Significant enhancement of high-order harmonics below 10 nm in a two-color laser field

Efficient high-order harmonic generation was obtained in helium below 10 nm using a linearly polarized two-color field consisting of a Ti:sapphire laser pulse (omega) with a 40-fs duration and its second harmonic (2 omega) with a 24-fs duration. The intensity of the two-color harmonics was enhanced more than that of the omega or 2 omega field alone by more than one order of magnitude along with the extension of the cutoff order and the generation of efficient even-order harmonics. This observation was explained by the contribution of the lower branch rather than the upper branch of the cutoff energy, which are split in the omega-2 omega field. The supercontinuum around 10 nm provides the bandwidth capable of attosecond pulse generation.