Giant Modulation of Second and Third Harmonic Generations in in-Plane Ferroelectric NbOI₂ via MoS₂ Atomic Layers

Probing and manipulating the intriguing nonlinear optical responses in new two-dimensional (2D) van der Waals ferroelectrics are of great significance to the development of 2D material-based nonlinear optical devices. Herein, the simultaneous detection of second-harmonic generation (SHG) and third-harmonic generation (THG) optical responses in a recently discovered 2D in-plane ferroelectric material, NbOI₂ is reported, as well as the giant modulation of SHG and THG in NbOI₂ by integrating with atomically thin MoS₂. Both experimental and theoretical results show that NbOI₂, ranging from few-layer to thick-layer dimensions, exhibits robust SHG and THG responses with pronounced dependence on excitation polarization and wavelength. This behavior is attributed to the anisotropic band structure and excitonic resonance effects in NbOI₂. By interfacing a few-layer NbOI₂ with a monolayer MoS₂, the amplitude and polarization of both SHG and THG signals are effectively modulated, which can be further tailored by excitation wavelength and twist angle. The nonlinear optical control in MoS₂/NbOI₂ heterostructures is correlated with polar symmetry coupling, which is well modeled via nonlinear electromagnetic theory. This study thus provides a new material platform based on van der Waals ferroelectric heterostructures for dynamic control of nonlinear light intensity and polarization, paving the path for developing advanced nonlinear photonic nanodevices.