Extension of cutoff in high harmonic by using doubly charged ions in a laser-ablation plume

We report high-order harmonic generation from doubly charged ions in laser-ablation plasma produced by different prepulse intensities. At the prepulse intensity of 3.5×1010 W cm−2, harmonics up to the 63 rd order (12.62 nm wavelength, 98.3 eV photon energy) were obtained by using a laser-ablation titanium plume. From analysis of the harmonics and visible radiation from the laser-ablation titanium plume at different prepulse intensities, we conclude that the highest harmonics near the cutoff region originated from doubly charged titanium ions. These studies show the effectiveness of using doubly charged ions to extend the cutoff energy of high-order harmonics.