“…[ 42 ] Nonlinear activation functions in PNNs have been implemented with creative combinations of classic photonic primitives, including microring resonators (MRRs), [ 43 ] Mach–Zehnder interferometers (MZIs), [ 44 ] semiconductor optical amplifiers (SOAs), [ 45 ] and electro‐optic modulators together with photodetectors. [ 43,46 ] In addition, typical biological features of the spiking neuron have been successfully emulated with different types of excitable lasers, i.e., vertical‐cavity surface‐emitting lasers (VCSELS), [ 47 ] micropillar lasers, [ 48 ] distributed feedback lasers (DFBs), [ 49 ] and quantum‐dot (QDs) lasers. [ 50,51 ] Meanwhile, phase‐change materials (PCMs), which can be flexibly switched between multiple states of different optical properties, [ 52,53 ] have emerged as a competitive alternative in hardware emulation of both the fundamental leaky integrate‐and‐fire (LIF) functionality of neurons and the plastic weighting process of synapses.…”