Giant pattern evolution in third-harmonic generation of strained monolayer WS₂ at two-photon excitonic resonance

Strong geometrical confinement and reduced dielectric screening of two-dimensional (2D) materials leads to strong Coulomb interaction and eventually give rise to extraordinary excitonic effects, which dominates the optical and optoelectronic properties. For nonlinear 2D photonic or optoelectronic applications, excitonic effects have been proved effective to tune the light-matter interaction strength. However, the modulation of excitonic effects on the other aspect of nonlinear response, i.e., polarization dependence, has not been fully explored yet. Here we report the first systemic study on the modulation of excitonic effects on the polarization dependence of second and third harmonic generation (SHG and THG) in strained monolayer WS₂ by varying excitation wavelength. We demonstrated that polarization-dependent THG patterns undergo a giant evolution near two-photon excitonic resonance, where the long-axis of the parallel component (originally parallel to the strain direction) has a 90° flip when the excitation wavelength increases. In striking contrast, no apparent variation of polarization-dependent SHG patterns occurs at either two- or three-photon excitonic resonance conditions. Our results open a new avenue to modulate the anisotropic nonlinear optical response of 2D materials through effective control of excitonic resonance states, and thus open opportunity for new designs and applications in nonlinear optoelectronic 2D devices.