Generation of submicrojoule high harmonics using a long gas jet in a two-color laser field

We have achieved very efficient high-harmonic generation in a two-color laser field using a long gas jet of He. With the optimization of laser parameters and target conditions, strong harmonics were produced at 2(2n+1)th orders in an orthogonally polarized two-color field. The strongest harmonic at the 38th order (21.6 nm) reached an energy of 0.6 mu J with a 6 mm gas jet, giving a conversion efficiency as high as 2x10(-4).