Optimal control of high-order harmonics for the generation of an isolated ultrashort attosecond pulse with two-color midinfrared laser fields

We present an efficient high-order-harmonic optimal control scheme for the generation of the ultrabroad supercontinuum spectrum and an isolated ultrashort attosecond pulse in gases with a two-color mid-IR laser field. The optimal control scheme is implemented using a derivative-free unconstrained optimization algorithm called NEWUOA (NEW Unconstrained Optimization Algorithm). For illustration, the high-order-harmonic generation (HHG) of a hydrogen atom is considered for optimization. It is shown that optimally shaped laser waveforms can greatly enhance and extend the HHG plateau and efficiently generate an isolated ultrashort attosecond pulse. Moreover, by performing accurate semiclassical simulations and a detailed wavelet time-frequency analysis, we found that the optimized supercontinuum harmonics corresponding to long-trajectory electrons are responsible for an isolated ultrashort 21-as pulse.