  We introduce the concept of an indirect photonic transition and demonstrate
its use in a dynamic delay line to alter the group velocity of an optical
pulse. Operating on an ultrafast time scale, we show continuously tuneable
delays of up to 20 ps, using a slow light photonic crystal waveguide only 300
$\mu$m in length. Our approach is flexible, in that individual pulses in a
pulse stream can be controlled independently, which we demonstrate by operating
on pulses separated by just 30 ps. The two-step indirect transition is
demonstrated here with a 30% conversion efficiency.

Optics