Synchronization of ultrafast pulses and pulse front tilt removal inside samples

Ultrafast imaging requires accurate synchronization of pump and probe pulses, as well as the removal of any pulse front tilt of the probe pulse. Indeed, any pulse front tilt is highly detrimental to the accuracy of the imaging technique. In addition, the "zero delay" between pump and probe is only true for a specific propagation distance. This is because along the path of pump and probe beams, the dispersion strongly modifies the retardance between the pulses. As an example, for pump and probe beams in a collinear configuration, the propagation through 100 μ m thick glass slide is enough to induce a shift by more than 37 fs between 800 nm and 400 nm pulses because of the difference in terms of refractive index. However, it is extremely difficult to characterize weak probe pulses.