Soft-matter (SM) is a complex system; this means that dynamic phases in SM are characterized by an exponentially large (in the system size) number of states, which are typically described in terms of a topologically structured landscape. A probe for this landscape is the measurement of the SM nonlinear susceptibility, which can surpass standard linear methods, like static and dynamic light scattering. Indeed nonlinear excitations provide information on those effects relying on the landscape topology, being the linear response only limited to small perturbations on local minima. The knowledge of the nonlinear optical susceptibility is of paramount importance for self-organization mechanisms: it measures the feedback effect between the driving field and the material.

Our running experiments show that the SM nonlinear optical susceptibility “ages”, at variance with the linear response; the aging process is the direct indication of a topologically structured landscape (see our recent work arXiv:cond-mat/0609659v1). The quantitative assessment of the nonlinear properties of most of the modern soft-materials is practically inexistent. Understanding the kind of nonlinear processes, their strength and their temporal dynamics in materials like, e.g., colloidal gels, liposomes, and block copolymers has a variety of applications ranging from the all-optical control of material synthesis or rheological properties, to novel diagnostic techniques for, e.g., monitoring of the blood coagulation process.

 Our methodology:

Dynamic and static light scattering and rheological measurements in out-of-equilibrium colloidal solutions are performed and the results compared with a computer assisted Z-scan measure of the nonlinear susceptibility. This will put into relation the SM structure, its dynamics and the nonlinear optical response.

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