Generation and Evolution of Spin-, Valley-, and Layer-Polarized Excited Carriers in Inversion-Symmetric ${\mathrm{WSe}}_2$

We report the spin-selective optical excitation of carriers in inversion-symmetric bulk samples of the transition metal dichalcogenide (TMDC) ${\mathrm{WSe}}_2$. Employing time- and angle-resolved photoelectron spectroscopy (trARPES) and complementary time-dependent density functional theory (TDDFT), we observe spin-, valley-, and layer-polarized excited state populations upon excitation with circularly polarized pump pulses, followed by ultrafast ($<100\mathrm{fs}$) scattering of carriers towards the global minimum of the conduction band. TDDFT reveals the character of the conduction band, into which electrons are initially excited, to be two-dimensional and localized within individual layers, whereas at the minimum of the conduction band, states have a three-dimensional character, facilitating interlayer charge transfer. These results establish the optical control of coupled spin-, valley-, and layer-polarized states in centrosymmetric materials with locally broken symmetries and suggest the suitability of TMDC multilayer and heterostructure materials for valleytronic and spintronic device concepts.

Figure 1

(a) Sketch of the local band structure and the spin-valley correlation at the $K$ and $K^{\prime }$ points of two noninteracting layers with $2H$ stacking. The two layers together comprise the unit cell of both bilayer and bulk crystal, in which the spin-polarization vanishes for every $k$ point. (b) Electronic band structures of bilayer ${\mathrm{WSe}}_2$ calculated by DFT. The red arrow and the shaded rectangle indicate the optical excitation and the experimentally investigated part of the electronic structure, respectively. (c) Electron intensity maps at various pump-probe delays obtained with linearly polarized pump pulses (logarithmic color scale). The overlaid valence and conduction bands are obtained by DFT calculated for a bilayer, with the conduction bands shifted 250 meV upwards to match the experimental observations.

Figure 2

(a) Energy distribution curves of the excited state signal at K and $K^{\prime }$, 15 fs after excitation with linearly and circularly polarized optical pump pulses. (b) Excited state photoelectron intensity at the $K$ point (green triangles) and the $K^{\prime }$ point (blue squares) at a fixed pump-probe delay as a function of the angle of a quarter-wave plate in the pump beam. The lower panel shows the measurement (squares) and fit (lines) of the parallel component of the pump polarization, from which the polarization state in dependence of the wave plate angle is deduced. (c) Evolution of excited state population at the $K$ and $K^{\prime }$ points in the upper half of a ${\mathrm{WSe}}_2$ bilayer for linear and circular (${\sigma }^{-}$) pump polarization from TDDFT simulations. The light gray oscillation indicates the applied electric field of the simulation.

Figure 3

(a) Temporal evolution of the excited state signal in the $K$ (squares) and $\mathrm{\Sigma }$ (circles) valleys for different pump polarizations. The solid lines are taken from a scattering model (see text). (b) Top: sketch of the BZ of bulk ${\mathrm{WSe}}_2$. Bottom: Dispersion of the conduction band states in bulk ${\mathrm{WSe}}_2$ along the $K\text{−}H$ and $\mathrm{\Sigma }\text{−}X$ directions as defined above. (c) Cut through the $\mathrm{\Gamma }\text{−}K\text{−}M$ plane of the Brillouin zone of a single ${\mathrm{WSe}}_2$ trilayer including $K$ and $\mathrm{\Sigma }$ valleys. The orange arrows indicate possible electronic scattering processes between the valleys.

Figure 4

Sketch of the electronic processes in reciprocal space (top panel) and real space electron densities integrated over one in-plane direction of the wave functions at the respective $k$ points in the conduction band (bottom panel). The large red arrows symbolize the electronic excitation as a consequence of ${\sigma }^{-}$-polarized pump pulses. Electronic scattering from the $K$ to the $\mathrm{\Sigma }$ valleys is indicated by the dashed blue arrows. The spin character of the bands is represented by the small blue and red arrows. The dashed box in the lower left panel marks the real space unit cell.