Narrow-band random Raman lasing from Rhodamine 6G assisted by cascaded stimulated Raman scattering effect.

This study reports the first experimental observation of cascaded stimulated Raman scattering (SRS) generation in a colloidal disordered medium. Generation of the cascaded effect requires both a high Raman gain and pump power in the disordered medium. Here, to extend effective path lengths of photons into the Raman gain medium for producing additional SRS processes, ZnO microspheres with abundant nano-protrusions as suitable scattering centers are proposed. It is explained that nano-protrusions on the surface of the spheres can act as nano reflectors and significantly provide potent feedback in the disordered system. This provided feedback via nano-protrusions boosts cascaded SRS generation to allow the appearance of higher Raman signals of Rhodamine 6G dye solution at a low scatterer concentration of 5 mg/ml. The threshold for the formation of the first Raman signal is measured at about 60 mJ/pulse. Also, the evolution of Raman signals under several fixed pump pulses is examined to investigate the stability from pulse to pulse. Our findings provide promising perspectives for achieving the single-frequency laser sources and generate desirable wavelengths for specific applications.