High harmonics from laser-ablated plumes are mostly generated from ionic species. We demonstrate that with ultrashort infrared (∼1.82  μm) driving lasers, high harmonics from laser-ablated manganese are predominantly generated from neutral atoms, a transition metal atom with an ionization potential of 7.4 eV. Our results open the possibility to advance laser-ablation technique to study the dynamics of neutral atoms of low ionization potential. Moreover, as manganese contains giant autoionizing resonance, intense and broadband high harmonics have been demonstrated from this resonance at energies from 49 to 53 eV. This opens the possibility to generate intense attosecond pulses directly from the giant resonances, as well as to study these resonances using high-harmonic spectroscopy.

In this study, we demonstrate intense extreme-ultraviolet optical vortices generated using laserablation plume as the nonlinear medium. We used two types of plumes that are known to generate intense high-order harmonics for driving lasers with Gaussian beam profiles, but through different mechanisms, namely carbon (diatomic carbon molecules) and tin (resonance with autoionizing state). We find that the harmonic fluxes for diatomic carbon molecules are similar for Gaussian and vortex driving fields. However, for harmonics from the autoionizing state of tin (~26.3 eV), the enhancement factor of the resonant harmonic intensity decreases by ~50% when using the vortex driving field. The intense extreme-ultraviolet optical vortices demonstrated in this study will be useful for many applications including a new material characterization technique known as optical angular momentum dichroism as well as the spectroscopy of spin-forbidden electronic transitions.

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ZnO thin-film loading of YZ-LiNbO3 is used to suppress all the harmonic generations of surface acoustic waves (SAW’s) in the substrate. An optical probing technique is used to measure the intensity variations of the harmonics as a function of acoustic power. Only the first-order diffracted light intensity is observed owing to the suppression of higher harmonic generations of SAW’s. This linearization of the first-order diffracted light intensity versus acoustic power is found to be independent of the interaction length (up to 5 mm) and the acoustic power (up to 4.3 W). The attenuation of SAW’s on YZ-LiNbO3 with a 2-μm ZnO layer was measured to be about 5 dB/cm at 325 MHz.

Resonant high-order harmonics, which result in quasi-monochromatic extreme ultraviolet light with coherent intensity enhancement involving autoionizing resonances, have been demonstrated from laser-ablated plumes in the tunnel-ionization regime. Here, we demonstrate resonant harmonics in the previously unexplored multiphoton-ionization regime. We demonstrate an intense resonant harmonic from gallium with an intensity enhancement ratio of 714 relative to the neighboring harmonics, achieved without the need for extreme ultraviolet filtering methods, thus preventing a typical photon flux loss of more than 70%. Three-dimensional time-dependent Schrödinger equation calculations reveal that this increase in the enhancement ratio is due to the low electron wave packet spreading in the multiphoton-ionization regime. These results reveal a method for increasing the intensity and monochromaticity of intense multimicrojoule femtosecond extreme ultraviolet light and will also facilitate understanding of the involvement of autoionizing resonances in generating resonant harmonics in the multiphoton-ionization regime.