Up-Conversion and Self-Stimulated Raman Laser in a Codoped Microsphere

In this letter, Yb³⁺-Eu³⁺ codoped sol-gel films are applied to the surface of silica microspheres. Under the excitation with 976 nm laser, we observe the luminescence of Eu³⁺ with relatively low pump power. The Yb³⁺ → Eu³⁺ energy transfer process and the pump power dependence are studied. Because of high energy density of the Yb³⁺-Eu³⁺ codoped microsphere (YECM), the threshold power for the downconversion (Yb³⁺:²F_5/2 → ²F_7/2) laser is as low as 8.99 μW. In addition, a self-stimulated cascaded Raman laser (SSCRL) is observed, which is stimulated by the down-conversion laser of the YECM.