Experimental demonstration of "fast light" in an absorptionless, non-reflective system using the birefringence of a photonic crystal

This study investigates the first experimental observation of faster-than-c group velocities in a passive system with neither absorption nor reflection. The investigation shows that superluminal and even negative group velocities can exist because of interference, instead of absorption or reflection. Results suggest that causal superluminal propagation can potentially be observed in any system in which input energy can escape into other modes. These results also demonstrate a particularly unusual application of the amplitude-phase Kramers-Kronig (K-K) relations; for the system being described, an infinitesimal adjustment of parameters radically affects the validity of the K-K transformation between amplitude and phase.