COMMUNICATIONof particular interest as their physical properties can be temporarily or permanently altered with light, thus giving the opportunity to provide similar conditions to propagating optical signals that exists for biochemical and electrical signal transduction in the brain. [18][19][20][21] The "photonic axon and synapse" proposed here is based on the gallium lanthanum oxysulphide (GLSO) fi ber. Here, photodarkening manifests itself as a volatile (transient) and nonvolatile (metastable) broadband reduction of transmissivity of the fi ber resulting from a fl ash of illumination at a subbandgap optical frequency. While the transient changes rapidly decay upon switching off the illumination, metastable photodarkening is permanent, but is reversible by annealing. [22][23][24][25][26] To emulate functions of the chemical synapse, the fi ber is illuminated from the side at a sub-bandgap wavelength of λ = 532 nm. Such a fl ash of light is analogous of the prespike that acts within a living neuron. The fl ash of illumination forms the "photonic synapse" at the exposure point (Figure 1 B). Conversely, the postspike function is delivered by light at λ = 650 nm, which is guided through the fi ber (in principle this wavelength could be anywhere within the transparency range of the fi ber). In this confi guration, the GLSO fi ber perfectly replicates, in the photonic regime, the data transmission characteristics of the biological axon, as well as its ability to be depolarized or hyperpolarized at any point across the membrane to form a synaptic junction. Unlimited photonic synapses could be created along the length of the fi ber, allowing changes induced in the transmission of the fi ber at any of the synaptic junctions to propagate to the next synaptic junction, or eventually power the analogue of a biological actuator.GLSO microfi bers are drawn using conventional fi ber fabrication techniques on a special adapted fi ber drawing tower, from a premelted polished glass preform (see Experimental Section). Their photodarkening characteristics are determined under sub-bandgap green illumination ( Figure 2 A,B). Broadband attenuation (depression) upon illumination of the fi ber is the sum of transient and metastable photodarkening phenomena. [ 22 ] As shown in thin fi lms, the attenuation can be modulated (depressed or potentiated) via transient photodarkening upon successive cycling of illumination. [ 25,26 ] In the GLSO fi ber, as much as 35% change in transmission is observed by subbandgap optical excitation (Figure 2 A).Transient and metastable changes occur for short illumination times and, in the absence of thermal annealing, the metastable changes are cumulative upon successive illumination. The transient photodarkening effect observed during illumination of chalcogenide glasses are time and intensity dependent ( Figure S2, Supporting Information), a phenomenon that involves nonradiative recombination of photoexcited charge carriers and defects, while metastable effects arise from The human brain, with all its co...

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An all-solid microstructured fiber based on two thermallymatched silicate glasses with a high index contrast has been fabricated for the first time. The microstructured cladding was shown to be essentially unchanged during fiber drawing. Fiber attenuation was measured as 5dB/m at 1.55µm by the cutback method. High nonlinearity 230 W -1 km -1 has been predicted and experimentally demonstrated in this fiber at 1.55µm. In addition, modeling predicts that near-zero dispersion can be achieved between 1.5-1.6µm in this class of high nonlinear fiber. Taylor, "Extruded soft glass photonic crystal fiber for ultrabroad supercontinuum generation," Opt. Express 10, 1520-1525 (2002), http://www.opticsexpress.org/abstract.cfm

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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.