Lanthanide-doped nanocrystals in high-Q microtoroids for stable on-chip white-light lasers

The plentiful energy states of lanthanide (Ln³⁺)-doped nanomaterials make them very promising for on-chip integrated white-light lasers. Despite the rapid progresses, the Ln³⁺-based white upconversion emissions are strongly restricted by their low upconversion quantum efficiency and the color stability. Herein, we combine the CaF₂:Yb₃₅Tm_1.5Er_0.5 nanocrystals and the high-Q microtoroids, and experimentally demonstrate the chip-integrated stable white-light laser. By optimizing the sizes, density, and distributions of Ln³⁺-doped nanocrystals, the Q factors of Ln³⁺-doped microtoroids are maintained as high as 5 × 10⁵. The strong light matter interaction in high-Q microtoroids greatly enhances the upconversion emission and dramatically reduces the laser thresholds at 652 nm, 545 nm, and 475 nm to similarly low values (1.89–2.10 mJ cm⁻²). Consequently, robust white-light microlaser has been experimentally achieved from a single microtoroid. This research has paved a solid step toward the chip-scale integrated broadband microlasers.