  Our ability to efficiently detect and generate far-infrared (i.e., terahertz)
radiation is vital in areas spanning from biomedical imaging to interstellar
spectroscopy. Despite decades of intense research, bridging the terahertz gap
between electronics and optics remains a major challenge due to the lack of
robust materials that can efficiently operate in this frequency range, and
two-dimensional (2D) type-II heterostructures may be ideal candidates to fill
this gap. Herein, using highly accurate many-body perturbation theory within
the GW plus Bethe-Salpeter equation approach, we predict that a type-II
heterostructure consisting of an electron rich C3N and an electron deficient
C3B monolayers can give rise to extraordinary optical activities in the mid- to
far-infrared range. C3N and C3B are two graphene-derived 2D materials that have
attracted increasing research attention. Although both C3N and C3B monolayers
are moderate gap 2D materials, and they only couple through the rather weak van
der Waals interactions, the bilayer heterostructure surprisingly supports
extremely bright, low-energy interlayer excitons with large binding energies of
0.2 ~ 0.4 eV, offering an ideal material with interlayer excitonic states for
mid-to far-infrared applications at room temperature. We also investigate in
detail the properties and formation mechanism of the inter- and intra-layer
excitons.
