Recently I read about something called photon up-conversion, in which a lower frequency of light is absorbed by a material that then proceeds to emit a higher frequency of light. When the mechanism for up-conversion is two-photon absorption, a molecule absorbs two lower-frequency photons, emits a higher-frequency photon, and then returns to it's initial state.

Why doesn't this violate the second law of thermodynamics?

Recently I read about something called photon up-conversion, in which a lower frequency of light is absorbed by a material that then proceeds to emit a higher frequency of light. When the mechanism for up-conversion is two-photon absorption, a molecule absorbs two lower-frequency photons, emits a higher-frequency photon, and then returns to it's initial state.

How would the change in entropy be calculated in a situation like this?