Chalcogenide Microfiber Photonic Synapses

Abstract

Optical axons and photonic synapses implemented using chalcogenide microfibers allow the generation and propagation of photonic action potentials which give rise to the demonstration of various neuromorphic concepts. Thus far, inorganic scalable neuromorphic systems and devices have been demonstrated using software and electronic configurations. However, as compared to biological systems based on organic axons and synapses, today's programmable inorganic computers are 6 to 9 orders of magnitude less efficient in complex environments. Simulating 5 seconds of brain activity takes 500 seconds and needs 1.4 MW of power [1,2,3,4]. Inspired by the emerging neuromorphic electronic systems and motivated by the potential of an all-optical cognitive platform, here we investigate and propose the use of amorphous chalcogenide microfibers as all-optical axons and synapses that exhibit brain-like functionality in the form of plasticity and data transmission in one scalable configuration.Optical fibers provide a mature mass manufacturable technology that has given rise to the complex network of interconnected nodes transferring information around the planet. They have been realized in a range of functional optical and electronic materials including amorphous, crystalline and semiconducting compounds [5,6,7]. In this work we realise an optical axon and photonic synapse based on neuromorphic chalcogenide microfibers of the alloy gallium lanthanum oxysulphide (GLSO) with an outer diameter of 150 µm, with a transmission window from 550 nm to 7 µm (Fig. 1). As a proof-of-concept, we demonstrate a variety of neurophysiological phenomena in the optical regime mimicking communication protocols in the mammalian central nervous system, including temporal and spatial summation, excitatory and inhibitory post synaptic potentials, and short and long term plasticity. Chalcogenide alloys are amorphous semiconducting media whose physical properties can be temporarily or permanently altered with light. Thanks to this, chalcogenide microfibers present an inorganic analogue of a biological neuron where signal propagation and processing is realised by optical confinement of light waves and photomodulation of their transmission properties, rather than by biochemical and electrical signal transduction. To implement an all-optical neuron in amorphous chalcogenide microfibers, we use photodarkening, which is a temperature dependent phenomenon that manifests itself in the form of a volatile (transient) and non-volatile (metastable) broadband attenuation in transparency and optical bandgap, brought about as a result of illumination with near or sub-bandgap light (Fig. 2). While the transient changes decay upon switching off the illumination, metastable photodarkening is non-volatile and reversible by annealing [8,9,10,11,12] providing the basis for short term and long term plasticity.

Photonic Synapses for Ultrahigh‐Speed Neuromorphic Computing

“…Gholipour et al developed purely photonic synapses based on amorphous gallium lanthanum oxysulfide (GLSO) microfibers . As we know, in the human brain, neurons are interconnected through synapses.…”

“…The fiber transmission can be reverted back to its original level by increasing the guided light intensity to optically anneal the fiber. Reproduced with permission . Copyright 2015, Wiley‐VCH.…”

“…Furthermore, when the synaptic weight is represented by optical parameters instead of electrical parameters, purely photonic synapses can then be realized. Given that photostructural changes can alter the optical transmissivity of the optical materials, the optical transmission through these materials may serve as synaptic weight . Recently, various photonic synapses have been realized by different technologies such as memristors, transistors, and photonic integrated circuit .…”

Recent Progress in Three‐Terminal Artificial Synapses: From Device to System

“…Another limitation is that these electrically stimulated devices have limited operating speed. Recently, optoelectronic synapses and photonic synapses have been demonstrated; they may lead to reduction in electrical energy loss, increase in bandwidth, robustness, and ultrafast signal transmission . The light‐stimulated synapse opens up a new opportunity for extending current neuromorphic systems in terms of system complexities and functionalities.…”

Recent Progress in Photonic Synapses for Neuromorphic Systems

“…Despite rapid advances, conventional synapses are based on the electronic technology, on which the speed of neuromorphic computing is limited on account of bandwidth connection density trade‐off. In contrast, synapses triggered by light pulses are more suitable for ultrahigh computing speed because they have unique advantages including high bandwidth, high interference immunity, and low power computation . In addition, photonic synaptic devices provide a noncontact writing method, which can facilitate the evolution of optical wireless communication.…”