Basic Principles of Upconversion

Another or the same Yb is excited (during the Ufetime of the excited Er ) 

This excited Yb ion interacts with the excited Er and, again, a non-radiative energy 

83 2 states, the latter two being in thermal equilibrium at room 

The the states decay radiatively to the ground state by the emission of 

There is also another pathway to arrive at the state that goes via the excited state of Er, generated through relaxation from the j j 2 state, through a non-resonant non-radialive energy transfer step by interaction with a nearby excited Yb ion. Which process is dominant is often a subtle result of several effects, such as crystal field splitting, and so on, and would require detailed (time-resolved and temperature-dependent) photoluminescence studies to unravel. Further discussion is beyond the scope of this review. 

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