Role of femtosecond prestructure of intense harmonic pulses in relativistic laser-solid interactions

Extreme intensity contrast is essential for ultraintense, femtosecond laser excitation of solids, in particular for studies with structured or ultrathin targets. In order to maximize the contrast in the nanosecond and picosecond range, a very effective approach is the use of second-harmonic generation. In this Letter, we use intense harmonic 400-nm pulses to explore experimentally the fundamental interactions of these pulses with flat and <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mi>sub</a:mi><a:mtext>−</a:mtext><a:mi>λ</a:mi></a:mrow></a:math> grating targets. We show that the presence of a femtosecond-scale structure in the rising edge of the harmonic laser pulse critically modifies the laser-solid interaction. It alters the local field structures and hence induces redirection in the electron trajectories and distributions. We show that it is of particular importance concerning resonant phenomena such as surface plasmon excitation in structured targets. Particle-in-cell 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.