(a–d) THG efficiency η of the highly doped semiconductor slab as a function of the incident angle θ and for different materials. Data reported in plots (a–c) are calculated taking into account distinct nonlinear sources: (a) is obtained considering only FE direct contributions; (b) refers to the FE cascaded THG; (c) to the THG originating from background lattice third-order susceptibility χ⁽³⁾. In plot (d) the curves are those predicted for a real experiment where all the contributions to THG are taken into account. (e–f) η as a function of the background permittivity, ε_∞, and of the effective mass, m, for a fixed angle of incidence θ = 40° in the case of FE-direct (e) and FE-cascaded (f) THG. Here, white diamond represent the semiconductors studied in correspondence of their values of ε_∞ and m. The level of doping and the wavelength of the fundamental field (FF) are fixed to n₀ = 10²⁰ cm⁻³ and , respectively, in all plots.