Aging of the nonlinear optical susceptibility in doped colloidal suspensions

Using $Z$-scan and dynamic light-scattering measurements, we investigate the nonlinear optical response of a colloidal solution undergoing dynamics slowing down with age. We study the high optical nonlinearity of an organic dye (Rhodamine B) dispersed in a water-clay (Laponite) solution, at different clay concentrations $(2.0–2.6\mathrm{wt}\%)$, experiencing the gelation process. We determine the clay platelet self-diffusion coefficient and, by its comparison with the structural relaxation time, we conclude that the gelation process proceeds through the structuring of interconnecting clay platelet network rather than through cluster growth and aggregation.

Figure 1

(A) DLS homodyne correlation functions and (B) $Z$-scan data of Laponite $2.2\mathrm{wt}\%$ dispersion in RhB $0.6\mathrm{mM}$ water solution for different waiting times $t_w$ and corresponding fits (full lines) (see text). In (B), RhB $0.6\mathrm{mM}$ scan (open circles) is also reported.

Figure 2

Mean relaxation time ${\tau }_m$ (A) vs waiting time $t_w$ and (B) vs the scaled variable ${\mu }_{\tau }{t}_w$ in logarithmic-linear scale at various clay concentrations; continuous lines in (A) are the fitted curves.

Figure 3

Nonlinear absorption coefficient ${\alpha }_2$ (A) vs waiting time $t_w$ and (B) vs the scaled variable ${\mu }_{\alpha }{t}_w$ at various clay concentrations; continuous lines in (A) are the fitted curves.