Real-Space Observation of Ultraslow Light in Photonic Crystal Waveguides

We show the real-space observation of fast and slow pulses propagating inside a photonic crystal waveguide by time-resolved near-field scanning optical microscopy. Local phase and group velocities of modes are measured. For a specific optical frequency we observe a localized pattern associated with a flat band in the dispersion diagram. During at least $3\mathrm{p}\mathrm{s}$, movement of this field is hardly discernible: its group velocity would be at most $c/1000$. The huge trapping times without the use of a cavity reveal new perspectives for dispersion and time control within photonic crystals.