Enhanced high-harmonic generation up to the soft X-ray region driven by mid-infrared pulses mixed with their third harmonic

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.

Parametric attosecond pulse amplification far from the ionization threshold from high order harmonic generation in He+

“…In the present study we investigate the amplification of attosecond pulses with specific central photon energies that are far from both the ionization energy of He (24.6 eV) and He + (54.4 eV). The results and the physical processes that we describe therefore essentially differ from other research on HHG yield and cutoff enhancement [19][20][21][22] or pulse amplification near the ionization threshold [23].…”

Optical Waveform Synthesis and Its Applications

“…On the experimental side, many studies have investigated HHG driven by two‐ and multi‐color fields with parallel [ 6,20,23,77,160,162,168–180 ] or perpendicular [ 181–186 ] polarizations. In particular, experiments with parallel‐polarized fields revealed HHG yield enhancement, energy cutoff extension, attosecond coherent control, and direct IAP generation.…”

“…[ 112,152–158 ] iii) The increase of the HHG single‐atom conversion efficiency by sculpting the driving waveform with sub‐cycle precision. [ 5,6,159–162 ] The latter approach can be implemented in the time‐domain by coherently combining different electric fields, that is, via WS technology, or in the frequency‐domain by shaping the phase of different spectral components via spectral phase modulators. [ 58,156,163 ] Recently, Yang et al.…”

“…These two challenges, the production of sub‐cycle pulses and the efficient production of coherent X‐rays, are actually connected since both theory and experiments show that driving HHG with multi‐color pulses may significantly increase HHG conversion efficiency. [ 3–7 ]…”

Towards Single Atom Computing via High Harmonic Generation

“…Computing on a single atom has more than just memetic appeal however -atomic systems exhibit one of the most extreme non-linear effects yet observed, a phenomenon known High Harmonic Generation (HHG) [32][33][34][35]. HHG occurs when a short and intense laser pulse generates responses of up to ≈ 100 times the frequency of the incident field.…”