Fig. 4: Exciton population and spin dynamics.

a Schematic of pump-probe Kerr measurement. A blue detuned (to X_A) circularly polarized light pulse for the pump and linearly polarized pulse for the probe were used. Upon reflection, the linearly polarized pulses turn elliptically polarized. By projecting the reflected elliptic pulse on a quarter waveplate and a Wollaston prism, we can separate out the relative strength of right and left circular components. The total probe intensity change is equivalent to net exciton density, and the intensity difference between left and right circular components gives the net spin-valley polarization. All the pump-probe measurements were done at 4 K. b, c False color map of transient reflection as a function of probe energy from domain I and II. White contours (without y-scale) are copied from Fig. 1d-inset for comparison. d, e Transient reflection at selected energies, marked by colored arrows in (b, c). The opposite trend in the amplitude of the TRΔR signal at the lower and higher energy states from the two domains is a direct consequence of opposite relative strength of these sub-features in the steady-state spectroscopic signal. f, g Traces of transient ellipticity at resonant energies in respective domains. The spin-valley lifetime in domain I is on the order of 10-15 ps, while in domain II the signal decays down to the detection noise-floor on sub-ps timescales.