Observation of slow-light dynamics in coupled periodic waveguides

The physics and applications of slow light are attracting increasing attention in recent years. Dramatic reduction of the pulse velocity by orders of magnitude was demonstrated experimentally in photonic structures with periodic modulation of refractive index. The benefits of slow-light regime include potential applications for optical delay lines and enhanced light-mater interactions. Whereas the key principles of achieving the individual slow-light states have been established, new concepts for light manipulation in photonic structures can be explored based on interactions between multiple optical waves if their velocities are reduced simultaneously. This work demonstrates theoretically and observe experimentally an efficient realization of slow-light regime for several optical modes in the same frequency region. The approach is based on coupled periodic waveguides, where two co-propagating modes exist with distinct dispersion characteristics. As the frequency is tuned close to the band-edge, the estimated group velocity is reduced below c/30. Slow-light propagates along a sinusoidal trajectory, exhibiting periodic tunneling between the waveguides. This happens due to the beating of two distinct Bloch modes which group velocities are reduced simultaneously, yet the phase velocities remain different.