High-harmonic generation with relativistic laser intensities

A method for producing hard x-rays via high-harmonic generation using ultra-intense lasers is proposed. The method relies on cavitation and ion channel formation by the ponderomotive force of a short, ultra-intense laser pulse or the space charge force of a dense, energetic electron beam. A second laser produces high harmonics in the electron-free cavity. A counter-propagating laser is used to eliminate the longitudinal motion owing to the magnetic component of the Lorentz force in the relativistic regime. A counter-propagating laser pulse train is proposed for quasi-phase matching. This method enables the reach of high-harmonic generation to be extended to the sub-\AA\ regime.