Harmonic generation by femtosecond laser-solid interaction: A coherent ''water-window'' light source?

The generation of harmonics by short, intense laser pulses reflected from a solid-density plasma is investigated using particle-in-cell simulation. High irradiance, obliquely incident p-polarized light generates harmonics via relativistic electrons dragged across the vacuum-solid interface. This mechanism does not exhibit the limitation previously predicted for lower intensities of a maximum harmonic ''cutoff'' n(max) = omega(p)/omega(0). For I lambda(2) > 10(19) W cm(-2) mu m(2) and modest shelf densities N-e/N-crit = 10, at least 60 harmonics can be generated with power efficiencies P-n/P-1 > 10(-6), suggesting coherent MW x rays with lambda similar to 4 nm could be generated with a KrF (248 nm) pump.