Efficient harmonic generation was achieved using a long argon gas jet pumped by intense 28 fs laser pulses. By balancing diffraction and plasma defocusing with nonlinear self-focusing, we could achieve a new optimum condition for efficient high-order harmonic generation. Under condition of a self-guided laser pulse, generated high-order harmonic signal was strong enough to saturate an x-ray CCD in one laser shot of a few mJ.

Generation of high harmonics in a femtosecond laser field with intensity far exceeding the saturation intensity for optical field ionization is investigated. As the laser intensity was increased, the time of strong high harmonic generation became earlier. As an atom is exposed to a larger electric field variation duration the harmonic emission, the emitted harmonics experience a blueshift due to a non-adiabatic effect. In addition the high harmonic spectra showed very complex structure, which was analyzed using a semi-classical theory based on the strong field approximation.