The recent progress of the high-order harmonic generation in laser-produced ions is presented. The experimental results obtained at RIKEN using a 500-fs KrF laser (248 nm) are summarized. Rare gas-like ions produced much higher-order harmonics than rare gases did when the laser intensity was increased to 1017 W/cm 2. In addition to rare gas-like ions, various metal ions were used as nonlinear media. The maximum harmonic orders are found to be simply proportional to the ionization potentials of the nonlinear media, including neutral rare gases. The maximum harmonic order observed was the 21st (11.8 nm) in Pb 2+. The advantages of using ions in laser-produced plasmas as nonlinear media for the high-order harmonic generation are discussed.
Wavelength and intensity dependence of recollision-enhanced multielectron effects in high-order harmonic generation