  The interlayer coupling is emerging as a new parameter for tuning the
physical properties of two-dimensional (2D) van der Waals materials. When two
identical semiconductor monolayers are stacked with a twist angle, the periodic
interlayer coupling modulation due to the moir\'e superlattice may endow exotic
physical phenomena, such as moir\'e excitons and correlated electronic phases.
To gain insight into these new phenomena, it is crucial to unveil the
underlying coupling between atomic layers. Recently, the rhombohedral-stacked
transition metal dichalcogenide (TMD) bilayer has attracted significant
interest because of the emergence of an out-of-plane polarization from
non-ferroelectric monolayer constituents. However, as a key parameter
responsible for the physical properties, the interlayer coupling and its
relationship with ferroelectricity in them remain elusive. Here we probe the
asymmetric interlayer coupling between the conduction band of one layer and the
valence band from the other layer in a 3R-MoS2 bilayer, which can be understood
as a result of a layer-dependent Berry phase winding. By performing optical
spectroscopy in a dual-gated device, we show a type-II band alignment exists at
K points in the 3R-MoS2 bilayer. Furthermore, by unraveling various
contributions to the band offset, we quantitatively determine the asymmetric
interlayer coupling and spontaneous polarization in 3R-MoS2.
