  We present few-femtosecond shadowgraphic snapshots taken during the
non-linear evolution of the plasma wave in a laser wakefield accelerator with
transverse synchronized few-cycle probe pulses. These snapshots can be directly
associated with the electron density distribution within the plasma wave and
give quantitative information about its size and shape. Our results show that
self-injection of electrons into the first plasma wave period is induced by a
lengthening of the first plasma period. Three dimensional particle in cell
simulations support our observations.
