Anisotropic third-harmonic generation of exfoliated TiS₃ nanoribbons

Van der Waals (vdW) materials have gained significant interest in the context of orientation-dependent linear and nonlinear optical properties, finding many potential applications in optoelectronics, thermoelectrics, and nanophotonics. Titanium trisulfide (TiS3) is a vdW layered material known for its strong structural in-plane anisotropy with quasi-one dimensional (1D) properties, but its nonlinear optical response is not well understood. Here, we report on a highly anisotropic third-harmonic generation (THG) response in TiS3 nanoribbons as a result of the reduced in-plane crystal symmetry. Furthermore, the third-order nonlinear susceptibility for TiS3 is retrieved from the thickness-dependent THG emission. Our results not only advance understanding in nonlinear light-matter interaction in anisotropic vdW materials, but also promote their usage in future electronic and optoelectronic innovations such as nonlinear optical signal processors and polarized-light photodetectors.