Phys. Rev. Lett. 108, 248002 (2012 )

Unlike most other lasers that use mirrors to confine light into the active region of the laser, random lasers trap light through disorder, where the light is scattered by the random structures of the laser medium. Thus, stimulated emission can occur as in any other laser. Viola Folli and colleagues have now studied how a random laser made with small metal spheres is influenced when these are shaken. The metal spheres are immersed in an organic dye solution that is the active laser medium. When this set-up is excited by an external pump light, the random laser emits light at 600 nm wavelength. If the spheres are shaken at this point, a weaker second laser peak at 620 nm appears for some single camera shots. This suggests a favourable arrangement of a number of the spheres that for brief moments can support a second lasing mode. Further study of this effect promises insight into the dynamics of granular matter and random lasers.