Broadband and ultra-compact all-optical nonlinear activation function enabled by VO ₂ -integrated photonics

Abstract We demonstrate a broadband and ultra-compact all-optical nonlinear activation function enabled by vanadium dioxide (VO 2 ) integrated into a silicon waveguide. The device exploits the reversible insulator-to-metal transition of VO 2 , yielding a triangular-shaped activation response with an activation power below 500 µ W and operation preserved over a measured 25 nm window (1540–1565 nm) within the telecom C-band. A 2 µ m-long VO 2 /Si structure was fabricated and experimentally validated, showing excellent agreement with thermal-optical simulations and a tunability of the activation threshold of −15 µ W °C −1 . Temporal characterization reveals activation dynamics governed by the interplay of thermal and phase-transition processes, with response times in the sub-microsecond regime. Finally, the proposed activation function was assessed through numerical simulations of a radial basis function neural network benchmarked on the MNIST dataset, achieving accuracies above 90%. These results place VO 2 -integrated photonics as a promising route toward compact all-optical nonlinear activation functions for photonic neural networks.