Conventionally, researchers study pulse propagation by coupling pulses into a medium and deducing what goes on inside from the radiation that is transmitted or reflected. Now a team from the University of Twente in Enchede, the Netherlands, has found a way to track the group and phase velocities of a femtosecond laser pulse inside a photonic structure using a heterodyne-detection, phase-sensitive photon scanning tunneling microscope.The researchers used a frequency-doubled, Ti:sapphire-pumped optical parametric oscillator to produce a 300-fs-long pulse inside a silicon nitride planar-channel waveguide. They captured snapshots of the pulse's position while moving the microscope's reference point along the medium with it. They then compiled the snapshots to form a full picture of what was happening to the pulse in the waveguide.The team expects this methodology to lend more insight into the dynamic behaviors of photonic crystals and integrated optical circuits. It reported its results in the Nov. 2 issue of Science.