Measurement of the gain volume in amplifying and scattering media by a two-beam spatial-correlation method.

We measured the size of the gain volume at the threshold of the spectral collapse in a solution of kiton red with colloidal suspensions of polystyrene spheres using a two-beam spatial-correlation method. In this method, the luminescence spectrum was measured as a function of the spatial separation of two excitation beams. It was found that the size of the gain volume at the threshold L(th), is proportional to the absorption length L(a), for different samples. Numerical simulations are also presented and the results are discussed on the basis of a relation L(th)=sqrt[3G(th)]sqrt[sigma(a)/sigma(s)]L(a), where sigma(s) and sigma(a) are stimulated emission and absorption cross sections of kiton red, respectively.