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 TiO<sub>2</sub> 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<i>Ɩ</i>. 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
(140.2050) Lasers and laser optics : Dye lasers
(140.3380) Lasers and laser optics : Laser materials
(140.3410) Lasers and laser optics : Laser resonators
(290.4210) Scattering : Multiple scattering
Gijs van Soest, Makoto Tomita, and Ad Lagendijk, "Amplifying volume in scattering media," Opt. Lett. 24, 306-308 (1999)