Stopped light? How? Why? Where does the energy go?

Recently physicists reduced the speed of light to 0 in, I think, a cloud of supercooled sodium atoms. They then ‘restarted’ the light, and brought its velocity back to a somewhat more reasonable level. Now, maybe I’m misunderstanding something here, but I was under the impression that light is pure energy. It has no mass, although it does have momentum. If this is so, then what is left when you reduce the speed, and therefore the kinetic energy of a photon to zero? Is the kinetic energy converted to some other kind of energy? The difference between a high energy photon and a low energy photon is, as I understand it, the frequency. So perhaps a stopped photon is a static pulse of energy, still vibrating at its original frequency. How do different mediums slow down light, anyway? And where does all that kinetic energy go?