Investigation of high-harmonic cutoff of metal ions driven by near-infrared laser

Harmonic Generation in Molecular Ag2S Plasma

The molecular laser-induced plasma (LIP) produced during the ablation of silver sulfide (Ag2S) was used as a medium for high-order harmonic generation in the extreme ultraviolet range. The role of LIP formation, the plasma components, and the geometry of plasma in the harmonic conversion efficiency was analyzed. We also analyzed the influence of the driving pulses (chirp, single-color pump, two-color pump, and delay between heating and converting pulses) on the harmonic yield in Ag2S LIP. The application of molecular plasma was compared with the application of atomic plasma, which comprised similar metallic elements (Ag) as well as other metal LIPs. The harmonics from the Ag2S LIP were 4 to 10 times stronger than those from the Ag LIP. The harmonics up to the 59th order were achieved under the optimal conditions for the molecular plasma.

Extension of high-order harmonic generation cutoff from laser-ablated tin plasma plumes

The high-order harmonic spectra from laser-ablated tin plasma plumes are investigated experimentally and theoretically at different laser wavelengths. It is found that the harmonic cutoff is extended to ∼84 eV and the harmonic yield is greatly improved by decreasing the driving laser wavelength from 800 nm to 400 nm. Appling the Perelomov-Popov-Terent'ev theory with the semiclassical cutoff law and one-dimensional time-dependent Schrödinger equation, the contribution of the Sn³⁺ ion to harmonic generation accounts for the cutoff extension at 400 nm. With the qualitative analysis of the phase mismatching effect, we reveal the phase matching caused by the dispersion of free electrons is greatly optimized in the 400 nm driving field relative to the 800 nm driving field. The high-order harmonic generated from laser-ablated tin plasma plumes driven by the short laser wavelength provides a promising way to extend cutoff energy and generate intensely coherent extreme ultraviolet radiation.

Comparative study of medium length-dependent high-harmonic generation from metal ions

We present an experimental study on the high-harmonic yields from the ions in the laser-ablated plumes of various metal targets (W, Mo, Cr, Cu, Ni, Fe, Ag and Mg) with the purpose of comparing their ion density and single-atom response. The harmonic yields as a function of medium length are measured and the results are fitted against a theoretical model to extract the coherence length, absorption length and strength single-atom response (in arbitrary units) of different harmonic orders for each target. It is found that the coherence lengths decrease monotonically as a function of harmonic order for all targets. Ion density of the generation media are estimated by the trend of the coherence length as a function of harmonic order. Qualitatively, targets with lower melting temperatures seem to produce laser-ablated plumes of higher ion density, vice versa. Also, the strength of the single-atom response of the metal ion species with only one electron in the outermost subshell are weak compared with the other targets considered in this study.