Isolated few-attosecond emission in a multi-cycle asymmetrically nonhomogeneous two-color laser field

By solving the three-dimensional time-dependent Schrödinger equation, we present a theoretical investigation of helium-based high-order harmonic generation (HHG) in both symmetric and asymmetric spatially nonhomogeneous laser fields. It is determined that the harmonic cutoff is extended in both symmetric and asymmetric nonhomogeneous laser fields, and the spatial symmetry of the nonhomogeneous field greatly influences the movement of electronic wave packets, which subsequently affect harmonic emission. In particular, the spatially asymmetric distribution of the laser field will induce asymmetric recombination of electronic wave packets between the two sides of a helium atom, which could reduce the interference in HHG emission for every optical cycle. In combination with the two-color technique, multi-cycle laser pulses with asymmetric spatially nonhomogeneous characteristics are capable of generating a coherent extreme ultraviolet supercontinuum, which can be used to produce isolated few-attosecond pulses.