Principle of a setup to measure the backscattering cone using a mirror (left) or a beam splitter (right). The sample is illuminated with a spatially broadened laser. In contrast to the setup using a mirror, the beam splitter setup allows the measurement of the intensity in direct back-reflection, however, needs a beam dump. The positive lens collects the light, which is then recorded with, e.g., a CCD-camera.
Illustration of the setup to measure the backscattered light up to high angles. 256 photodiodes are mounted on a detection arc with a diameter of 1.2 m. The arc is set up horizontally, so that the sample can be placed upright. Close to the back-reflection, eight photodiode arrays are positioned as indicated by the black lines are distributed. The photodiodes at higher angles are indicated by black dots. A positive lens produces divergent light, and a focal point in the plain of the detectors 0.6 m distant from the sample. The whole setup is put in a black box to shield stray light. The combination of two circular polarizing foils (CP), is used to suppress singly scattered light.
The response of ten typical photodiodes against the intensity of the incident laser. The curves show the same characteristic with a difference in the gradient. This is due to the fact that the diodes differ in the detection angle and hence see a different part of the background.
Two measurements of the coherent backscattering enhancement of one sample. The sample material was a powder. The solid lines show the theoretical fit from Ref. 7 both with a of 6.3, which is the corrected value taking internal reflections into account (Ref. 7). The agreement is excellent for the whole angular detection range. The enhancement factor for the configuration is 0.95, where for an enhancement of 0.75 is achieved.
Measurement of a sample with with angles up to 85°. As can be seen, an enhancement of 0.95 is observed and the cone shape is reasonably well described by the theory of Akkermans et al. 7 over the full angular range. The inset shows a magnification around small angles, where the wide angle setup overlaps with the small angle setup in order to determine the enhancement.
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