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...

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.

Abstract-We report on the development of three systems intended to provide fast, non-intrusive measurement of cross sectional distributions of pollutant species within gas turbine exhaust flows, during ground-based testing. This research is motivated by the need for measurement systems to support the introduction of technologies for reducing the environmental impact of civil aviation. Tomographic techniques will allow estimation of the distributions of CO2, unburnt hydrocarbons (UHC), and soot, without obstruction of the exhaust, bypass or entrained flows, from measurements made in a plane immediately aft of the engine.We describe a CO2 imaging system that performs wavelength modulation spectroscopy (WMS) simultaneously on 126 beam paths. Its novel architecture uses a Tm-doped fiber amplifier to generate sufficient optical power for the entire beam array (> 3 W) from a single 1997.2 nm diode-laser seed, reducing cost and enabling fully parallel detection and signal recovery. Various optical propagation issues are considered, including those arising from the varying degrees of interaction with the exhaust flow that exist within the beam array, as well as pointing errors arising from the limited rigidity of the measurement system's structure.We also report first steps towards a similar UHC measurement system, operating in the mid-infrared (MIR) region and targeting partially decomposed or oxidized fuel constituents, including formaldehyde and propene. Progress towards the chalcogenide glasses and fibers, needed for light delivery and/or amplification at these wavelengths is described. Finally, we report on the development status of a tomographic soot imaging system, based on laser induced incandescence (LII). We have demonstrated both long (192 ns) and short (17 ns) pulse variants of LII using fiber laser sources. Single path tests on a laboratory soot generator and, in the long pulse case, on a jet engine have confirmed that the energy and beam quality available from the fiber lasers is sufficient to enable an autoprojection approach, using just two intensified CCD cameras having 'near-orthogonal' views, with respect to the excitation laser.

Metal tips are emerging plasmonic structures that can offer high field intensity at the tip apex and high confinement in the nanoscale. The fabrication though of smooth metal tips with well-defined geometrical characteristics, crucial for optimizing the performance of the plasmonic structure, is not trivial. Furthermore pure metal tips are exposed to the environment and fragile, thus, complicating their use in real applications. The proposed platform based on hybrid composite glass metal microwires can offer the required robustness for device development. An optimized fabrication process of high quality all-fiber plasmonic tips by tapering such hybrid metal core/dielectric cladding microfibers is proposed and demonstrated experimentally. The presence of the dielectric cladding offers continuous re-excitation of the plasmon modes due to repeated total internal reflection at the glass/air interface which can dramatically reduce the high losses induced by the metal core. This enables direct light coupling from the distal end of fiber instead of side excitation of the tip, allowing thus their integration in optical fiber and planar circuits. Plasmonic tips were successfully demonstrated in a highly controllable manner and their performance was related to simulation results predicting high field enhancement factors up to 10 5 .

Gallium lanthanum sulfide glass (GLS) has been widely studied in the last 40 years for middle‐infrared applications. In this work, the results of the substitution of selenium for sulphur in GLS glass are described. The samples are prepared via melt‐quench method in an argon‐purged atmosphere. A wide range of compositional substitutions are studied to define the glass‐forming region of the modified material. The complete substitution of Ga2S3 by Ga2Se3 is achieved by involving new higher quenching rate techniques compared to those containing only sulfides. The samples exhibiting glassy characteristics are further characterized. In particular, the optical and thermal properties of the sample are investigated in order to understand the role of selenium in the formation of the glass. The addition of selenium to GLS glass generally results in a lower glass transition temperature and an extended transmission window. Particularly, the IR edge is found to be extended from about 9 µm for GLS glass to about 15 µm for Se‐added GLS glass defined by the 50% transmission point. Furthermore, the addition of selenium does not affect the UV edge dramatically. The role of selenium is hypothesized in the glass formation to explain these changes.