Superradiant guided modes in flashlamp pumped capillary dye lasers

Superradiant self-guiding emission from capillary dye lasers is reported. Capillary cells with very small inner diameter (50–200 μm) and standard large bore (2–6 mm) flashlamps have been used. The heat released by the pumping pulse to the active medium (Rhodamine 6G in ethanol) produces a lens-like distribution of the refractive index, which in turn gives rise to low-loss guided propagation in the cell. Near-field and far-field patterns turn out to be almost uniform at low dye concentration and become markedly ring-shaped at high dye concentration, denoting the presence of ‘whispering gallery’ modes. A theoretical analysis of the mode propagation in lens-like active media is carried out, and the field intensity distribution of Gaussian modes are evaluated for some cases of interest. The mode selection mechanism which determines the excitation of the modes is discussed.