  Extreme intensity contrast is considered essential for ultraintense,
femtosecond laser excitation of solid targets, in particular for studies with
structured or ultra-thin targets. Second-harmonic generation has been used to
maximize the contrast in the nanosecond and picosecond timescales but the
resulting pulses can have intense broad femtosecond structures in the rising
edge of the pulse. We show that femtosecond scale structures that arise in this
process critically modify the interaction, by altering the local field
structures and hence redirecting the electron trajectories and distributions,
especially concerning resonant phenomena such as surface plasmon excitation in
structured targets. Particle-in-cell (PIC) simulations fully support and give
further insight into our experimental results. Our findings have important
implications not only for the use of harmonic pulses on solid targets but also
for two-color schemes based on second harmonic pulses.
