Injection and population inversion in electrically induced p-n junction in graphene with split gates

We study electron and hole injection processes in a forward biased p-n junction electrically induced in a graphene heterostructure with split gates and calculate the ac conductivity associated with the interband and intraband transitions under the conditions of population inversion. It is shown that the net conductivity can be negative in the terahertz range of frequencies, so that the electrically induced p-n junctions in graphene heterostuctures might be used in sources of coherent terahertz radiation.