  Dynamic light scattering techniques are routinely used for numerous
industrial and research applications, because they can give access to the
motion spectrum of micro- and nano-objects, and therefore to particle sizes or
visco-elastic properties. However, measurements are impossible when samples do
not scatterer light enough, i.e. when there are too few scattering events due
to excessively small scattering cross-sections and/or low concentrations of
scatterers. Here, we propose to amplify light scattering efficiency by placing
weakly scattering samples inside a Lambertian cavity with high reflectance
walls. It produces a 3D isotropic and homogeneous light field that effectively
elongates the scattering pathlength by 2 to 3 orders of magnitude, and leads to
a dramatic increase in sensitivity. We could indeed measure the diffusion
coefficient and size of particles ranging from 5nm to 20 microns with volume
fractions as low at 10^(-9) in volumes as low as 100 microliters, and in
solvents with refractive index mismatches down to 0.01. With a 10^(4) fold
increase in sensitivity compared to classical techniques, we considerably
expand the applications of light scattering to highly diluted samples,
miniaturized microfluidics samples, and samples practically deemed
non-scattering. Beyond the realm of current applications of light scattering
techniques, our Cavity Amplified Scattering Spectroscopy method (CASS) and its
outstanding sensitivity represent a major methodological step towards the study
of problems such as the ballistic limit of Brownian motion, the internal
dynamics of proteins, or the low frequency dielectric dynamics of liquids.
