Fig. 1.

Dispersion of an ultrashort electron packet. At t = t_o, the packet is created from a photocathode and travels with a velocity v₀. As it propagates along the x axis it disperses, with the faster electrons traveling further, and the slower ones trailing for a given propagation time t. At t = 0 a parabolic potential is pulsed on, giving an impulsive “kick” to the dispersed electron packet. After the potential is turned off, t > τ, the trailing electrons now have a greater velocity than the leading electrons. After a propagation time t = t_i, the pulse is fully compressed.