Direct coupling of light to valley current.

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Electronics

Background:

• Two-dimensional (2D) semiconductors offer unique electronic properties based on the valley degree of freedom.
• Current methods using circularly polarized light primarily couple to valley charge, not valley current, limiting valleytronic device control.
• Lightwave manipulation of valley current is crucial for advanced valleytronic applications.

Purpose of the Study:

• To demonstrate direct lightwave control of valley current in 2D materials.
• To explore the underlying physical mechanism enabling valley current manipulation.
• To investigate the potential for ultrafast valleytronic devices.

Main Methods:

• Theoretical modeling using minimal tight-binding models for transition metal dichalcogenides and biased bilayer graphene.
• Simulations employing time-dependent density functional theory (TD-DFT) with transient excitonic effects.
• Analysis of the emergent vectorial character of few-cycle circularly polarized light pulses.

Main Results:

• Few-cycle circularly polarized light directly couples to valley current, contradicting previous understanding.
• An emergent momentum space valley dipole is identified as the key physical mechanism.
• Complete control over valley current direction and magnitude is achieved via the valley dipole.
• Valley currents are generated on femtosecond timescales (1-14 fs), potentially overcoming quantum decoherence.

Conclusions:

• Ultrafast light-matter interactions can unlock new degrees of freedom in 2D materials.
• The direct manipulation of valley current opens avenues for novel valleytronic devices.
• Emergent phenomena in the ultrafast regime offer exciting possibilities for quantum electronics.