Highly Efficient High-Harmonic Generation in an Orthogonally Polarized Two-Color Laser Field

Highly efficient high-harmonic generation was achieved in helium using a two-color laser field that consisted of the fundamental and the second harmonic fields of a femtosecond Ti:sapphire laser. By applying a high intensity second harmonic, the harmonics generated in the orthogonally polarized two-color field were stronger than those obtained in the fundamental field by more than 2 orders of magnitude, and even stronger than those of the parallel polarization case. A conversion efficiency as high as 5×10^-5 was obtained for the 38th harmonic at 21.6 nm. The physical origin of this enhancement was deduced by analyzing the electron behavior in the two-color field.