Optical Dispersion Of Valley-Hybridised Coherent Excitons With Momentum-Dependent Valley Polarisation In Monolayer Semiconductor

2D excitons in transition-metal dichalcogenides (TMDCs) offer interesting effects related to the valley pseudo-spin degree of freedom and long-range exchange interactions, as well as the coupling with light states. Several theoretical predictions have claimed that the neutral exciton of TMDCs splits into a transversal and longitudinal exciton branch, with the longitudinal one, which is the upper branch, exhibiting an extraordinary strong dispersion in the meV range within the light cone. Historically, this was linked for semiconductor quantum wells to strong far-field optical dipole coupling, or strong electronic long-range exchange interactions. Recently, experiments utilizing Fourier-space spectroscopy have shown that the exciton (exciton-polariton) dispersion can indeed be measured for high-quality hexagonal-BN-encapsulated WSe2 monolayer samples and can confirm the energy scale. Here, the exciton fine-structure's pseudo-spin and the valley polarisation are investigated as a function of the centre-of-mass-momentum and excitation-laser detuning. For quasi-resonant excitation, a strong dispersion featuring a pronounced momentum-dependent helicity is observed. By increasing the excitation energy step-wise towards and then above the electronic band gap and the B-exciton level, the dispersion and the helicity systematically decrease due to contributions of incoherent excitons and emission from plasma. The decline of the helicity with centre-of-mass momentum can be phenomenologically modelled by the Maialle-Silva-Sham mechanism using the exciton splitting as the source of an effective magnetic field. By contributing to a better understanding of valley decoherence effects and the role of hybridised states in the optoelectronic properties, polarisation-sensitive Fourier-space investigations can support the development of future 'optical-valleytronic' devices.