Blue upconversion laser based on thulium-doped silica m icrocavity

Whispering gallery mode optical resonant cavities fabricated from rare-earth-doped silica glasses have demonstrated lasing from the visible through the near-IR. However, achieving lasing in the blue has been elusive. In this Letter, thulium-doped silica films are synthesized and used to fabricate toroidal optical microcavities with quality factors in excess of 10 million. Despite the high phonon energy of silica, the high circulating optical intensities present in the microcavities enable upconversion of the thulium, resulting in emission in the blue and near-IR with microwatt threshold powers that scale linearly with the concentration of the thulium.