There are two pictures that you could take of the situation. Classically, where the medium is smooth continuous stuff with a permeability and a permittivity, th

There are two pictures that you could take of the situation. Classically, where the medium is smooth continuous stuff with a permeability and a permittivity, there are no photons, only waves. Quantum mechanically, where the medium is made up of lots of little particles, there are photons indeed, but no smooth medium, and no region where the speed is less than C (because there is no "region," only lots of separate particles). So there's one picture missing "photons," and the other missing "speed less than c," so you can't capture "photons in a region where the speed is less than c," in a single image. I guess if you wanted to pose that question in a framework where a medium where the speed was less than c, and photons, could exist simultaneously, you would want an effective field theory (maybe). But you will have to get someone else who knows more than me to explain that. :-)