Driven by machine-learning tasks neural networks have demonstrated useful capabilities as nonlinear hypothesis classifiers. The underlying technologies performing the dotproduct multiplication, the summation, and the nonlinear thresholding on the input data in electronics, however, are limited by the same capacitive challenges known from electronic integrated circuits. The optical domain, in contrast, provides low delay interconnectivity suitable for such node-distributed non-Von Neumann architectures relying on dense node-to-node communication. Thus, once the neural network's weights are set, the delay of the network is just given by the time-of-flight of the photon, which is in the picosecond range for photonic integrated circuits. However, the functionality of memory for storing the trained weights does not exists in optics, thus demanding a fresh look to explore synergies between photonics and electronics in neural networks. Here we provide a roadmap to pave the way for emerging hybridized photonic-electronic neural networks by taking a detailed look into a single node's perceptron, discussing how it can be realized in hybrid photonic-electronic heterogeneous technologies. We show that a set of materials exist that exploit synergies with respect to a number of constrains including electronic contacts, memory functionality, electro-optic modulation, optical nonlinearity, and device packaging. We find that the material ITO, in particular, could provide a viable path for both the perceptron 'weights' and the nonlinear activation function, while simultaneously being a foundry process-near material. We finally identify a number of challenges that, if solved, could accelerate the adoption of such heterogeneous integration strategies of emerging memory materials into integrated photonics platforms for real-time responsive neural networks.
Introduction:While the scientific community still lacks a full understanding of the operation of the human brain, we yet can draw some parallels to compute-systems with respect to operating efficiency. Machine learning (ML) tasks performed by neural networks (NN) can be used for such computer-vs.-brain comparison, since both are examples of nonlinear hypothesis systems that can be trained to classify patterns. Interestingly, one of the fastest computers 1 are only able to simulate ~1% of human brain activity in requiring about one hour demanding massive compute infrastructure overheads (e.g. 82,000 processors, and 1.73 billion virtual nerve cells connected by 10.4 trillion synapses) consuming about 10 megawatts, while the human brain operates on 10's of Watts 2 . In the aim to compensate this disparity and to match some of the brain's computational and energy efficiency, the recent efforts to develop non-Von Neumann neuromorphic hardware are capable of efficiently implementing artificial NN operations, in particular vector matrix multiplication (VMM) and backpropagation 3-5 . However, unlike