  We propose and evaluate the heterostructure based on the graphene-layer (GL)
with the lateral electron injection from the side contacts and the hole
vertical injection via the black phosphorus layer (PL) (p$^+$PL-PL-GL
heterostructure). Due to a relatively small energy of the holes injected from
the PL into the GL (about 100 meV, smaller than the energy of optical phonons
in the GL which is about 200 meV), the hole injection can effectively cool down
the two-dimensional electron-hole plasma in the GL. This simplifies the
realization of the interband population inversion and the achievement of the
negative dynamic conductivity in the terahertz (THz) frequency range enabling
the amplification of the surface plasmon modes. The later can lead to the
plasmon lasing. The conversion of the plasmons into the output radiation can be
used for a new types of the THz sources.
