Taking this result, we have stated the mechanism of UC emission of Pr3+ sensitized by Yb3+ with the help of schematic energy level diagram in Figure 2f. When pumped by 980 nm CW laser, the Yb3+ can strongly absorb the incident 980 nm photon in its ground state (2F7/2) and be promoted to excited state (2F5/2). And then Yb3+ emitting broad band resonant phonons help the electrons in the Pr3+: 3H4 level to be excited to the 1G4 and 3P0 level by ground state absorption (GSA) and excited state absorption (ESA).