Generation of efficient high harmonics in an orthogonally polarized two-color laser field

Highly efficient high-harmonic generation was achieved in helium using an orthogonally polarized two-color laser field that consisted of the fundamental and second harmonic of a femtosecond Ti:sapphire laser. By applying a higher intensity second harmonic, the harmonics in the two-color field were stronger than those obtained in the fundamental field by more than two orders of magnitude. The conversion efficiency was as high as 5/spl times/10/sup -5/ for the 38/sup th/ harmonic at 21.6 nm.