Generation of intense isolated sub-40-as pulses from a coherent superposition state by quantum path control in the multicycle regime

We theoretically investigate high-order harmonic generation and attosecond pulses by numerically solving the three-dimensional time-dependent Schroedinger equation from a helium ion in a two-color laser field, which is synthesized by adding a 1600-nm laser pulse to a multicycle 800-nm laser pulse. The numerical results show that the short quantum path selection and broadband continuum spectra are achieved by adjusting the relative phase between two laser pulses, and isolated attosecond pulses can be generated successfully. Compared with the case of He+ ions from the 1s ground state, the emission efficiency of the continuous harmonics and the intensity of the isolated attosecond pulse are enhanced approximately thirteen orders of magnitude by preparing He+ ions in a coherent superposition of the states 1s and 2s. Furthermore, the bandwidth of the continuum spectrum is further broadened by increasing the intensity of the 1600-nm laser pulse, and an intense 38-as isolated pulse with a bandwidth of 109 eV is straightforwardly obtained.