The particular form of the refractive index was sort of interesting - for some frequencies, it was less than 1. That implied that the phase velocity of a sine wave would be faster than the speed of light. Here's the group velocity as a function of frequency for the refractive index I posted last time (c = 300 nm/fs):Away from that little wiggly feature in the refractive index, the group velocity is a nice and sedate sub-300 figure. What it boils down to is the fact that a material doesn't have separate refractive index curves and absorption curves. Systems which behave in a causal way produce dispersion curves that satisfy the Kramers-Kronig relation.