Highly efficient high-harmonic generation was achieved in helium using a two-color laser field that consisted of the fundamental and second harmonic of a ferntosecond Ti:sapphire laser. The harmonics generated in an orthogonally polarized two-color field were stronger than those obtained in a fundamental field by more than two orders of magnitude, and were even stronger than those of the parallel polarization case. To gain physical understanding of this highly efficient harmonic generation, the behavior of electron in the two-color laser field was analyzed. A two-color laser field with suitable relative phase could select the short quantum path component which has denser electron wave packet at the time of recombination. Consequently, the orthogonally polarized two-color laser field could generate harmonics much more strongly than the fundamental field.
We observed the effects of the mixed field from the spectral structure of high-harmonics by controlling the relative phase and time delay between two-color laser fields. As the temporal overlap between the two fields becomes better, the intensity of, 2(2n)th harmonics, purely mixing orders was enhanced; all even order harmonics became strong. When the phase matching condition is optimized, the reabsorption by the harmonic generation medium becomes the main limiting factor to the efficiency enhancement of high harmonics. Using a long gas medium, we obtained more efficient high-harmonics through the absorption-limited generation. This ultrafast soft-x-ray source emits approximately $3 \times 10^{10}$ photons per shot at 21.6 nm, reaching the conversion efficiency of $1 \times 10^{-4}$.
The relative simplicity and effectiveness of this two-color method for strong high harmonic and attosecond pulse generation will be of great use in practical applications such as ultrafast soft-x-ray spectroscopy for atoms, molecules and attosecond physics.
