Multi-speckle autocorrelation spectroscopy - a new strategy to monitor ultraslow dynamics in dense and nonergodic media

We present a modification of the conventional dynamic light scattering set-up which allows to monitor the intensity fluctuations of many independent spatial Fourier components of the density fluctuations, i.e. ''speckles'', simultaneously by using a charge-coupled device (CCD) camera as area detector. By averaging over the intensity autocorrelation function the final 10-20% decay of the intermediate scattering function in very dense colloidal dispersions is obtained with much higher accuracy. At the same time this multi-speckle autocorrelation spectroscopy provides an alternative route for constructing ensemble-averaged intermediate scattering functions in nonergodic media by replacing the average over many independent sample volumes by an average over independent spatial Fourier components of the density fluctuations. We will survey the methods proposed so far to generate ensemble averages in nonergodic media and discuss their merits and limits. We then demonstrate the advantages of the new technique, taking as an example a colloidal dispersion where in the glassy state long-lived density fluctuations superimpose on the frozen ones. Finally, we make a direct comparison with another ''speckle-averaging'' technique, the ''interleaved sampling'' method, which has been proposed and applied to the same system recently.