Manipulation of valley pseudospin in WSe2 monolayer by ultrafast laser pulses

Time-dependent density functional theory (TDDFT) combined with linear-response treatment Is used to study the propagation of light and valley pseudospin in the WSe2 monolayer. Electromagnetic field analysis is done by calculating the transmission and reflection. To get an understanding of valley pseudospin, we calculate the k-resolved electron populations. We observed that the valley degeneracy is lifted and only K or Kʹ electron is excited by changing the helicity of the laser.OPTO Symposium on Photon and Beam Science 202

Spin–valley Hall phenomena driven by Van Hove singularities in blistered Graphene

Using first-principles calculations, we investigate the possibility of realizing valley Hall effects (VHE) in blistered graphene sheets. We show that the Van Hove singularities (VHS) induced by structural deformations can give rise to interesting spin–valley Hall phenomena. The broken degeneracy of spin degree of freedom results in spin-filtered VH states and the valley conductivity have a Hall plateau of ±e2/2h, while the blistered structures with time-reversal symmetry show the VHE with the opposite sign of σK/K′xy (e2/2h) in the two valleys. Remarkably, these results show that the distinguishable chiral valley pseudospin state can occur even in the presence of VHS induced spin splitting. The robust chiral spin–momentum textures in both massless and massive Dirac cones of the blistered systems indicate significant suppression of carrier back-scattering. Our study provides a different approach to realize spin-filtered and spin-valley contrasting Hall effects in graphene-based devices without any external field

Linear and nonlinear optical response of WSe2 monolayer by chiral resonant pulses

We have studied the linear and nonlinear optical response of WSe2 monolayer by the classical Maxwell equations combined with TDKS equations to describe the propagation of electromagnetic fields in thin 2D layers. WSe2 monolayer is chosen because of strong SOC and its most promising phenomena of valleytronics. Laser intensity dependence of the valley polarization and light propagation in terms of the transmitted and reflected high harmonic generation (HHG) is also investigated.OPIC 202

Valley polarization control in WSe2 monolayer by a single-cycle laser pulse

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 WSe_{2} 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^{\u27}-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

Optical response and valley pseudospin of WSe2 monolayer: 2D Maxwell scheme

We present an effective method based on the Maxwell-TDDFT scheme, the combined classical Maxwell plus time-dependent Kohn-Sham (TDKS) equations to describes the propagation of electromagnetic fields in the weak field limit. The present scheme has a great advantage to analyze electron dynamics as well as electromagnetic field analysis with ultrashort pulse without extra computational cost. Our theoretical de-scription named 2D Maxwell is appropriate for extremely thin layers. Results of electron dynamics ob-tained by 2D Maxwell agree well with the results of TDKS which validates our description.ViCPEAC-202