Effect of Ultrahigh Contrast on Absorption and Surface Plasma Wave Excitation in Relativistic Laser-Plasma Interactions

Intense, femtosecond laser pulses with ultrahigh contrast, provide the advantage of "instantaneous" excitation of solid density matter as the solid surface becomes an overdense plasma with a sharp density gradient. This facilitates the generation of homogenous high-energy-density matter, laser-driven particle acceleration in ultrathin foil targets and efficient interactions with various structured targets where it is important for the structures to survive until the femtosecond peak of the laser pulse reaches the target. However, the absorption mechanisms of such temporally clean pulses with solids are not well explored, especially in experiments.

 

We have produced ultrahigh contrast (better than 10^-12), relativistic laser pulses, using efficient second-harmonic generation in an LBO crystal and characterized them using self-diffraction frequency-resolved optical gating (SD-FROG). Here, we report absorption measurements of 400 nm, 60 fs, relativistic laser pulses in flat targets on one hand, and sub-λ grating targets on the other, which allow resonant excitation of surface plasma waves. We also present the corresponding hot-electron angular distributions and energy spectra. We present particle-in-cell (PIC) simulations which, along with the measurements, give critical insights into understanding the ultrahigh-contrast laser absorption mechanisms, and the hot-electron transport in solids.