multi-keV harmonic generation from relativistically oscillating plasma surfaces

High order harmonic generation (HOHG) from intense laser- solid density interactions has emerged as a promising route to the generation of attosecond pulses extending to keV energies. With efficiency of the nth harmonic order, jn, scaling as ~ n-2.5-n-3 in the relativistic limit up to a maximum harmonic order nmax ~ 8 1/2i3 (where i is the maximum relativistic Lorentz factor of the oscillating plasma surface), the potential for a bright solution to attosecond science is a distinct possibility. Here we present the first demonstration of nmax ¶ i3 cut off scaling for both 500fs pulses and, excitingly, ultra short pulses (<50fs), giving clear indication that bright soft x-ray pulses can be generated using even modestly sized ‘table-top ’ generation lasers. The angular distribution of the HOHG signal is also investigated. For ultrashort pulses there is distinct behaviour observed between the recently described coherent wakefield emission (CWE) process for harmonic generation and relativistic oscillating mirror (ROM) harmonics. Near diffraction limited performance for ROM HOHG is displayed with clear indication of source size shrinking for the cut-off orders (up to 40). CWE is observed to be emitted into cone angles several times that expected for diffraction limited performance. The close relation of the angular distribution o