Multi-μJ harmonic emission energy from laser-driven plasma

We report on simultaneous efficiency and divergence measurements for harmonics from solid targets generated by the relativistic oscillating mirror mechanism. For a value of the normalized vector potential of a L ≃ 1.5 , we demonstrate the generation of 30 μJ high-harmonic radiation in a 17 ± 3 mrad divergence cone. This corresponds to a conversion efficiency of ≳ 10 −4 in the 17–80 nm range into a well-confined 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 efficiencies of a few percent at higher laser intensities.