All-optical valley switch and clock of electronic dephasing

Area of Science:

• Condensed Matter Physics
• Materials Science
• Nanotechnology

Background:

• Two-dimensional (2D) materials with broken inversion symmetry utilize valley pseudospin for conducting carriers.
• Circularly-polarized light can achieve high valley polarization, enabling valleytronics.
• Short valley lifetimes hinder practical applications due to challenges in switching valley polarization.

Purpose of the Study:

• To introduce a coherent control protocol for an ultrafast optical valley switch.
• To enable switching of valley polarization on timescales faster than decoherence and depolarization.
• To theoretically demonstrate the protocol for hexagonal boron nitride (hBN) and molybdenum disulfide (MoS₂) monolayers.

Main Methods:

• Utilized trains of attosecond laser pulses with controlled phase and polarization.
• Developed a coherent control protocol for valley polarization manipulation.
• Performed first-principles calculations for hBN and MoS₂ monolayers.
• Employed time-delayed, perpendicularly polarized, linearly-polarized pulses to measure dephasing time.

Main Results:

• Demonstrated an ultrafast optical valley switch capable of turning on, off, and switching valley polarization.
• Achieved switching on timescales faster than electron-hole decoherence and valley depolarization.
• Successfully extracted the electronic dephasing time (T₂) from the valley Hall conductivity measurement.

Conclusions:

• The proposed coherent control protocol offers a viable method for ultrafast valley switching in 2D materials.
• This advancement is crucial for the practical implementation of valley-based transistors and other valleytronic devices.
• The technique provides a novel approach to measure fundamental electronic properties like dephasing time.