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I thought that two-photon absorption is the underlying mechanism of upconversion, but according to Wikipedia (1, 2),

Upconversion should be distinguished from two-photon absorption [...]

and

Two-photon absorption (TPA) is the simultaneous absorption of two photons of identical or different frequencies in order to excite a molecule from one state (usually the ground state) to a higher energy electronic state.

So what's exactly the difference?

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These terms are the beginning and the end of the process that you're thinking about. Lets look at examples where they don't occur together, to see the distinction.

  1. A photon excites an electron from A to B. Later, another excites the electron from B to C. If the electron then drops from C to A, the resulting photon is higher energy than either incident photon. This is upconversion, but not TPA, because the photons did not arrive at the same time (and state B is a real state).

  2. Two low energy photons excite an electron from A to B. The electron doesn't drop down, but instead a voltage is used to ionize it (from B to infinity). The ionization is easier because of the TPA that happened. Upconversion is not happening here.

I hope this makes the distinction clear. Upconversion is only the emission of a higher frequency photon, and TPA is only the simultaneous absorption of two photons.

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