High Harmonic Generation from Solid Targets at High Repetition Rate

Harmonics of 800 nm light up to the 18th order are generated from solid targets by focusing 2 mJ, 50 fs pulses to a spot size of 1.7 $\mu $m (FWHM). This is the first demonstration of high-harmonic generation with a very short focal length paraboloid (f /1.4). The harmonics have a low divergence ($<$ 4{\_}) compared to the driving beam and conversion efficiencies ($>$ 10-7 per harmonic) comparable to gas harmonics. No contrast enhancement techniques are employed and the system is capable of operating at 500 Hz.