Multispeckle diffusing-wave spectroscopy: a tool to study slow relaxation and time-dependent dynamics

摘要

A multispeckle technique for efficiently measuring correctlyensemble-averaged intensity autocorrelation functions of scattered light fromnon-ergodic and/or non-stationary systems is described. The method employs a CCD camera as a multispeckle light detector and acomputer-based correlator, and permits the simultaneous calculation of up to500 correlation functions, where each correlation function is started at adifferent time. The correlation functions are calculated in real time and are referenced to aunique starting time. The multispeckle nature of the CCD camera detector means that a true ensembleaverage is calculated; no time averaging is necessary. The technique thus provides a "snapshot" of the dynamics, making itparticularly useful for non-stationary systems where the dynamics are changingwith time. Delay times spanning the range from 1 ms to 1000 s are readily achieved withthis method. The technique is demonstrated in the multiple scattering limit wherediffusing-wave spectroscopy theory applies. The technique can also be combined with a recently-developed two-celltechnique that can measure faster decay times. The combined technique can measure delay times from 10 ns to 1000 s. The method is peculiarly well suited for studying aging processes in softglassy materials, which exhibit both short and long relaxation times,non-ergodic dynamics, and slowly-evolving transient behavior.