Generalization of the thermal lens model formalism to account for thermodiffusion in a single-beam Z-scan experiment: determination of the Soret coefficient

A generalization of the thermal lens model formalism is proposed to introduce and characterize the thermodiffusion phenomenon. The theory treats the case in which local heating generates a temperature gradient in a single-phase binary sample (a colloid, for instance) that yields, through thermodiffusion, concentration gradients in an initially homogeneous sample. The treatment generalizes the concept of a thermal lens to a material lens as a result of the coupling of a concentration variation with the optical properties of the medium. This formalism permits the use of the Z-scan technique to determine the Soret coefficient of samples. Applying this theory to the results of a Z-scan experiment with an ionic ferrofluid sample gives values that agree with those obtained from forced Rayleigh scattering measurements of the same material.