Isolated attosecond pulse generated by spatial shaping of femtosecond laser beam

We study numerically the time-dependent HHG phase-matching in the laser beam having a flat-top radial intensity profile. A flat-top profile is the key to produce similar ionization degree at the axis and at the periphery and thus to achieve simultaneous phase-matched generation. Such a profile can be obtained experimentally by using two concentric phase plates that introduce a specific phase shift between the central and the outer part of a focused Gaussian beam. We find realistic laser field parameters and medium density that allow obtaining (after spectral filtering) single attosecond pulse using 10 fs driving laser pulse. Our technique provides isolated attosecond pulse emission almost insensitive on the CEP of the laser pulse. Moreover, the technique is effective both for the mid-plateau and the cut-off spectral range. In particular, the XUV from Ar target in the cut-off spectral range (compatible with the Zr filter) provides isolated attosecond pulse with 185 as duration at a central energy of 92 eV. Using achievable multi mJ 10 fs laser pulses this technique could provide attosecond pulses approaching the μJ energy range