as you know, higher excitation density is needed for upconversion emission. when it lower than threhold value, no emission is obtained. different materials have different threhold value. why?
If i understand your question correctly, the threshold value is necessary to establish a certain population in an intermediate state.
Energy migration in upconversion materials can be quite complicated with different paths depending on the energy levels of the emitter and of the sensitizer (when such an help is used). The corresponding kinetics governs the upconversion process and since different materials will have different electronic levels, one may understand why the threshold energy density is different.
Dear Jean-Claude G Bünzli thank you for your reply.
That upconversion emisson need higher excitation density is hampering its applying in biology. the lower excitation density with stronger emission is needed. our data showed that increasing teh concentration of the center of luminescence Er3+ can improve the emission intensity with the lower excitation density. but differnt hosts have different threshold value of excitation density. for examples La2S3
May the paper attached is helpful.
Huang, S., S.T. Lai, L. Lou, W. Jia and WM Yen. "Upconversion in LaF3: Tm^{3+}." Physical Review B 24, no. 1 (1981): 59.
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