Generation of highly coherent submicrojoule soft x rays by high-order harmonics

We demonstrated the energy scaling of high-order harmonics using a self-guided beam under the phase-matched condition. By adjusting the argon gas density and pump laser focusing condition, a total output harmonic energy as high as 0.7 \ensuremath{\mu}\mathrm{J}was obtained in the spectral region of 34.8 to 25.8 nm (the corresponding order of the 23rd to 31st harmonic), while the 27th-order harmonic (29.6 nm) energy attained was as high as...