Two-photon Emission
The opposite process of TPA is two-photon emission (TPE), which is a single electron transition accompanied by the emission of a photon pair. The energy of each individual photon of the pair is not determined, while the pair as a whole conserves the transition energy. The spectrum of TPE is therefore very broad and continuous. TPE is important for applications in astrophysics, contributing to the continuum radiation from planetary nebulae (theoretically predicted for them in ref] and observed in ref]). TPE in condensed matter and specifically in semiconductors was only recently observed, with emission rates nearly 5 orders of magnitude weaker than one-photon spontaneous emission, with potential applications in quantum information.
Read more about this topic: Two-photon Absorption
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