Anisotropic Coulomb potential (grey surface) of the excitons. The isoenergy contours (white ellipses) and the projections (black lines) onto the two crystallographic directions highlight the large in-plane anisotropy. Although the a and b axes share a common 1 s level (light grey line), the degeneracy of the 2 p states is lifted. Consequently, the 1 s 2 p a and 1 s 2 p b transitions (vertical red lines) feature different energies that are interrogated by MIR transients (red waveforms) of orthogonal polarization ( E MIR || a and E MIR || b , respectively).
