  Van der Waals heterostructures formed by two different monolayer
semiconductors have emerged as a promising platform for new optoelectronic and
spin/valleytronic applications. In addition to its atomically thin nature, a
two-dimensional semiconductor heterostructure is distinct from its
three-dimensional counterparts due to the unique coupled spin-valley physics of
its constituent monolayers. Here, we report the direct observation that an
optically generated spin-valley polarization in one monolayer can be
transferred between layers of a two-dimensional MoSe2-WSe2 heterostructure.
Using nondegenerate optical circular dichroism spectroscopy, we show that
charge transfer between two monolayers conserves spin-valley polarization and
is only weakly dependent on the twist angle between layers. Our work points to
a new spin-valley pumping scheme in nanoscale devices, provides a fundamental
understanding of spin-valley transfer across the two-dimensional interface, and
shows the potential use of two-dimensional semiconductors as a spin-valley
generator in 2D spin/valleytronic devices for storing and processing
information.
