Enhancement in optically induced ultrafast THz response of MoSe₂MoS₂ heterobilayer

THz conductivity of large area MoS₂ and MoSe₂ monolayers as well as their vertical heterostructure, MoSe₂MoS₂ is measured in the 0.3-5 THz frequency range. Compared to the monolayers, the ultrafast THz reflectivity of the MoSe₂MoS₂ 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 MoS₂ bandgap energy. A mechanism accounting for these observations is proposed.