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Proceedings Paper

Numerical modelling of pump-wavelength dependence of high harmonic generation efficiency (Conference Presentation)
Author(s): Samuel M. Senior; William S. Brocklesby; Peter Horak

Paper Abstract

High harmonic generation (HHG) provides a table-top source of extreme ultraviolet (XUV) and soft x-ray radiation. HHG pump-wavelength dependence is of significant practical interest for laser system design as HHG efficiency scales with pump wavelength to the power of P. First experiments suggested P=-6.5 while theoretical models predict P=-4.7 to -6.0. These investigations exploited single-atom models; insight into efficiencies for full experimental setups will further guide HHG laser designs. We developed a model that simulates the HHG process in full for an argon-filled capillary including all Ti:sapphire pump pulse and XUV propagation effects. With this we compare HHG of two geometries: a thin slice of argon, and an argon-filled capillary. For the thin slice with pump wavelengths 820-1890nm we found P=-4.5 scaling when the harmonic energies were integrated between 16 and 45eV. However, further analysis revealed a dependence of P=-6.4 for longer pump wavelengths (1500-1890nm), but P=-4.0 for shorter wavelengths (820-1500nm). By contrast, HHG in a 7-cm long capillary was found to scale with P=-3.4 (800-1850nm). We attribute this to phase-matching effects over longer propagation distances and nonlinear pump propagation distorting the pulse. Different scaling is observed when the energy of a single harmonic is calculated. In the thin slice the energy in the first harmonic above 20eV yields P=-6.1 (820-1890nm), P=-5.7 (820-1500nm), and P=-7.8 (1500-1890nm). For the whole capillary the corresponding value is P=-4.1 (800-1850nm). High-energy harmonics also exhibit very different scaling with pump wavelength as they cross the classical harmonic cutoff energy. For example, for the first harmonic beyond 41eV no value of P provides a good fit to the simulated HHG efficiencies, neither for the thin slice nor the whole capillary. Our simulations highlight pump-wavelength dependence of HHG efficiency is complex, with many contributing factors such as exact experimental geometry, optical nonlinearity, phase matching, and classical cutoff.

Paper Details

Date Published: 1 April 2020
Proc. SPIE 11358, Nonlinear Optics and its Applications 2020, 1135815 (1 April 2020); doi: 10.1117/12.2554554
Show Author Affiliations
Samuel M. Senior, Univ. of Southampton (United Kingdom)
William S. Brocklesby, Univ. of Southampton (United Kingdom)
Peter Horak, Univ. of Southampton (United Kingdom)

Published in SPIE Proceedings Vol. 11358:
Nonlinear Optics and its Applications 2020
Neil G. R. Broderick; John M. Dudley; Anna C. Peacock, Editor(s)

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