(color online) (a) Helicity-resolved transient reflectance response (Δ R/ of the direct exciton at temperature of 10 K. The same (SCP, black trace) and opposite (OCP, red trace) circularly-polarized Δ R/R is recorded by σ⁺- and σ⁻-polarized probes under σ⁺-polarized pumping, respectively. The blue trace is the difference between the SCP and OCP signal, which is fitted by a single-exponential decay function shown in the magenta line. (b) Schematic of the spin flip and intervalley scattering process that induces spin relaxation in WSe₂ bilayer, under excitation of σ⁺-polarized light (red thick arrow). The red or blue thin arrow represents the spin up or down configuration of electrons in the conduction band.