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Numerical modelling of pump-wavelength dependence of high harmonic generation efficiency

Numerical modelling of pump-wavelength dependence of high harmonic generation efficiency
Numerical modelling of pump-wavelength dependence of high harmonic generation efficiency
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.
SPIE
Senior, Samuel, Martin
993b2864-8eea-4440-9609-37ee04ef7d48
Brocklesby, William
c53ca2f6-db65-4e19-ad00-eebeb2e6de67
Horak, Peter
520489b5-ccc7-4d29-bb30-c1e36436ea03
Senior, Samuel, Martin
993b2864-8eea-4440-9609-37ee04ef7d48
Brocklesby, William
c53ca2f6-db65-4e19-ad00-eebeb2e6de67
Horak, Peter
520489b5-ccc7-4d29-bb30-c1e36436ea03

Senior, Samuel, Martin, Brocklesby, William and Horak, Peter (2020) Numerical modelling of pump-wavelength dependence of high harmonic generation efficiency. In Nonlinear Optics and its Applications 2020. vol. 1135815, SPIE.. (doi:10.1117/12.2554554).

Record type: Conference or Workshop Item (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.

Text
2003 SPIE-Photonics-EU_TechnicalAbstract_SamuelSenior4 - Accepted Manuscript
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Published date: 6 April 2020
Venue - Dates: SPIE Photonics Europe 2020, France, 2020-03-29 - 2020-04-02

Identifiers

Local EPrints ID: 439350
URI: http://eprints.soton.ac.uk/id/eprint/439350
PURE UUID: 31dd0ea4-b371-4aa9-aff3-866c0793ee53
ORCID for William Brocklesby: ORCID iD orcid.org/0000-0002-2123-6712
ORCID for Peter Horak: ORCID iD orcid.org/0000-0002-8710-8764

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Date deposited: 17 Apr 2020 16:30
Last modified: 21 May 2020 00:28

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