Ultra-high amplitude isolated attosecond pulses generated in the transmission regime from ultrathin foil

A coherent-synchrotron-emission regime of forward high-order harmonics generation (HHG) is proposed for the emission of an isolated unipolar half-cycle attosecond pulse by a three-color laser pulse impinging on ultrathin foil. A theoretical model is proposed for the electron nanobunching mechanism and the forward radiation, which is consistent with the numerical-simulation results. As the forward HHG does not need to penetrate from the front to the rear side of the target, the spectrum of the forward HHG has no low-frequency cutoff. The robustness of this regime is verified with different laser and foil plasma conditions as well as the two-dimensional effects. The robustness is also checked with similarity theory, which confirms that isolated attosecond pulses can be efficiently generated when the plasma density and the laser amplitude change simultaneously such that their ratio remains unchanged.