Yellow stimulated emission from Dy$^{3+}$-doped silica glass microspheres
Dy$^{3+}$-doped silica glass whispering gallery mode microspheres are fabricated by fiber fusion splicing. They present an almost ideal spherical morphology with a radius ranging from 60 to 67 $μ$m as determined by confocal laser microscopy. The host composition of the microsphere is close to that of the fiber core. The dopant Dy$^{3+}$ ions are uniformly distributed across the microsphere as evidenced by $μ$-luminescence studies and present a luminescence lifetime. The Dy$^{3+}$-doped glass microspheres were excited via evanescent field coupling using a half-tapered fiber and a blue 450-nm GaN laser diode (direct pumping scheme). The yellow fluorescence of Dy$^{3+}$ ions is filtered by the whispering gallery modes (free spectral range: 0.5 nm for 67-$μ$m radius microsphere). The onset of stimulated emission is further observed highlighting the potential of such microresonators for narrow-linewidth light sources directly emitting visible light.