What are dark excitons and how to find them? I am reading about excitons and I encountered a few times the term "dark exciton" but I have a hard time finding a good definition. I tried to google it but I only find scientific articles where the term is used but not really introduced.
Is the dark exciton just an exciton that does not emit a photon? If so, why do some excitons emit light and other not (what are the conditions that decide what happens)? And also - how can we observe a dark exciton if it doesn't emit a photon that we could measure?
 A: Dark excitonic states or dark excitons describe excitonic transitions that are optically 'dark', which means that they can neither be probed by either photon-absorption or photon-emission directly. A dark exciton sate could be thought of in terms of the band structure as the interaction of an electron and a hole located at distinct momenta $\mathbf{k}$. In order to create or anneal a dark exciton one needs to not only transfer energy to one of the constituents, (which mostly comes from the photon) but also transfer momentum to either the electron or the hole (which comes from either phonon absorption or phonon emission). Dark excitons can therefore actually be thought of as three-particle complexes consisting of an electron, a hole and a phonon.
As experiments are conducted at finite temperatures there will always be a phonon which makes the dark excitons visible (in for example an absorption or photoluminescence spectrum). Nevertheless, three-body interactions are quite unlikely to occur compared to bright excitons, which is why the observed dark exciton peaks are mostly quite small.
Sometimes people call spin-forbidden optical transitions also dark excitons or spin-dark excitons.
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