It is mentioned in this article It seems to be a Bose-Einstein condensate of some kind, but it is not exactly clear how one can create a BEC with just photons
Light consists of tiny indivisible portions, the photons. Under certain conditions, they, too, can condense, if they are cooled enough. Many thousands of these light packets then suddenly fuse into a kind of super-photon with unusual characteristics – a so-called Bose-Einstein condensate.
How do you "cool light"???
Read more at: http://phys.org/news/2016-04-capacity-condensed.html#jCp