A recent discovery at MIT further supports the idea that photons may be dielectric, and capable of a phase shift into massive matter.
Researchers found that photons can stick together into pairs and triplets when passed through a cloud of ultracold rubidium atoms.
The structures formed had mass, and were therefore slowed down a lot. They moved at a speed 100,000 times slower than ordinary light.
The light appears to have gone through a phase shift similar to that observed for electron-positron pair production. However, in this case, we got the additional effect of photons latching onto each other to create structure.
Light was, as it were, condensed into matter.
This goes a long way towards proving that photons are dielectric. After all, it is in the nature of dielectric matter to interact and form structures.