In layered semiconductors, such as transition metal dichalcogenides (TMDs), the electron-electron interaction is strongly enhanced by both quantum confinement and reduced screening. Furthermore, in these materials the valley polarization can be optically controlled by means of circularly polarized light. Here we discuss the ultrafast optical response of TMDs, focusing on two aspects: time-resolved measurements of exciton dynamics and spin/valley relaxation processes. We first study exciton dynamics in single-layer (1L) MoS2 by broadband femtosecond transient absorption spectroscopy combined with time-resolved ab-initio simulations based on the non-equilibrium Green's functions and density-functional theory. This comparison indicates that the non-equilibrium optical properties of TMDs are influenced by the renormalisation of both band gap and exciton binding energies caused by photo-excited charge carriers. We further use sub-20-fs pump pulses tuned above the electronic gap to study the exciton formation process in 1L-MoS2, and determine a build-up time of ≈ 30 fs. This is consistent with a scenario where free carriers, initially excited above the quasi-particle gap, relax towards lower energy states and finally form the exciton state via the strong Coulomb interaction. The exciton valley relaxation dynamics is investigated by time-resolved Faraday rotation. We observe a double exponential decay, with an initial fast (~200fs) decay due to scattering of spin-polarized excitons from K to K’ valley. This is in good agreement with the time scale predicted by the Maialle-Silva-Sham electron/hole exchange interaction mechanism, which can be interpreted as a virtual annihilation of a bright exciton in one valley followed by the creation of an exciton in the opposite valley. Finally, use two-colour helicity-resolved pump-probe spectroscopy in order to disentangle the intervalley and intravalley spin-flip processes of electrons in the conduction band of 1L-WS2. Spin-polarized carriers are injected by a circularly polarized pump pulse resonant with the A exciton, while the co-circularly polarized probe pulse is tuned around the B excitonic peak. The scattering of the electrons from the upper to the lower conduction band level (where they cannot radiatively recombine) is detected by measuring the build-up dynamics of the bleaching signal around the B exciton caused by Pauli blocking.

Ultrafast carrier and spin dynamics in two-dimensional semiconductors

256th National Meeting (2018)

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