Stimulated emission in scattering and composite dielectric media (random lasers): effect of particle size (Key Lecture)

Stimulated emission can be obtained in small volumes of scattering laser materials without cavity or any special optical design. Such sources of stimulated emission are known as random lasers. In random lasers, amplifying laser medium provides for gain, and scatterers (powder particles, air gaps between particles, etc.) provide for stimulated emission feedback. Above certain threshold pumping energy, the emission characteristics of random lasers change dramatically: the emission spectrum collapses to one or several narrow lines and one or several short emission pulses appear in response to a relatively long pumping pulse. Solid-state random lasers based on rare-earth doped dielectrics, dielectrics with color centers, semiconductors, scattering polymers, etc., offer challenging and not yet completely understood physics as well as promising applications, including express testing of laser materials, identification, and information processing. The focus of our presentation is on optically pumped neodymium random lasers. In particular, we discuss the dependence of the photon mean free path lt, the threshold energy density E th /S and the slope efficiency in neodymium random lasers as a function of the mean particles size s. The experimental results are compared with the predictions of the developed models.