  Graphene and other two-dimensional crystals can be combined to form various
hybrids and heterostructures, creating materials on demand, in which the
interlayer coupling at the interface leads to modified physical properties as
compared to their constituents. Here, by measuring Raman spectra of shear
modes, we probe the coupling at the interface between two artificially-stacked
few-layer graphenes rotated with respect to each other. The strength of
interlayer coupling between the two interface layers is found to be only 20% of
that between Bernal-stacked layers. Nevertheless, this weak coupling manifests
itself in a Davydov splitting of the shear mode frequencies in systems
consisting of two equivalent graphene multilayers, and in the intensity
enhancement of shear modes due to the optical resonance with several optically
allowed electronic transitions between conduction and valence bands in the band
structures. This study paves way for fundamental understanding into the
interface coupling of two-dimensional hybrids and heterostructures.
