Generation of isolated sub-20-attosecond pulses from He atoms by two-color midinfrared laser fields

We propose an efficient method for the generation of ultrabroadband supercontinuum spectra and isolated ultrashort attosecond laser pulses from He atoms with two-color midinfrared laser fields. High-order harmonic generation (HHG) is obtained by solving the time-dependent Schrodinger equation accurately by means of the time-dependent generalized pseudospectral method. We found that the optimizing two-color midinfrared laser pulse allows the HHG cutoff to be significantly extended, leading to the production of an ultrabroadband supercontinuum. As a result, an isolated 18-attosecond pulse can be generated directly by the superposition of the supercontinuum harmonics. To facilitate the exploration of the ultrashort attosecond generation mechanisms, we perform both a semiclassical simulation and a wavelet time-frequency transform.