Fig. 4: Effective propagation-induced dephasing in THz HHG.

For all panels, we plot the exact output HHG spectrum computed via Maxwell’s equations as solid red curves, and the output HHG spectrum computed via Eq. (1) fitted to an effective scattering time τ_eff —while neglecting propagation effects—as blue dashed-dotted curves. a, b, c show the spectra when 3D DSM films of thicknesses 0.8 μm, 1 μm, and 2.5 μm, respectively are driven by a peak field of 2 MV/m. d, e, f show the spectra for film thicknesses of 2 μm, 2.5 μm, and 5 μm respectively, driven by a peak field of 10 MV/m. Our fitted values of τ_eff are in good agreement with the exact spectra, implying that the reduced HHG can be captured by an effective dephasing time and is thus due to propagation-induced dephasing effects. As expected, larger film thicknesses correspond to shorter τ_eff. We consider the same parameters as in Figs. 1 and 3. Here, we normalize all angular frequencies of the output ω against the fundamental driving frequency \(\omega _0\).