Superluminal group velocities in a bulk two-dimensional photonic band gap crystal

The superluminal propagation of wave packets with faster-than-c, infinite, and negative group velocities, has been observed in a wide range of physical systems, including both passive and active optical media, we have experimentally demonstrated that superluminal (including infinite and even negative) group velocities can exist for analytic signals whose spectral bandwidth lies within the band gap of a two-dimensional hexagonal photonic crystal. We believe that our result calls for a generalization of the Kramers-Kronig relations including not only absorption but also other possible "dissipation" channels.