  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.
