Multi- µj harmonic emission energy from laser-driven plasma

We report on simultaneous effciency and divergence measurements for harmonics from solid targets generated by the relativistic oscillating mirror mechanism. For a value of the normalized vector potential of al — 1.5, we demonstrate the generation of 30 (jJ high-harmonic radiation in a 17 ± 3 mrad divergence cone. This corresponds to a conversion effciency of >10~⁴ in the 17-80 nm range into a well-confned beam. Presuming phase-locked harmonics, our results predict unprecedented levels of average power for a single attosecond pulse in the generated pulse train. Results of PIC simulations raise the prospect of attaining effciencies of a few percent at higher laser intensities.