Coherent Light-Matter Interactions in Monolayer Transition-Metal Dichalcogenides by Edbert Jarvis Sie

Author:Edbert Jarvis Sie
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

The observed spectra include both the coherent signals from the optical Stark effect and the incoherent signals from the exciton population that is unavoidably generated by the above-resonance optical pumping [10, 11, 18–25]. The coherent signals are known to appear only within the pump pulse duration, whereas the incoherent signals remain after the pulsed excitation. Their different time dependences allow us to separate them by monitoring the temporal evolution of Δα. Figure 5.5a, b shows the time traces after the excitation with 200 fs laser pulses at photon energy hν = 2.03 eV. At pump-probe delay Δt > 1 ps, Δα is similar for both valleys, with positive value at the probe energy of 1.84 eV but negative value at 1.95 eV. These features correspond to the exciton population effects, where the slow rise in Fig. 5.5a shares the same timescale with the intervalley scattering. This suggests that the initial population-induced dynamics arises from the same scattering mechanism, which can be mediated by defects and electron-phonon interactions. At zero pump-probe delay, however, the two valleys exhibit significantly different response. The difference can be attributed to the optical Stark effect, a coherent process that follows the pump pulse intensity profile. At probe energy 1.84 eV, the coherent contribution is particularly prominent and can be readily separated from the incoherent background by direct extrapolation (insets of Figs. 5.5a, b and 5.10 in the Supplementary Sect. 5.6.2).

Fig. 5.5(a, b) Time traces of Δα induced by σ − pump pulses (hν = 2.03 eV) and monitored at probe energy of 1.84 and 1.95 eV with different helicities. The top inset shows the curve-fitting decomposition of the coherent and incoherent signals. The bottom inset shows the valley contrast of the signals, Δα(σ +) − Δα(σ −), where the two curves are offset for clarity

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