Fig. 2: Extreme subwavelength phase-flip of the induced current in highly nonlinear materials.

The colormap shows the phase shift in the 3rd harmonic current for a 2500 nm-thick, non-dispersive dielectric film as a function of its linear and third-order susceptibilities, χ⁽¹⁾ and χ⁽³⁾ respectively, driven by an external field of 10 MV/m. We find that the extreme subwavelength phase-flip of the induced current occurs within a regime of nonlinear optics accessible only by highly nonlinear materials like 3D Dirac semimetals (DSMs); no such phase-flip occurs at extreme subwavelength propagation distances in conventional nonlinear materials, which have weaker nonlinearity (χ⁽¹⁾ and χ⁽³⁾ values⁴⁸ of conventional materials plotted as white crosses). We use the same THz pulse parameters as Fig. 1.