Resonant optical Stark effect in monolayer WS₂.

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Optics

Background:

• Monolayer transition metal dichalcogenides exhibit valley degeneracy, exploitable for valleytronics.
• The valley-selective optical Stark effect (OSE) is key for manipulating valley superposition states.
• Existing models for OSE often use a two-level dressed-atom approximation, assuming noninteracting particles.

Purpose of the Study:

• To investigate the validity of the two-level dressed-atom model for OSE in monolayer WS2 near exciton resonance.
• To determine if many-body interactions are necessary to describe OSE in this regime.
• To propose an improved theoretical framework for OSE in low-dimensional semiconductors.

Main Methods:

• Experimental study of OSE in monolayer WS2.
• Excitation of the material near its exciton resonance.
• Comparison of experimental results with predictions from both two-level and excitonic models.

Main Results:

• The two-level dressed-atom model fails to accurately describe OSE near exciton resonance in monolayer WS2.
• An excitonic model, incorporating many-body Coulomb interactions, is required for accurate OSE description.
• Experimental data confirms the prediction of a dominant blue-shift due to virtual exciton interactions when detuned by less than the exciton binding energy.

Conclusions:

• The standard two-level model is insufficient for describing OSE in monolayer transition metal dichalcogenides near resonance.
• Many-body Coulomb interactions, specifically involving virtual excitons, play a critical role in OSE.
• The findings are likely applicable to other low-dimensional semiconductors with bound excitons and significant many-body effects.