Observation of dark doublets induced by spin-flip processes in WSe₂

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Optics

Background:

• Transition metal dichalcogenides (TMDs) are 2D materials with potential for single-photon emitters (SPEs).
• Defect-induced excitons in TMDs typically exhibit bright, spin-allowed transitions.
• Spin-forbidden transitions are usually absent in flat 2D materials.

Purpose of the Study:

• To engineer curved WSe2 monolayers to investigate novel excitonic phenomena.
• To identify and characterize dark doublets associated with spin-flip transitions of intravalley defect excitons.
• To explore the influence of material curvature on exciton behavior and optical properties.

Main Methods:

• Fabrication of curved WSe2 monolayers using nanopillar-induced curvature.
• Optical spectroscopy to characterize the emission properties of defect-induced excitons.
• Polarization-resolved measurements to analyze the emission characteristics of bright and dark excitons.

Main Results:

• Direct identification of dark doublets linked to spin-flip transitions in curved WSe2.
• Observation of unique linear polarization for dark doublets, differing by a 45° phase from bright doublets.
• Emergence of new fine structure splitting due to exchange interactions coupling bright and dark intravalley transitions.
• Evidence of mixed in-plane and out-of-plane magnetic field effects due to curvature, resulting in spin-flip effects even in out-of-plane configurations.
• Determination of a large in-plane g-factor (4.5) indicative of the mixed magnetic configuration.

Conclusions:

• Material curvature enables the observation of previously inaccessible dark excitonic states in TMDs.
• The coupling between dark and bright excitonic states is significantly influenced by engineered nanostructuring.
• These findings provide a deeper understanding of exciton dynamics in curved 2D materials, paving the way for advanced quantum applications.