Band narrowing and gain spectra of laser dye solutions with scattering TiO2 nanoparticles

The spectra of amplified spontaneous emission (ASE) intensity and gain have been measured for solutions of laser dye Rhodamine-640 in glycerin with light scattering TiO2 nanoparticles. Such solutions typically manifest random lasing in backscattering geometry. In this work, the conventional transverse pump geometry is used, and the ASE gain spectra are measured using a variation of the length of the illuminated (pumped) stripe. In this geometry, with increasing concentration of nanoparticles, the "unidirectional" gain along the stripe decreases dramatically. At the same time, strong narrowing of the ASE bandwidth is observed even in the absence of net gain. The experimental results help one to understand the role of the scattering particles in the light amplification. On the one hand, the particles increase a zigzag type of the light path within the amplifying medium, which results in bandwidth narrowing and strong amplification of chaotic light beams. On the other hand, the amplification of a seed beam propagating in a selected direction is dramatically suppressed. The results are discussed using simple modeling of ASE amplification and qualitative arguments.