ABSTRACT
High-order harmonic generation using femtosecond high-power lasers is very attractive as an intense coherent short wavelength source in the XUV region [1]. Its high peak power is promising to induce nonlinear optical phenomena. Multiphoton ionization of rare-gas atoms in the VUV region has been reported already using high harmonics [2, 3]. However, observation of mutliphoton and/or nonlinear processes in the soft x-ray region (< 30 nm) is still a challenging issue.
