Kroh, TobiasJin, ChengKrogen, PeterKeathley, Philip D.Calendron, Anne-LaureSiqueira, Jonathas P.Liang, HoukunFalcão-Filho, Edilson L.Lin, C. D.Kärtner, Franz X.Hong, Kyung-Han2019-04-172019-04-172018-06-25http://hdl.handle.net/2097/39545Citation: Kroh, T., Jin, C., Krogen, P., Keathley, P. D., Calendron, A.-L., Siqueira, J. P., … Hong, K.-H. (2018). Enhanced high-harmonic generation up to the soft X-ray region driven by mid-infrared pulses mixed with their third harmonic. Optics Express, 26(13), 16955–16969. https://doi.org/10.1364/OE.26.016955We 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.en-US© 2018 Optical Society of America. This Item is protected by copyright and/or related rights. You are free to use this Item in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s).Harmonic generation and mixingSoft x-raysStrong field laser physicsUltravioletText