Direct Optical Evidence of Free Excitons in a Monolayer Quantum Material and Effective-Mass Measurements

While Fourier-space-resolved spectroscopy is an established technique for detecting and analysing exciton-polariton branches and effective polariton masses [1,2], it has not been explicitly used to determine effective masses for excitons in semiconductors owing to their high effective masses. While other techniques may provide electron or hole effective masses [3], signatures of excitons [4,5], that allow to infer indirectly an effective exciton mass, our measurements demonstrate the unambiguous existence and presence of free excitons with centre-of-mass propagation in a high-quality monolayer sample probed optically under proper conditions. Here, we determine the two-dimensional exciton's effective mass as a function of the excitation conditions and temperature for a WSe₂ monolayer encapsulated by few-layer h-BN (cf. fig.1).