Single-layer Transition Metal Dichalcogenides (TMDs) are two-dimensional semiconductors attracting a great interest for ultrathin optoelectronic devices because of their unique electronic and optical properties. The present talk discusses the ultrafast optical response of these materials, focusing on two aspects: time-resolved measurements of exciton dynamics and spin/valley relaxation processes.
In the first part of the talk will present a time-resolved study of exciton dynamics in single-layer MoS2 (1L-MoS2) by ultrafast transient absorption spectroscopy. The relaxation dynamics of the photo-excited states is measured on a broad energy range. The equilibrium optical response of TMDs is strongly renormalized by excitonic effects which are enhanced by the reduced Coulomb screening. The non-equilibrium optical response is characterized by prominent features, each consisting of a bleaching at the energies of the excitonic transitions and a red-shifted photoinduced absorption. The experimental data are compared with first-principle simulations that combine non-equilibrium Green's functions with density-functional theory (DFT) methods. The comparison shows that the mere Pauli blocking effect of the photoexcited transition cannot account for the experimental results. Instead, a Band Gap Renormalization (BGR) process, i.e. a transient reduction of the quasi-particle optical gap and the exciton binding energy caused by the presence of photo-excited carriers, can quantitatively reproduce the MoS2 non-equilibrium optical response .
The second part of the talk deals with the emerging concept of valleytronics, which has been recently introduced in TMDs in order to describe the possibility to use the valley index as a way to store information. The spin/valley relaxation dynamics in 1L-MoS2 is measured combining time-resolved Faraday rotation (TRFR) and time-resolved circular dichroism (TMCD). Both the experiment clearly show that the depolarization dynamics occurs on an extremely fast time scale (i.e. 200 fs). On a slower timescale, a residual component of the valley polarization, lasting few ps, is detected. Further studies of the spin/valley relaxation dynamics in 1L-WS2 are presented: by using two-colours helicity-resolved pump probe spectroscopy we directly measure the intravalley spin-flip process of electrons in the conduction band of single-layer WS2. This process occurs on a sub-ps time scale and it is the first direct measurements of the bright-to-dark exciton transition occurring in W-based 2D materials.
 E.A.A. Pogna et al. ACS Nano, 2016, 10 (1), pp 1182–1188
 S. Dal Conte, et al., Phys. Rev. B 92 (2015) 235425
 Z. Wang, S. Dal Conte et al. Manuscript in preparation