Fast revealing the spatial and energy distribution of exciton complexes in WSe₂ and WS₂ using multiphoton microscopy

Kuang-I Lin^1*, Shiue-Yuan Shiau², Shu-Bai Liu¹, Chien-Liang Tu³, Chang-Hsiao Chen³

¹Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan, Taiwan
²Physics Division, National Center for Theoretical Sciences, Hsinchu, Taiwan
³Department of Electrical Engineering, National Sun Yat-sen University, Kaohsiung, Taiwan

* Presenter:Kuang-I Lin, email:kilin@mail.ncku.edu.tw

Two-dimensional (2D) transition metal dichalcogenides (TMDs) possess fascinating optical nonlinear properties associated with exciton complexes that can be exploited for developing rapid imaging technology [1-5]. In this work, we investigate various exciton complexes that exist in WSe₂ and WS₂ monolayers, with particular emphasis on the spatial and energy distribution of these bound states. By using a multiphoton laser scanning microscope to fast generate second-harmonic generation (SHG) and two-photon excited photoluminescence (2P-PL) images, we reveal the spatially varying resonant intensities of the exciton complexes in these triangular monolayers. The exciton and trion resonances appear in SHG images, but the naively expected enhancement at the biexciton energy is absent in SHG, despite a prominent biexciton signature in 2P-PL; this peculiar absence is explained using time-dependent perturbation theory. SHG also provides fast mapping regardless of the optical band gap and resonates with the band nesting energy, which cannot be implemented in conventional PL. We then analyze the crystal structure and growth dynamics of WS₂ monolayers using high-angle annular dark-field scanning transmission electron microscope, and 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 made possible by multiphoton microscopy to study the spatial characteristics and excitation energy dependence of the exciton complexes in TMD monolayers proves crucial for providing a deeper understanding of 2D materials and their different carrier densities.
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2. Rosa, H. G. et al., Adv. Opt. Mater. 6, 1701327 (2018).
3. Karvonen, L. et al., Nat. Commun. 8, 15714 (2017).
4. Lin, K.-I. et al., Nano Lett. 18, 793 (2018).
5. You, J. W. et al., Nanophotonics 8, 63-97 (2019).

Keywords: transition metal dichalcogenides, second-harmonic generation, exciton complexes, rapid imaging, defects