  Monolayers of transition metal dichalcogenides are semiconducting materials
which offer many prospects in optoelectronics. A monolayer of molybdenum
disulfide (MoS2) has a direct bandgap of 1.88 eV. Hence, when excited with
optical photon energies below its bandgap, no photocarriers are generated and a
monolayer of MoS2 is not of much use in either photovoltaics or photodetection.
Here, we demonstrate that large size MoS2 monolayer sandwiched between two
graphene layers makes this heterostructure optically active well below the band
gap of MoS2. An ultrafast optical pump-THz probe experiment reveals in
real-time, transfer of carriers between graphene and MoS2 monolayer upon
photoexcitation with photon energies down to 0.5 eV. It also helps to unravel
an unprecedented enhancement in the broadband transient THz response of this
tri-layer material system. We propose possible mechanism which can account for
this phenomenon. Such specially designed heterostructures, which can be easily
built around different transition metal dichalcogenide monolayers, will
considerably broaden the scope for modern optoelectronic applications at THz
bandwidth.
