Abstract

We investigate the influence of the excitation spot diameter on the laser threshold of a scattering amplifying medium. Fluorescence spectra are recorded from a suspension of ${\mathrm{TiO}}_2$ scatterers in Sulforhodamine B dye. The threshold pump intensity becomes larger by a factor of 70 if the excitation beam diameter gets close to the mean free path $l$. This increase is explained by use of a simple model describing diffusion out of the amplifying volume and is confirmed by a Monte Carlo simulation.

© 1999 Optical Society of America

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