Tracking femtosecond laser pulses in space and time

We show that the propagation of a femtosecond laser pulse inside a photonic structure can be directly visualized and tracked as it propagates using a time-resolved photon scanning tunneling microscope. From the time-dependent and phase-sensitive measurements, both the group velocity and the phase velocity are unambiguously and simultaneously determined. It is expected that this technique wilt find applications in the investigation of the local dynamic behavior of photonic crystals and integrated optical circuits.