[PatrickHenry]

... now, if it is possible to 'slow down' light and have that energy captured as a fuel for use in an engine ...

I don't think I fully understand you. Anyway, there's not all that much "fuel for use in an engine" to worry about. For an idea of what light might be useful for, see: Solar sails.

[Pistolshot]

In this dense medium, Schneider explained, light is slowed to three-fourths of its speed in a vacuum. In a nuclear reactor, charged particles flying off the radioactive rods through the water they are submerged in exceed this reduced speed. Because these particles contain an electric charge, they emit energy, called Cherenkov radiation. Any particles they bump into become radioactive, giving the water a characteristic blue glow.

I was referring to this explanation of how light moves in a dense medium. If there is energy being released in any kind of form, it is possible to use as fuel or an energy supply to convert to fuel. The Solar Sail is an idea, but what if you could use this extra 'energy' to help push the craft?