Understanding the relevant interactions in materials and the dynamic response to ultrafast electromagnetic excitation has been a strong focus in condensed matter research. Driving pulses in the terahertz (THz) range are now attracting attention given that many elementary excitations like lattice or magnetic resonances are located here.
2D spectroscopy, recently extended to the THz frequency range from 1 THz to 25 THz, shows good potential for exciting and probing low-energy excitations. Recent demonstrations studied electronic and lattice coupling, 2-phonon coherence and magnon nonlinearities in different materials.
I will present the principle of 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 dominates the nonlinear optical response in the THz range. Using 2D THz spectroscopy, I will show that we can follow the continuous ballistic trajectory of the out-of-equilibrium electron population in the ( → X, K)-plane of InSb. By separating different contributions to the nonlinear response, we show that we can highlight some bandcurvature features like anisotropy. To better understand the system response at times when the pulses overlap, we simulate our results using the finite-difference time-domain technique (FDTD).