High-energy isolated attosecond pulses required for the most intriguing nonlinear attosecond experiments as well as for attosecond-pump/attosecond-probe spectroscopy are still lacking at present. Here we propose and demonstrate a robust generation method of intense isolated attosecond pulses, which enable us to perform a nonlinear attosecond optics experiment. By combining a two-colour field synthesis and an energy-scaling method of high-order harmonic generation, the maximum pulse energy of the isolated attosecond pulse reaches as high as 1.3 μJ. The generated pulse with a duration of 500 as, as characterized by a nonlinear autocorrelation measurement, is the shortest and highest-energy pulse ever with the ability to induce nonlinear phenomena. The peak power of our tabletop light source reaches 2.6 GW, which even surpasses that of an extreme-ultraviolet free-electron laser.

We demonstrate the generation of a coherent water window x ray by extending the plateau region of high-order harmonics under a neutral-medium condition. The maximum harmonic photon energies attained are 300 and 450 eV in Ne and He, respectively. Our proposed generation scheme, combining a 1.6 microm laser driver and a neutral Ne gas medium, is efficient and scalable in output yields of the water window x ray. Thus, the precept of the design parameter for a single-shot live-cell imaging by contact microscopy is presented.

We report on the multiphoton ionization processes in the soft-x-ray region ͑ഛ30 nm͒. On the basis of the measured time-of-flight spectra for both ions and electrons obtained using intense soft-x-ray pulses produced by high-order harmonics, the cross sections of the two-photon double ionization and above-threshold ionization of He are estimated. The high-intensity soft-x-ray radiation achieved by phase-matched high-order harmonics enables the investigation of these nonlinear optical processes, which were beyond the reach of conventional light sources.

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