Fast Real-Space Imaging of the Exciton Complexes in WSe₂ and WS₂ Monolayers Using Multiphoton Microscopy

Two-dimensional transition metal dichalcogenides possess unprecedentedly strong optical nonlinear properties associated with exciton complexes, which are highly promising for developing novel optoelectronics and nanophotonics. In this work, we investigate various exciton complexes that exist in WSe₂ and WS₂ triangular monolayers, through second-harmonic generation (SHG) and two-photon excited photoluminescence (2P-PL) images that are generated rapidly by multiphoton laser scanning microscopy. These large-scale images taken at different photon energies reveal the spatial distribution of the exciton complexes. The SHG images capture the exciton and trion resonances but not the biexciton resonance, despite a prominent biexciton signature in 2P-PL. The peculiar absence of biexciton signature is explained using time-dependent perturbation theory. SHG can also resonate with the band nesting states to produce images with high contrast, which is out of reach in conventional PL. By analyzing the crystal structure and growth dynamics of WS₂ monolayers using a high-angle annular dark-field scanning transmission electron microscope, we further establish the link between the oxidized triangular holes induced by S atom vacancies and the formation of biexcitons at the triangle edges. This newfangled approach using multiphoton microscopy shows that the spatial characteristics and excitation energy dependence of the exciton complexes are crucial for a deep understanding of the interactions between excitons and charged carriers and the two-dimensional materials in general.