A broadband supercontinuum generated by helium atom exposed to combined mid-infrared laser field

By solving the one-dimensional time-dependent Schrodinger equation with split-operator method, we study the high harmonics generated by helium atom, and analyse the characteristics of the attosecond pulses generated by the high-order harmonics. By adopting a mid-infrared driving pulse (with wavelength of 2000 nm and pulse duration of 12.5 fs) combined with a short UV controlling pulse, a smooth supercontinuum with the bandwidth of 230 eV is observed. By adjusting the delay between the mid-infrared and ultraviolet fields, the contribution of the short quantum trajectory can be efficiently enhanced and an isolated 100 as pulse seems achievable. We found that the supercontinuum is not sensitive to the intensities of the two lasers, which facilitates the experimental implementation for the isolated attosecond pulse generation.