  When a coherent laser beam impinges on a random sample (e.g. a colloidal
suspension), the scattered light exhibits characteristic speckles. If the
temporal coherence of the light source is too short, then the speckles
disappear, along with the possibility of performing homodyne or heterodyne
scattering detection or photon correlation spectroscopy. Here we investigate
the scattering of a so-called "skewed coherence beam", i.e., a short-coherence
beam modified such that the field is coherent within slabs that are skewed with
respect to the wave fronts. We show that such a beam generates speckles and can
be used for heterodyne scattering detection, despite its short temporal
coherence. When applied to quite turbid samples, the technique has the
remarkable advantage of suppressing the multiple scattering contribution of the
scattering signal. The phenomenon presented here represents a very effective
method for measuring the coherence skewness of either a continuous wave or a
pulsed beam. Another field of application concerns X-rays. The observation of
speckles is usually limited to synchrotron radiation and FELs. Our experiment
suggests that a short-coherence X-ray source can also be used, with no
monochromator filtration, provided that the coherence is suitably skewed. Such
a technique will also enable heterodyne scattering detection with standard
short-coherence light sources.
