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

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.

References in corpus (12)

• Exciton Binding Energy and Nonhydrogenic Rydberg Series in Monolayer WS2
• Excitons in atomically thin transition metal dichalcogenides
• Dielectric screening in two-dimensional insulators: Implications for excitonic and impurity states in graphane
• Excitonic linewidth approaching the homogeneous limit in MoS2 based van der Waals heterostructures : accessing spin-valley dynamics
• In-plane Propagation of Light in Transition Metal Dichalcogenide Monolayers: Optical Selection Rules
• Bright excitons in monolayer transition metal dichalcogenides: from Dirac cones to Dirac saddle points
• Fine Structure and Lifetime of Dark Excitons in Transition Metal Dichalcogenide Monolayers
• Dark excitons and the elusive valley polarization in transition metal dichalcogenides
• Influence of the effective layer thickness on the groundstate and excitonic properties of transition-metal dichalcogenide systems
• Dependence of the hBN Layer Thickness on the Band Structure and Exciton Properties of Encapsulated WSe2 Monolayers
• Resonance profiles of valley polarization in single-layer MoS${}_2$ and MoSe${}_2$
• Observation of macroscopic valley-polarized monolayer exciton-polaritons at room temperature