Anisotropic Enhancement of Second-Harmonic Generation in Monolayer and Bilayer MoS₂ by Integrating with TiO₂ Nanowires

The ability to design and enhance the nonlinear optical responses in two-dimensional (2D) transition-metal dichalcogenides (TMDCs) is both of fundamental interest and highly desirable for developing TMDC-based nonlinear optical applications, such as nonlinear convertors and optical modulators. Here, we report for the first time a strong anisotropic enhancement of optical second-harmonic generation (SHG) in monolayer molybdenum disulfide (MoS₂) by integrating with one-dimensional (1D) titanium dioxide nanowires (NWs). The SHG signal from the MoS₂/NW hybrid structures is over 2 orders of magnitude stronger than that in the bare monolayer MoS₂. Polarized SHG measurements revealed a giant anisotropy in SHG response of the MoS₂/NW hybrid. The pattern of the anisotropic SHG depends highly on the stacking angle between the nanowire direction and the MoS₂ crystal orientation, which is attributed to the 1D NW-induced directional strain fields in the layered MoS₂. A similar effect has also been observed in bilayer MoS₂/NW hybrid structure, further proving the proposed scenario. This work provides an effective approach to selectively and directionally designing the nonlinear optical response of layered TMDCs, paving the way for developing high-performance, anisotropic nonlinear photonic nanodevices.