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 $I{\lambda }^2>{10}^{19}\mathrm{W}{\mathrm{cm}}^{-2\mathrm{}}\mu {\mathrm{m}}^2$ and modest shelf densities $N_e{/N}_{\mathrm{crit}}=10\mathrm{}$, at least 60 harmonics can be generated with power efficiencies $P_n{/P}_1>{10}^{-6\mathrm{}}$, suggesting coherent MW x rays with $\lambda \sim 4\mathrm{nm}$ could be generated with a KrF $\left(248\mathrm{nm}\right)$ pump.

DOI:https://doi.org/10.1103/PhysRevLett.76.50