High harmonics from relativistically oscillating plasma surfaces - A high brightness attosecond source at keV photon energies

An intense laser pulse interacting with a near discontinuous plasma vacuum interface causes the plasma surface to perform relativistic oscillations. The reflected laser radiation then contains very high order harmonics of fundamental frequency and - according to current theory - must be bunched in radiation bursts of a few attoseconds duration. Recent experimental results have demonstrated x-ray harmonic radiation extending to 3.3 (3.8 keV, order n > 3200) with the harmonic conversion efficiency scaling as η(n) n -2.5 over the entire observed spectrum ranging from 17 nm to 3.3 . This scaling holds up to a maximum order, nRO 81/2γ 3, where γ is the peak value of the Lorentz factor, above which the harmonic efficiency decreases more rapidly. The coherent nature of the generated harmonics is demonstrated by the highly directional beamed emission, which for photon energy h ν > 1 keV is found to be into a cone angle ∼4°, significantly less than that of the incident laser cone (20°). © 2007 IOP Publishing Ltd