  There has been a recently renewed interest in neuromorphic photonics, a field
promising to access pivotal and unexplored regimes of machine intelligence.
Progress has been made on isolated neurons and analog interconnects;
nevertheless, this renewal has yet to produce a demonstration of a silicon
photonic neuron capable of interacting with other like neurons. We report a
modulator-class photonic neuron fabricated in a conventional silicon photonic
process line. We demonstrate behaviors of transfer function configurability,
fan-in, inhibition, time-resolved processing, and, crucially, autaptic
cascadability -- a sufficient set of behaviors for a device to act as a neuron
participating in a network of like neurons. The silicon photonic modulator
neuron constitutes the final piece needed to make photonic neural networks
fully integrated on currently available silicon photonic platforms.
