Fig. 4.

Change of time zero with spatial position. (A) Without tilted pulses, time zero (■) changes by the dispersion of 10 ps/mm, reducing the overall time resolution by the range indicated (25 ps for a 2-mm sample). The solid line is a fit including data points outside the displayed range. (B) With tilted pulses, time zero does not change significantly across the 2-mm range. The difference between A and B in position range is due to the footprint used. The dotted lines give a better than 1-ps velocity match, and <50 fs is the limit (see text).