Velocity of light Velocity of light decreases as it enters a more optically denser medium. So how far can the velocity of light be reduced? Which object has the highest refractive index?
 A: You can store light up to one minute so far. Basically you make a crystal transparent (low OD) at a predefined desired wavelength. When the light pulse goes in, you turn the crystal opaque (high OD). You retrieve the pulse by making it transparent again at the right time.
The material is some Pr-doped crystal. For this purpose, it is hard to find a material with a tunable operating wavelength.
Note that, in general, the refractive index might depend strongly on the wavelength. So, when you ask for the deceleration of light in a medium, a wavelength is implicitly assumed from the context. Broadband light will easily get dispersed, as, by today, I only know of objects with a roughly flat refractive index in the visible spectrum to have low OD.
A: Index of refraction depends on frequency of the light. Indices can be very high in the far infrared. CO2 lasers operate at a wavelength of 10.6 um. This is about 20 times longer than visible light. The highest index I know of is GE, n = 4. See this for more. 
It is possible to "stop" light in a Pr-doped crystal. But this really sets the state of the crystal to match the state of light at a particular instant, and then later emits light in the same state. 
Also see this. What really cause light/photons to appear slower in media?
