Sarah Houver, Université Paris Cité, CNRS, Matériaux et Phénomènes Quantiques, F-75013 Paris, France

The next LPEM seminar (hybrid format) will be given Thursday June 8 at 2:00 pm (Paris time)
Room Holweck, Building C, 1st Floor

Exploring electron dynamics with 2D-THz spectroscopy

Former : Institute of Quantum Electronics, ETH Zürich, Switzerland

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, has attracted increasing attention given that many elementary interactions like lattice or magnetic resonances are 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. 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. Using 2D-THz spectroscopy, I will show that we can follow the continuous ballistic trajectory of the out-of-equilibrium electron population in InSb, highlighting some band-curvature features, like the band anisotropy along different crystallographic axes. If time, I will conclude showing some preliminary results obtained in the study of topological semimetals, in particular the Weyl semimetal TaAs.

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