How far into the infrared can a colour centre in diamond emit?

Infrared emission has been observed for the nitrogen-vacancy (NV ^-) colour centre in diamond, in addition to the characteristic red emission of this centre [1]. However, this infrared emission zero-phonon line at 1046 nm is about four orders of magnitude weaker than the red emission. This is somewhat surprising, as a third of the population is known to decay without the red emission and the infrared emission is considered to be associated with the alternative decay path. The most obvious explanation is a competing efficient non-radiative decay. There are few reports of diamond emitting at wavelengths longer than 1000 nm. Colour centres in diamond have strong electron-phonon coupling and, of course, the phonon energies in diamond are high. These properties suggest that non-radiative decay could dominate the transition whenever the electronic energy lies in the infrared. This would be a general phenomena and would account for the weak IR emission observed for this centre. It would also account for why there are so few reports of emission from diamond in the infrared.