2D semiconductors: Probing by broadband femtosecond continuum pulses

Transition metal dichalcogenides (TMDCs) are the focus of fundamental research and technological applications due to their novel electronic and optical properties. We demonstrate an effective microspectroscopy technique by tracing the dispersion of second-order nonlinear susceptibility χ⁽²⁾ to characterize the monolayers of WSe₂, WS₂, MoS₂, and MoSe₂ within the photon energy range of 2.4-3.2 eV. We then estimate, with a fairly good precision, the fundamental bandgap and exciton binding energy of these semiconductors. We also provide the absolute value of χ ⁽²⁾. To perform the experiment, ultra-broadband continuum pulses served as the fundamental beam while its second harmonic generation (SHG) spectrum in visible and ultraviolet (UV) was detected and analyzed with better than 0.3 nm spectral resolution (<2 meV). Our results show that the experimental dispersion data provide the clear resolution of the near band-gap exciton states in comparison with traditional linear methods that target measurement of the linear dielectric function.