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'' ${n}_{\mathrm{max}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}{\ensuremath{\omega}}_{p}/{\ensuremath{\omega}}_{0}$. For $I{\ensuremath{\lambda}}^{2}>{10}^{19}\mathrm{W}{\mathrm{cm}}^{\ensuremath{-}2}\ensuremath{\mu}{\mathrm{m}}^{2}$ and modest shelf densities ${N}_{e}{/N}_{\mathrm{crit}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}10$, at least 60 harmonics can be generated with power efficiencies ${P}_{n}{/P}_{1}>{10}^{\ensuremath{-}6}$, suggesting coherent MW x rays with $\ensuremath{\lambda}\ensuremath{\sim}4\mathrm{nm}$ could be generated with a KrF $(248\mathrm{nm})$ pump.

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