficient is D = 16500 m2 /s,
as suggested by the time of flight experiments (see sec. 4.2.2). With eqn. 2.16 one obtains
a FWHM of about 0.01 ◦ , which corresponds to 12 pixels on the CCD chip. [47, 49] c report
experimentally determined cone widths of about 0.03 ◦ , which would correspond to 35 pixels
in our setup.
This however is not that much wider than the intensity profile of the focus spot (see sec. 3.3.4),
which therefore will have a considerable influence on the shape of the intensity enhancement.
Consequently, the measured backscattering cone of teflon can be expected to be significantly
widened, and the convolution of the fitted theory with the focus spot profile will become
essential to yield correct results.
The accuracy of the setup is for the most part determined by the divergence of the incoming
laser beam (which should be as low as possible) and the correct position of the focussing lens
in front of the camera. The former seems to be comparatively uncritical, as the beam can easily
be adjusted to have a divergence lower than ±0.01 ◦ , which is the range where no significant
change in the cone shape seems to occur.
The appropriate distance between lens and camera is more difficult to determine. Fig. 5.12
shows that the fitted values for the transport mean free path can fluctuate strongly, even
if the lens–camera distance is set correctly. Hence it is reasonable to measure a series of
backscattering cones where the setting is deliberately varied slightly for each take. The correct
[c] In [49] there is obviously a typing error at this point.
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