To model the optical absorption of MoS2/WS2 heterostructures, we treated the MoS2 and WS2 layers as independent, with in-plane dielectric functions of 2.5 + 30i and 9 + 23.5i (25) and an out-of-plane dielectric function of 10 at 3.1 eV. The results were not sensitive to the choice of the out-of-plane dielectric function (fig. S5). We used nominal thicknesses of 0.615 and 0.618 nm for MoS2 and WS2 monolayers, respectively, and values of 3.19 and 3.16 for the in- and out-of-plane dielectric function of the c-cut sapphire substrate (44). We obtained the absorption of the heterostructure for light incident at 60° and varying polarization. This analysis replicated the measured dependence of the THz radiation on the polarization of the excitation beam, as shown in Fig. 2B. The same calculation also provided the reflectance of the heterostructure: 37.5% for s-polarization and 1.5% for p-polarization. The reflectance for p-polarization was much smaller because the incident angle is close to the Brewster’s angle. We expected a reflectance of 36.4% for the near s-polarized excitation (10° toward p-polarization) in the high fluence measurements (Fig. 4), in reasonable agreement with the experimental value of 32% (fig. S9).

We applied the same method to calculate the absorption of MoS2/MoSe2 heterostructures using dielectric functions of 2.5 + 30i and 5.0 + 15.8i for 3.1 eV, and 21 + 0i and 20.6 + 12.3i for 1.55 eV (25). We used a nominal thickness of 0.646 nm for the MoSe2 monolayer and values of 3.10 and 3.06 for the in- and out-of-plane dielectric function of the sapphire substrate for 1.55 eV. For the measurements in Fig. 5, both the 1.55- and 3.1-eV excitation beams were p-polarized. The calculated p-polarization absorbance for 3.1 eV (17.8%) was about six times that for 1.55 eV (3.0%) and was taken into account when normalizing the THz waveforms.

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