Fig. 5: Upconversion lasing with Tm3+.

a Upconversion luminescence and upconversion lasing in an LQUM doped with Yb3+ and Tm3+ (20 and 2%). The color inset shows an optical microscopy image of an illuminated LQUM. b Emission spectrum in the blue emission band around the lasing threshold of 1.5 kW cm−2. c A SEM image of an LQUM measuring 1.99 μm in diameter doped with Yb3+ and Tm3+.