  The efficient optical second-harmonic generation (SHG) of two-dimensional
(2D) crystals, coupled with their atomic thickness that circumvents the
phase-match problem, has garnered considerable attention. While various 2D
heterostructures have shown promising applications in photodetectors, switching
electronics, and photovoltaics, the modulation of nonlinear optical properties
in such hetero-systems remains unexplored. In this study, we investigate
exciton sensitized SHG in heterobilayers of transition metal dichalcogenides
(TMDs), where photoexcitation of one donor layer enhances the SHG response of
the other as an acceptor. We utilize polarization-resolved interferometry to
detect the SHG intensity and phase of each individual layer, revealing the
energetic match between the excitonic resonances of donors and the SHG
enhancement of acceptors for four TMD combinations. Our results also uncover
the dynamic nature of interlayer coupling, as evidenced by the dependence of
sensitization on interlayer gap spacing and the average power of the
fundamental beam. This work provides insights into how interlayer coupling of
two different layers can modify nonlinear optical phenomena in 2D
heterostructures.
