Computational modelling of high harmonic generation in gas-filled hollow capillaries

Abstract

High harmonic generation uses ultrashort intense laser pulses to generate extreme ultraviolet and soft x-ray radiation. This provides a table-top source of this radiation with uses within imaging and spectroscopy that is an alternative to large and costly facilities. This process is highly complex and difficult to simulate in detail. Here, for the first time, we developed a full, three-dimensional, a priori computer model that runs on a high performance computing cluster and describes all aspects of the physics involved: the ultrashort nonlinear laser pulse propagation, the medium response including ionisation and high harmonic generation, and the propagation of these high harmonics. This model was then validated against experimental and simulation results from the literature. This model was then used to investigate two experimentally important questions. The first being which pump laser wavelength should be used, and the second being which state of the art short-pulse laser system should be used, where powers, pulse lengths, wavelengths, and repetition rates are varied. On the wavelength question, we investigated three different geometries, a single atom, a sheet of atoms, and a gas-filled hollow capillary. When accounting for full three-dimensional effects of quantum diffusion we found the harmonic generation for the single atom to scale with wavelength as λ⁻⁵ and for the sheet of atoms and capillary we found it to scale as λ⁻⁶ instead. For the choice of laser system, we investigated six different systems, testing capillary lengths of up to 7 cm. We found laser pulse mode beating, capillary length, and capillary gas pressure profile to all greatly affect the harmonic generation, with reductions in capillary length from 7 cm significantly increasing the resulting high harmonic generation.

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Identifiers

ORCID for Samuel, Martin Senior: orcid.org/0000-0002-3428-9215

ORCID for Peter Horak: orcid.org/0000-0002-8710-8764

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