Speaker
Description
We present Spin, Time-, and Angle-Resolved Photoemission Spectroscopy (STARPES) measurements of photoexcited free carriers in centrosymmetric bulk 2H-WSe2. This setup combines a high harmonic generation (HHG) source with tunable repetition rate, pulse duration, photon energy, and polarization, and a hemispherical analyzer with a 3D VLEED spin detector.
The electron spin polarization dynamics in photoexcited 2H-WSe2 have been studied in different regimes. In the excitonic case$^{1}$, resonant pumping near the K-valley optical bandgap with a fluence below the critical Mott density creates bound excitons. Strong spin-integrated circular dichroism is then reported, along with valley and helicity dependence of the bright excitons’ (Κ, Κ’) spin polarization decaying within 100fs. In contrast, the momentum-forbidden dark exciton (electrons scattered into the Σ valleys) exhibits a time- and helicity-independent spin polarization, acting as a momentum-space spin-reservoir. At higher fluence$^{2}$, the system breaks into an electron-hole plasma, where Σ-valley spin polarization becomes valley- and helicity-dependent. Additionally, a spin polarization flip at longer timescales is observed for a single pump helicity, attributed to the intrinsic conduction band spin polarization. Both regimes yield distinct spin polarization behaviors, yet remain constrained by optical selection rules imposed by resonant pumping.
Here, we study the free carrier regime, accessed by pumping well above resonance with a 2.4eV, 350fs pulse. We observe free carriers in K and Σ valleys, finding a decay time at K of an order of magnitude slower than in the resonant case, comparable to that at Σ. Due to the large number of allowed transitions, scattering processes and spin polarization dynamics are expected to be significantly more complex than in the resonant case.
At time zero, the Κ and Σ spin polarization sign and magnitude are valley dependent but helicity independent. Furthermore, no spin polarization reversal is observed at longer timescales either at Κ or Σ. These results differ from both the excitonic and electron-hole plasma resonant regimes, providing a better insight into the complex spin dynamics in 2H-WSe2.
References
- Fanciulli et al., Phys. Rev. Lett. 131(6), 066402 (2023)
- Hedwig et al., Phys. Rev. Lett. 135(18), 186903 (2025)