Non-Centrosymmetric $γ$-Phase GaS Nanobelts for On-Chip Nonlinear Photonic Applications

Second-order nonlinear optical processes in van der Waals (vdW) semiconductors offer a compelling route toward compact, integrable photon-conversion platforms. Among III-VI vdW semiconductors, GaS is particularly attractive owing to its wide bandgap suppressing two-photon absorption under near-infrared laser excitation. However, bulk GaS typically crystallizes in the centrosymmetric $\beta$ phase, which eliminates second-order nonlinearity and severely limits its application in nonlinear photonics. Here, we demonstrate that GaS nanobelts synthesized via self-catalyzed vapor-liquid-solid growth predominantly crystallize in non-centrosymmetric $\gamma$-phase stacking. This behavior originates from edge-selective growth kinetics at the Ga catalyst interface, which stabilizes the $\gamma$ phase and enables deterministic in-plane dipole moment alignment. The GaS nanobelts exhibit strong second-harmonic generation (SHG) with intensities comparable to those of GaSe, a widely used nonlinear optical material. Furthermore, we integrate the nanobelts onto SiN waveguides and demonstrate efficient on-chip SHG and sum-frequency generation. These results establish $\gamma$-GaS nanobelts as a transferable one-dimensional nonlinear materials well suited for on-chip photonic integration and indicate their strong potential for nonlinear optical applications.