Pulse shortening via Relativistic Transparency of Nanometer Foils

Intense lasers drive plasma electrons to velocities approaching light-speed. Increase of the electron mass causes optical transparency in otherwise classically over-dense plasma. Simulations indicate relativistic transparency can produce near-single-cycle rise time light pulses. It also lies enables a new mechanism for laser-based ion-acceleration yielding energy increases over earlier approaches. A direct signature of transparency is pulse-shortening thru over-dense plasmas in which relativistic intensity induces transmission. Using nm C foils (LMU) and the high-contrast Trident laser (LANL) we have made auto-correlation measurements showing $>$2x transmitted pulse duration reduction at intensities corresponding to $\sim $20-fold increase in electron mass. Spectral measurements agree with pulse shortening thru the target, and 1-D particle-in-cell simulations support the measurements.