Abstract
We systematically study the efficiency enhancement of high-harmonic generation (HHG) in an Ar gas cell up to the soft X-ray (SXR) range using a two-color laser field composed of 2.1 μm (ω) and 700 nm (3ω) with parallel linear polarization. Our experiment follows the recent theoretical investigations that determined two-color mid-infrared (IR) pulses, mixed with their third harmonic (ω + 3ω), to be close to optimal driving waveforms for enhancing HHG efficiency in the SXR region [Jin et al., Nature Comm. 5, 4003 (2014)]. We observed sub-optical-cycle-dependent efficiency enhancements of up to 8.2 of photon flux integrated between 20 - 70 eV, and up to 2.2 between 85 - 205 eV. Enhancement of HHG efficiency was most pronounced for the lowest tested backing pressure (≈ 140 mbar), and decreased monotonically as the pressure was increased. The single-color (ω)-driven HHG was optimal at the highest backing pressure tested in the experiment (≈ 375 mbar). Our numerical simulations based on single-atom response and 3D pulse propagation show good qualitative agreement with experimental observations. The lower enhancement at high pressure and higher photon energy indicates that phase matching of two-color-driven HHG is more sensitive to ionization rate and pulse propagation effects than the single-color case. We show that with further improvements to the relative phase jitter and the spatio-temporal overlap of the two beams, the efficiency enhancement could be further improved by at least a factor of ≈ 2.
Highlights
The use of mid-infrared (IR) pulses for driving high-harmonic generation (HHG) in gases is found to be the most effective way of extending the harmonic photon energy into the soft Xray (SXR) range, including the water-window and even keV regions [1,2,3,4], as the cutoff energy scales with the driving wavelength squared (λ2)
We systematically study the efficiency enhancement of high-harmonic generation (HHG) in an Ar gas cell up to the soft X-ray (SXR) range using a two-color laser field composed of 2.1 μm (ω) and 700 nm (3ω) with parallel linear polarization
We demonstrated experimentally and with simulations that two-color, ω + 3ω, mid-IR pumping of HHG has the potential to boost the HHG efficiency extending to photon energies in the SXR, if ionization is kept sufficiently low to avoid dephasing of the two-color driver
Summary
The use of mid-infrared (IR) pulses for driving high-harmonic generation (HHG) in gases is found to be the most effective way of extending the harmonic photon energy into the soft Xray (SXR) range, including the water-window and even keV regions [1,2,3,4], as the cutoff energy scales with the driving wavelength squared (λ2). Brizuela et al [32] used a booster gas cell to mix belowthreshold odd harmonics with the high-power fundamental pulse centered at ≈800 nm They observed enhanced HHG up to 80 eV with an intensity-dependent enhancement factor. We tuned the relative phase between these pulses with accuracy sufficient to manipulate the HHG process with sub-optical-cycle control, and study the macroscopic enhancement compared to HHG driven by only the ω-pulses, i.e., single-color case. We performed full 3D simulations of the HHG process, including macroscopic phase-matching effects and experimental parameters, and compared these results to our experimental findings, determining that phase-matching and timing jitter between the two colors pose the greatest limitations to achieving optimal efficiency enhancements
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