Figure 1:

Na EM response. (a) Schematic of the considered Na-based thin film with thickness d, illuminated by a pump field with intensity I ₀ and vacuum wavelength λ, producing forward and backward THG at λ _THG = λ/3. (b) Dependence of real (red curve) and imaginary (dashed blue curve) parts of Na relative dielectric permittivity ϵ(λ). The ENZ wavelength of Na λ _ENZ where Re[ϵ(λ _ENZ)] = 0 is indicated by the black solid circle in the figure inset. (c) Pump vacuum wavelength-dependence of the real (red curve) and imaginary (dashed blue curve) parts of the THG susceptibility ${\chi }_{\text{coll}}^{(3\omega )}(\lambda )$ . (d) Wavelength dependence of the THG NL susceptibility modulus $|{\chi }_{\text{coll}}^{(3\omega )}(\lambda )|$ for several distinct electron thermalization rates γ _th = 1/300, 1/100, 1/50 fs⁻¹, indicated by full, dashed, and dotted curves, respectively.