Orientation-Related Giant Third Harmonics Generation in Quasi-One-Dimensional ZrS₃ Crystals

Exploring strong nonlinear optical (NLO) response and high NLO anisotropy materials is crucial for nonlinear nanophotonic devices with a polarization function. Here, we report a quasi-one-dimensional (quasi-1D) ZrS₃ nanobelt exhibiting ultrastrong third harmonics generation (THG) with a maximum third-order susceptibility (χ⁽³⁾) of 67.9 × 10^-18 m²/V² for a 24.3 nm thickness, which is approximately 3 orders of magnitude higher than most 2D materials. This thickness-dependent NLO coefficient (χ⁽³⁾) decreases from 4.88 × 10^-18 to 1.14 × 10^-18 m²/V² with the variation in nanobelt thickness from 15.6 to 131 nm, which is explained by the phase matching confinement effect. More importantly, an anisotropic ratio of THG as large as 61.5 is confirmed, with the b-axis being the high THG polarization orientation. This orientation-dependent giant THG in quasi-1D ZrS₃ crystals will provide potential for the development of future nonlinear nanophotonics with a polarization function.