Phys. Rev. Lett. 108, 183903 (2012)

The use of slow-light in periodic structures is an appealing technique for enhancing inherently weak light–matter interactions. However, Jure Grgić and co-workers in Denmark have now raised concerns regarding the magnitude of the anticipated enhancement. The researchers numerically analysed the modification to the dispersion relation in the gain media of periodic structures such as coupled-resonator optical waveguides, Bragg stacks and photonic crystal waveguides. They found that the photonic density of states is not affected regardless of whether the net gain factor is positive (gain) or negative (loss). They also discovered that when the gain is increased, the group index decreases to a constant value at the Brillouin zone edge. Considering all these factors together, the researchers conclude that the slow-light enhancement is considerable for a low gain factor but may not be present in all cases. For quantum-dot-based materials, whose gain factor is in the range of 10–45 cm−1, a slow-light enhancement factor of around 60–130 is anticipated.