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

In this letter, Yb
³⁺
-Eu
³⁺
codoped sol-gel films are applied to the surface of silica microspheres. Under the excitation with 976 nm laser, we observe the luminescence of Eu
³⁺
with relatively low pump power. The Yb
³⁺
→ Eu
³⁺
energy transfer process and the pump power dependence are studied. Because of high energy density of the Yb
³⁺
-Eu
³⁺
codoped microsphere (YECM), the threshold power for the downconversion (Yb
³⁺
:
²
F
_5/2
→
²
F
_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.