In extra ops 49 (perfect stealth), i think there's a POW you have to save to get metamaterial skill

One possibility is that the medium the light is supposed to refract from is a metamaterial, an artificially created material with a negative refractive index.

No, as a practical thing. Yes, look up "metamaterials". Metamaterials can produce a limited form of invisibility around a very small area. However to "someone" made invisible by being inside a metamaterial "cloak" to them everything else is invisible. Metamaterials have been built, tested, and shown to operate for electromagnetic frequencies up to microwave and for sound and other mechanical waves. One major problem that prevents metamaterials from being a practical invisibility cloak is that they are extremely frequency sensitive (i.e. shift the frequency of the radiation only a small amount and the metamaterial loses allof its "meta" properties and what is inside is visible).

A frequency selective surface is a structure or material that selectively transmits, reflects, or absorbs electromagnetic waves based on their frequency. These surfaces are often used in antennas and radar systems to control wave propagation. By manipulating the geometry and materials of the surface, specific frequency bands can be isolated or blocked.