Time-dependent nonlinear optical susceptibility of an out-of-equilibrium soft material.

We investigate the time-dependent nonlinear optical absorption of a clay dispersion (Laponite) in an organic dye (rhodamine B) water solution displaying liquid-arrested state transition. Specifically, we determine the characteristic time tauD of the nonlinear susceptibility buildup due to the Soret effect. By comparing tauD with the relaxation time provided by standard dynamic light scattering measurements we report on the decoupling of the two collective diffusion times at the two very different length scales during the aging of the out-of-equilibrium system. With this demonstration experiment we also show the potentiality of nonlinear optics measurements in the study of the late stage of arrest in soft materials.