We theoretically investigate harmonic generation driven by intense midinfrared lasers. A valleylike structure is observed at very low-order harmonics because of a low-energy photoelectron suppression effect. Further, at the beginning of a broad supercontinuum, a convex structure appears that is distributed from the tail in a very narrow energy band in time-frequency maps. Surprisingly, our quantum dynamics calculations demonstrate the beneficial wavelength scaling of the harmonic yield to be ${\lambda }^{4.6}$ for He and ${\lambda }^{5.1}$ for Ne over selected energy windows. The bandwidth of the harmonic plateau with only a single quantum trajectory contribution can be further extended by adding a controlling laser field, and the harmonic efficiency is found to be significantly enhanced after macroscopic propagation. In addition, ultrashort isolated attosecond pulses can be obtained by properly superposing the harmonics in the plateaus of both the He and Ne systems.

• Received 12 September 2013

DOI:https://doi.org/10.1103/PhysRevA.89.023825