Nonlinear spectroscopy of excitonic states in transition metal dichalcogenides

Methods in nonlinear optics attract much interest because they capture physical effects beyond linear spectroscopy. Here, the authors demonstrate that second harmonic generation (SHG) can be used to investigate details of the many-body effects, particularly the structure of excitonic states in low-dimensional materials. The dependence of the SHG signal on the polarization angle changes qualitatively with the frequency and intensity of the excitation light, which allows users to employ SHG to probe the fine structure of strongly bound excitonic states.