<p>All-optical deep neural networks offer significant improvements in processing speed and energy efficiency compared with traditional electronic systems. However, achieving broadband all-optical signal transmission with ultrafast processing speed in conventional photonic waveguides remains challenging. In this work, we present an integrated photonic circuit incorporating vertically grown molybdenum disulfide (MoS₂), which is also compatible with wafer-scale manufacturing. By leveraging the saturable absorption properties of MoS₂, this platform enables all-optical nonlinear activation over both the telecommunication O-band and C-band, with response speeds as fast as ~ 10 ps. This broadband operability enables flexible deployment as an optical activation function for neural network inference. Using a standardized evaluation framework, we benchmarked task-level performance against mainstream digital activation functions across diverse network architectures and quantization settings. These findings highlight the potential of van der Waals materials like MoS₂ for enabling in-situ nonlinearity in all-optical neural network hardware, advancing the development of scalable photonic computing systems.</p>