What slows light down the most?
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That's equivalent to asking what material has the highest index of refraction, n. n = c/v where c is the speed of light in a vacuum and v is the speed of light in the substance. n for water is 1.33. Typical glass is about 1.5. Sapphire is about 1.7. Diamond is about 2.4. Silicon is around 3.4 (for infrared light - silicon is opaque in the visible.) While higher than the previous materials they are by no means the highest, contrary to the previous answer: Rutile has two indices, 2.6 and 2.9, depending on polarization. Germanium is about 4 (infrared only.) Recently work has been done on slow light - look it up. They've used supercooled rubidium gas to slow light to a fast walk!
A diamond and silicon slow down light the most. For more information, check out this page:
