Tracking ultrashort pulses through dispersive media: experiment and theory

We report on the direct visualization of a femtosecond pulse propagating through a dispersive waveguide at a telecom wavelength. The position of a propagating pulse is pinpointed at a particular point in space and time using a scanning probe based measurement. The actual propagation of the pulse is visualized by changing the reference time. Our phase-sensitive and time-resolved measurement provides local information on all properties of the light pulse as it propagates, in particular its phase and group velocity. Here, we show that the group velocity dispersion can be retrieved from our measurement by developing an analytical model for the measurements performed with a time-resolved photon scanning tunneling microscope. As a result, interesting and useful effects, such as pulse compression, pulse spreading, and pulse reshaping, become accessible in the local measurement.