Quasi-Linear and Nonlinear High-Harmonic Generation from Ultraintense Ultrafast Laser Plasmas

High-harmonic generation from ultra-intense laser-solid interaction has the potential for efficient generation of attosecond pulses, from a plasma so thin that phase-matching issues need not arise. However, extremely difficult constraints on laser-pulse contrast for solid interaction have routinely meant that new effects of nonlinear optical physics cannot be teased out from the hydrodynamic effects. New beamline-engineered systems of double plasma-mirrors have changed this completely, and we have discovered that the process of high-harmonic generation in laser-solid interactions is actually comprised of two disparate processes, a quasi-linear process of Coherent Wakefield Emission (efficient for ultrafast laser pulses down to I = 10$^{15}$ W/cm$^{2})$, as well as the nonlinear Relativistic Oscillating Mirror mechanism. Both depend crucially on aspects of optical and plasma-wave phase-control during the interaction.