Enhancement in optically induced ultrafast THz response of MoSeMoS heterobilayer.

THz conductivity of large area MoS and MoSe monolayers as well as their vertical heterostructure, MoSeMoS is measured in the 0.3-5 THz frequency range. Compared to the monolayers, the ultrafast THz reflectivity of the MoSeMoS heterobilayer is enhanced many folds when optically excited above the direct band gap energies of the constituting monolayers. The free carriers generated in the heterobilayer evolve with the characteristic times found in each of the two monolayers. Surprisingly, the same enhancement is recorded in the ultrafst THz reflectivity of the heterobilayer when excited below the MoS bandgap energy. A mechanism accounting for these observations is proposed.