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arXiv · 2003.00277

The imaginary part of the high-harmonic cutoff

Emilio Pisanty·Marcelo F. Ciappina·Maciej Lewenstein
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Abstract

High-harmonic generation - the emission of high-frequency radiation by the ionization and subsequent recombination of an atomic electron driven by a strong laser field - is widely understood using a quasiclassical trajectory formalism, derived from a saddle-point approximation, where each saddle corresponds to a complex-valued trajectory whose recombination contributes to the harmonic emission. However, the classification of these saddle-points into individual quantum orbits remains a high-friction part of the formalism. Here we present a scheme to classify these trajectories, based on a natural identification of the (complex) time that corresponds to the harmonic cutoff. This identification also provides a natural complex value for the cutoff energy, whose imaginary part controls the strength of quantum-path interference between the quantum orbits that meet at the cutoff. Our construction gives an efficient method to evaluate the location and brightness of the cutoff for a wide class of driver waveforms by solving a single saddle-point equation. It also allows us to explore the intricate topologies of the Riemann surfaces formed by the quantum orbits induced by nontrivial waveforms.

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Emilio Pisanty, Marcelo F. Ciappina, Maciej Lewenstein. 2020-07-28. The imaginary part of the high-harmonic cutoff. https://doi.org/10.1088/2515-7647%2Fab8f1e

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