HarP: a simulation tool for high-order harmonic generation and propagation in gases

Accurately simulating high-order harmonic generation (HHG) for attosecond science requires connecting the microscopic single-atom quantum dynamics and the macroscopic propagation of intense driving laser fields. HarP is an open-source Python code designed to address this multiscale problem. It solves the wave equations for both the driving laser field and the generated harmonics, using precomputed dipole tables for the nonlinear polarization density. This tabulation enables the direct integration of accurate microscopic models while avoiding costly calculations during the propagation. The methodology builds upon the approach of [L'Huillier et al. (1992) PRA 46, 2778] and has already facilitated multiple studies on phase matching, propagation effects and HHG optimization. This paper presents the underlying physical model, its numerical implementation, and comparison against alternative simulation frameworks.