Amplification and diffusion of spontaneous emission in strongly scatteringmedium

We experimentally examined the threshold of the spectral collapse in dye em bedded in a strongly scattering medium as a function of the excitation beam diameter and the transport mean free path in order to find a condition to minimize the threshold. We found a critical transport mean free path, below which the threshold pulse energy is almost independent of the mean free pa th. Experimental observations are well explained by a theoretical model bas ed on the rate and diffusion equations, which shows that the luminescence s pectra are also dependent on the position in the medium. (C) 2000 American Institute of Physics.[S0021-8979(00)05111-2].