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 $L_{\mathrm{th}},$ 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 $L_{\mathrm{th}}=\sqrt{{3G}_{\mathrm{th}}}\sqrt{{\sigma }_a/{\sigma }_s}{L}_a,$ where ${\sigma }_s$ and ${\sigma }_a$ are stimulated emission and absorption cross sections of kiton red, respectively.