Benchmark calculations for high-order harmonic generation in helium.

We report a detailed study of high-order harmonic generation (HHG) in helium. When comparing predictions from a single-active-electron model with those from all-electron simulations [1], such as R-matrix with time-dependence (RMT) [2] and the ATTOMESA code currently under development [3], both of which can include different numbers of states in the close-coupling expansion, it seems imperative to generate absolute numbers for the HHG spectrum in a well-defined framework. While qualitative agreement in the overall frequency dependence of the spectrum, including the cut-off frequency predicted by a semi-classical model [4,5], can be achieved by many models in arbitrary units, only absolute numbers can be used for benchmark comparisons between different approaches. This is particularly important when obtaining appropriate estimates for the conversion efficiency, i.e., the area under the HHG plateau compared to the peaks due to the fundamental frequency and the low-order harmonics.

[1] A. T. Bondy et al., https://arxiv.org/abs/2311.03677 (2023)

[2] A. C. Brown et al., Comput. Phys. Comm. 250 (2020) 107062.

[3] N. Douguet, unpublished (2024)

[4] K. J. Schafer, B. Yang, L. F. DiMauro, and K. C. Kulander, Phys. Rev. Lett. 70 (1993) 1599.

[5] P. B. Corkum, Phys. Rev. Lett. 71 (1993) 1994.