Optical Measurement of Pseudo-Spin Texture of the Exciton Fine-Structure in Monolayer WSe2 within the Light Cone

TL;DR

This study uses optical spectroscopy to measure the pseudo-spin texture of excitons in monolayer WSe2, revealing momentum-dependent helicity and dispersion that depend on excitation energy, advancing understanding of exciton fine-structure in 2D materials.

Contribution

It provides the first experimental measurement of exciton pseudo-spin and valley polarization dispersion in monolayer WSe2, confirming theoretical predictions and modeling the helicity behavior.

Findings

Strong momentum-dependent helicity observed at quasi-resonant excitation.

Dispersion and helicity decrease with increasing excitation energy.

Model aligns with Maialle-Silva-Sham mechanism for exciton splitting.

Abstract

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, describing two sides of the same coin. 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 polarization 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, 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.