S. Houver – 7 novembre

In condensed matter systems, the different degrees of freedom, the electrons, the lattice or the spins can be coupled. The typical energy scale of these couplings, is such that their interactions fall in the picosecond down to femtosecond time scale. Understanding these interactions and their dynamics has been a strong focus in ultrafast condensed matter research. A typical scheme to investigate ultrafast processes involves the use of pump-probe techniques where a first sub-picosecond electromagnetic excitation (typically in the vis-NIR range) drives the system out of equilibrium, followed by an equally short probe of  the system excitation. By tuning the relative delay of the two pulses we can map the excitation and subsequent relaxation mechanisms of the system. More recently, using pulses in the terahertz (THz) range, both as pump and probe of the excitation, is attracting increasing attention given that many elementary interactions like lattice or magnetic resonances are located in this range.

In this seminar, I will give a general introduction to ultrafast processes in condensed matter systems and how terahertz light in particular can be used to study them. I will then focus my presentation on the 2D-THz spectroscopy technique which has recently shown interesting potentialities in exciting and probing low-energy interactions. Recent demonstrations highlighted electron-lattice coupling, 2-phonon coherence, magnon nonlinearities and phonon anharmonic coupling.
In particular I will focus on 2D-THz experiments in the range between 1-10 THz, for investigating the electronic band-nonlinearities for a low-bandgap semiconductor: InSb. In the very first picoseconds after excitation, coherent motion of electrons, driven by the THz electric field, dominates the nonlinear optical response. Using 2D THz spectroscopy, I will show that we can follow the continuous ballistic trajectory of the out-of-equilibrium electron population in the (Gamma → X, K)-plane of InSb. By separating different contributions to the nonlinear response, we show that we can highlight some band-curvature features, like the band anisotropy along different crystallographic axes. To better understand the system response in the early stage of the excitation process, we simulate the THz-material interaction using the finite-difference time-domain technique (FDTD).
I will then conclude showing some recent preliminary results obtained in the study of topological materials, such as Cd3As2 and TaAs.