The discovery of graphene, with its extraordinary mechanical and optical properties, has triggered an intense research activity on two-dimensional (2D) materials, such as transition metal dichalcogenides (TMDs), which promise a whole new class of applications in optoelectronics and photonics. Of key importance for the design and optimization of devices based on 2D materials is a fundamental understanding of their ultrafast photophysics, i.e. of their complex non-equilibrium response following light excitation. In this talk I will present several examples of studies of the ultrafast optical response of 2D materials.

For single-layer (1-L) graphene two-colour pump-probe spectroscopy with 10-fs time resolution will be used to study the primary relaxation process following photoexcitation in the Dirac cone, i.e. thermalization due to electron-electron interaction, giving rise to an equilibrated Fermi-Dirac electron distribution. For the prototypical TMD MoS₂, broadband transient absorption microscopy on 1-L flakes, combined to ab initio simulations, allows to establish the dominant role of bandgap renormalization, stemming from the partial compensation of electronic band-gap shrinkage and exciton binding energy reduction induced by the photo-excited carriers. Finally, valley relaxation dynamics is studied in 1-L MoS2 by time-resolved Faraday rotation, which evidences a loss of valley polarization on the 200-fs timescale, in agreement with a model including the intervalley electron-hole Coulomb exchange as the dominating mechanism.


Monday, October 5, 2015, 12:00. Seminar Room

Hosted by Prof. Simon Wall