[Paper Review] Valley polarization control in WSe2 monolayer by a single-cycle laser pulse

Arqum Hashmi, Shunsuke Yamada+3 more|arXiv (Cornell University)|Oct 13, 2021

2D Materials and Applications34 references4 citations

TL;DR

This study demonstrates ultrafast valley pseudospin control in monolayer WSe2 using a single-cycle linearly polarized laser pulse with tunable carrier-envelope phase (CEP). By leveraging nonlinear optical responses and quantum interference via the massive Dirac model, the researchers achieve robust, CEP-dependent valley polarization that can be tuned by laser intensity, enabling sub-cycle manipulation of electron momentum in momentum space with potential for femtosecond valleytronic devices.

ABSTRACT

Abstract The valley degree of freedom in two-dimensional materials provides an opportunity to extend the functionalities of valleytronics devices. Very short valley lifetimes demand the ultrafast control of valley pseudospin. Here, we theoretically demonstrate the control of valley pseudospin in WSe2 monolayer by single-cycle linearly polarized laser pulse. We use the asymmetric electric field controlled by the carrier-envelope phase (CEP) to make the valley polarization between K and K'-point in the Brillouin zone (BZ). Time-dependent density functional theory with spin-orbit interaction reveals that no valley asymmetry and its CEP dependence is observed within the linear-optical limit. In the nonlinear-optical regime, linearly polarized pulse induces a high degree of valley polarization and this polarization is robust against the field strength. Valley polarization strongly depends and oscillates as a function of CEP. The carrier density distribution forms nodes as the laser intensity increases, our results indicate that the position of the carrier density in the BZ can be controlled by the laser intensity. From the analysis by the massive Dirac Hamiltonian model, the nodes of the carrier density can be attributed to the Landau-Zener-Stückelberg interference of wave packets of the electron wave function.

Motivation & Objective

• To achieve ultrafast, all-optical control of valley pseudospin in WSe2 monolayer on femtosecond timescales.
• To overcome the challenge of short valley lifetimes (10^3–10^6 fs) by enabling sub-cycle optical control.
• To investigate the role of laser intensity and carrier-envelope phase (CEP) in inducing and tuning valley polarization.
• To understand the origin of node formation in carrier density distribution in momentum space under strong laser fields.
• To establish a link between observed valley polarization and quantum interference mechanisms such as Landau-Zener-Stückelberg interference.

Proposed method

• Employed time-dependent density functional theory (TDDFT) with spin-orbit coupling (SOC) to simulate electron dynamics under intense laser fields.
• Used the open-source SALMON code to solve the TDDFT equations for electron and electromagnetic field dynamics in 2D WSe2.
• Applied single-cycle linearly polarized laser pulses with variable carrier-envelope phase (CEP) and intensity to excite the system.
• Performed analysis using a two-band massive Dirac Hamiltonian model to interpret quantum interference effects in the electron wave packets.
• Tracked valley polarization via population difference in conduction bands at K and K' points in the Brillouin zone.
• Calculated excitation probabilities and phase evolution to identify interference patterns linked to Stückelberg oscillations.

Experimental results

Research questions

• RQ1 Can a single-cycle linearly polarized laser pulse induce significant valley polarization in WSe2 monolayer?
• RQ2 How does the valley polarization depend on the carrier-envelope phase (CEP) of the laser pulse?
• RQ3 What is the role of laser intensity in modulating valley polarization and inducing node structures in the carrier density distribution?
• RQ4 Can the massive Dirac model explain the observed quantum interference patterns and node formation in momentum space?
• RQ5 Is the valley pseudospin control robust against variations in laser field strength?

Key findings

• Valley polarization is induced only in the nonlinear optical regime and shows strong dependence on the carrier-envelope phase (CEP), with oscillatory behavior as a function of CEP.
• Valley polarization increases with laser intensity and reaches substantial levels (up to ~0.6 at 1×10^12 W/cm²), remaining robust against field strength variations.
• Distinct nodes in the carrier density distribution emerge in the Brillouin zone at high intensities, indicating momentum-space localization of electron wave packets.
• The node formation is attributed to Landau-Zener-Stückelberg interference of electron wave packets, confirmed by the two-band massive Dirac model.
• The phase of valley polarization undergoes a sign reversal at specific intensities due to interference between excitation pathways at K and K' points.
• Spin polarization mirrors valley polarization trends, indicating simultaneous control of both valley and spin degrees of freedom via optical excitation.