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I have read this:

https://en.wikipedia.org/wiki/Photon_upconversion

And it says that upconversion is a process where subsequential absorption of two or more photons leads to emission of light with shorter wavelength then the excitation wavelength.

Two photon absorption is an absorption of two photons of identical or different frequencies, in order to excite a molecule.

Upconversion should be distinguished from two-photon absorption and second-harmonic generation. These two physical processes have a similar outcome to photon upconversion (emission of photons of shorter wavelength than the excitation) but the mechanism behind is different.

But it does not give an explanation why the two differ.

Question:

  1. What is the difference (the different mechanism) between upconversion and two-photon absorption?
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