Continuous Wave Sum Frequency Generation and Imaging of Monolayer and Heterobilayer 2D Semiconductors

Optical nonlinearities in two-dimensional transition metal dichalcogenides (TMDs) are greatly enhanced by excitonic resonances. Here we report continuous-wave second harmonic and sum frequency generation from two-dimensional TMD monolayers and their heterostructures, with pump irradiances several orders of magnitude lower than conventional pulsed experiments. The high nonlinear efficiency originates from above-gap excitons in the band nesting regions, as revealed by wavelength-dependent second order optical susceptibilities quantified in four common monolayer transition metal dichalcogenides. Using sum frequency excitation spectroscopy and imaging, we identify and distinguish one- and two-photon resonances in both monolayers and heterobilayers. Data for heterostructures reveal responses from constituent layers accompanied with nonlinear signal correlated with interlayer transitions. Furthermore, we demonstrate spatial mapping of heterogeneous interlayer coupling by sum frequency and second harmonic confocal microscopy on heterobilayer MoSe₂/WSe₂.