An ultralow-threshold microlaser

In our experiment Nd³⁺ ions are pumped with a diode laser at 807nm using a resonant WG mode. The coupling to this mode is achieved via frustrated total internal reflection using a high index prism. The emission from the ions is then coupled out via the Same prism and is diagnosed using a spectrometer and a Fabry-Perot. The figure shows a lasing threshold of about 200nW in a sphere with a diameter of 56μm, a doping level of about O.l%, and at a cold cavity Q of 2x10⁸. Furthermore, the evanescent wave coupling allows us to investigate the microlaser action over a cold cavity Q range of more than one order of magnitude by varying the sphere-prism distance. We discuss future experiments with lower doping levels and at cryogenic temperatures aimed at the reduction of the homogeneous linewidth. Such an approach could lead to the laser action from a single ion permanently trapped in a solid medium.