Xiaohui Gao

Ultraviolet surprise: Efficient soft x-ray high-harmonic generation in multiply ionized plasmas.

Short wavelengths birth shorter ones The shortest laser pulses—with durations measured in attoseconds—arise from a process termed high-harmonic generation (HHG). Essentially, a longer, “driving” pulse draws electrons out of gaseous atoms like a slingshot, and, when they ricochet back, light emerges at shorter wavelengths. Most HHG has been carried out using light near the visible/infrared boundary for the driving pulse. Popmintchev et al. used an ultraviolet driving pulse instead, which yielded an unexpectedly efficient outcome. These results could presage a more generally efficient means of creating x-ray pulses for fundamental dynamics studies as well as technological applications. Science, this issue p. 1225 Ultraviolet pulses show unexpected efficiency in generating the higher-frequency emission underlying attosecond spectroscopy. High-harmonic generation is a universal response of matter to strong femtosecond laser fields, coherently upconverting light to much shorter wavelengths. Optimizing the conversion of laser light into soft x-rays typically demands a trade-off between two competing factors. Because of reduced quantum diffusion of the radiating electron wave function, the emission from each species is highest when a short-wavelength ultraviolet driving laser is used. However, phase matching—the constructive addition of x-ray waves from a large number of atoms—favors longer-wavelength mid-infrared lasers. We identified a regime of high-harmonic generation driven by 40-cycle ultraviolet lasers in waveguides that can generate bright beams in the soft x-ray region of the spectrum, up to photon energies of 280 electron volts. Surprisingly, the high ultraviolet refractive indices of both neutral atoms and ions enabled effective phase matching, even in a multiply ionized plasma. We observed harmonics with very narrow linewidths, while calculations show that the x-rays emerge as nearly time-bandwidth–limited pulse trains of ~100 attoseconds.

Spatio-temporal profiling of cluster mass fraction in a pulsed supersonic gas jet by frequency-domain holography

We present an in-depth study of a rapid, noninvasive, single-shot optical method of determining cluster mass fraction fc(r, t) at specified positions r within, and at time t after opening the valve of, a pulsed high-pressure pulsed supersonic gas jet. A ∼2 mJ, 40 fs pump pulse ionizes the monomers, causing an immediate drop in the jets refractive index njet proportional to monomer density, while simultaneously initiating hydrodynamic expansion of the clusters. The latter leads to a second drop in njet that is proportional to cluster density and is delayed by ∼1 ps. A temporally stretched probe pulse measures the 2-step index evolution in a single shot by frequency-domain holography, enabling recovery of fc. We present a model for recovering fc from fs-time-resolved phase shifts. We also present extensive measurements of spatio-temporal profiles fc(r,t) of cluster mass fraction in a high-pressure supersonic argon jet for various values of backing pressure P0 and reservoir temperature T0.

Picosecond ionization dynamics in femtosecond filaments at high pressures

We observe a 3-fold increase in the electron density within 30 picoseconds after the filamentary propagation of femtosecond pulses in 60-bar argon gases. This suggests that electron-impact ionization dominates on this time scale.

In situ measurement of cluster mass fraction in supersonic gas jets by frequency domain holography

We determine cluster mass fraction fc in supersonic gas jets by measuring femtosecond evolution of the jets refractive index by single-shot frequency domain holography. Variations of fc with time, position and backing pressure are measured.

Enhanced Harmonic Generation in Gas Jets with Expanding Clusters

We report femtosecond-time-resolved enhancement and anisotropy of thirdharmonic generation in highly ionized clustered argon at intensities exceeding 1015 W/cm2.Results suggest a path to phase-match high-order harmonic generation in dense plasmas with ultrahigh intensity.

Laser-driven Acceleration in Clustered Plasmas

We propose a new approach to avoid dephasing limitation of laser wakefield acceleration by manipulating the group velocity of the driving pulse using clustered plasmas. We demonstrated the control of phase velocity in clustered plasmas by third harmonic generation and frequency domain interferometry experiments. The results agree with a numerical model. Based on this model, the group velocity of the driving pulse in clustered plasmas was calculated and the result shows the group velocity can approach the speed of light c in clustered plasmas.