Fig. 4.

Origin for PL polarization in bilayer. a The requirement for the switching of light helicity can be thought of as interconversion between circularly polarized excitons, ${\sigma }_{+}^{\mathrm{ex}}$ and ${\sigma }_{-}^{\mathrm{ex}}$. The emission helicity is tied to the spin of the electron in the exciton, which is formed out of electron–hole pairs in both valleys/layers. The hole spin is labeled by thick vertical arrows and in our convention have spin, which is opposite to that of the electron. Owing to spin–valley-layer locking, both spin and valley need to be flipped for the switching of helicity, in the absence of interlayer hopping. b At positive magnetic field, the selective conversion of ${\sigma }_{-}^{\mathrm{ex}}$ to ${\sigma }_{+}^{\mathrm{ex}}$ is energetically favorable due to the Zeeman splitting