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 = root 3G(th)root sigma (a)/sigma L-s(a), where sigma (s) and sigma (a) are stimulated emission and absorption cross sections of kiton red, respectively.