Demonstration of Yb³⁺-doped and Er³⁺/Yb³⁺-codoped on-chip microsphere lasers

Rare-earth-doped on-chip microlasers are of great significance in both fundamental research and engineering. To the best of our knowledge, this is the first report of Yb³⁺-doped and Er³⁺/Yb³⁺-codoped on-chip microsphere lasers fabricated via sol-gel synthesis. Laser emissions were observed in a band around 1040 nm in both Yb³⁺-doped and Er³⁺/Yb³⁺-codoped resonators pumped at 980 nm and had measured ultralow thresholds of 5.2 µW and 0.6 µW, respectively. Both single-mode and multi-mode emissions were recorded around 1040 nm in these lasers. Single-mode and two-mode emissions were obtained at 1550 nm in the Er³⁺/Yb³⁺-codoped lasers when pumped at 980 nm and 1460 nm, respectively. Furthermore, quality factors induced by different loss mechanisms in the microsphere lasers are theoretically estimated. These resonators are expected to contribute to the high-density integration of on-chip silica-based microlasers.