Optically-triggered deterministic spiking regimes in nanostructure resonant tunnelling diode-photodetectors

This work reports a nanostructure resonant tunnelling diode-photodetector (RTD-PD) device and demonstrates its operation as a controllable, optically-triggered excitable spike generator. The top contact layer of the device is designed with a nanopillar structure (500 nm in diameter) to restrain the injection current, yielding therefore lower energy operation for spike generation. We demonstrate experimentally the deterministic optical triggering of controllable and repeatable neuron-like spike patterns in the nanostructure RTD-PDs. Moreover, we show the device's ability to deliver spiking responses when biased in either of the two regions adjacent to the negative differential conductance region, the so-called 'peak' and 'valley' points of the current–voltage (I–V) characteristic. This work also demonstrates experimentally key neuron-like dynamical features in the nanostructure RTD-PD, such as a well-defined threshold (in input optical intensity) for spike firing, as well as the presence of spike firing refractory time. The optoelectronic and chip-scale character of the proposed system together with the deterministic, repeatable and well controllable nature of the optically-elicited spiking responses render this nanostructure RTD-PD element as a highly promising solution for high-speed, energy-efficient optoelectronic artificial spiking neurons for novel light-enabled neuromorphic computing hardware

Artificial optoelectronic spiking neurons with laser-coupled resonant tunnelling diode systems

We report a spiking artificial optoelectronic neuron based on a resonant tunnelling diode (RTD) coupled to a photodetector (receiver) and a vertical cavity surface emitting laser (VCSEL, transmitter). We experimentally realize this O/E/O system, and demonstrate optical spiking with a well-defined, adjustable excitability threshold

Tuneable presynaptic weighting in optoelectronic spiking neurons built with laser-coupled resonant tunneling diodes

Optoelectronic spiking neurons are regarded as highly promising systems for novel light-powered neuromorphic computing hardware. Here, we investigate an optoelectronic (O/E/O) spiking neuron built with an excitable resonant tunnelling diode (RTD) coupled to a photodetector and a vertical-cavity surface-emitting laser (VCSEL). This work provides the first experimental report on the control of the amplitude (weighting factor) of the fired optical spikes directly in the neuron, introducing a simple way for presynaptic spike amplitude tuning. Notably, a very simple mechanism (the control of VCSEL bias) is used to tune the amplitude of the spikes fired by the optoelectronic neuron, hence enabling an easy and high-speed option for the weighting of optical spiking signals in future interconnected photonic spike-processing nodes. Furthermore, we validate the feasibility of this layout using a simulation of a monolithically-integrated, RTD-powered, nanoscale optoelectronic spiking neuron model, confirming the system's potential for delivering weighted optical spiking signals at very high speeds (GHz firing rates). These results demonstrate the high degree of flexibility of RTD-based artificial optoelectronic spiking neurons and highlight their potential towards compact, high-speed and low-energy photonic spiking neural networks for use in future, light-enabled neuromorphic hardware

Artificial optoelectronic spiking neuron based on a resonant tunnelling diode coupled to a vertical cavity surface emitting laser

Excitable optoelectronic devices represent one of the key building blocks for implementation of artificial spiking neurons in neuromorphic (brain-inspired) photonic systems. This work introduces and experimentally investigates an opto-electro-optical (O/E/O) artificial neuron built with a resonant tunnelling diode (RTD) coupled to a photodetector as a receiver and a vertical cavity surface emitting laser as a transmitter. We demonstrate a well-defined excitability threshold, above which the neuron produces optical spiking responses with characteristic neural-like refractory period. We utilise its fan-in capability to perform in-device coincidence detection (logical AND) and exclusive logical OR (XOR) tasks. These results provide first experimental validation of deterministic triggering and tasks in an RTD-based spiking optoelectronic neuron with both input and output optical (I/O) terminals. Furthermore, we also investigate in simulation the prospects of the proposed system for nanophotonic implementation in a monolithic design combining a nanoscale RTD element and a nanolaser; therefore demonstrating the potential of integrated RTD-based excitable nodes for low footprint, high-speed optoelectronic spiking neurons in future neuromorphic photonic hardware

Towards an Excitable Microwave Spike Generator for Future Neuromorphic Computing

This paper describes the systematic approach to develop low power consumption excitable neuromorphic spike generators using nano-sized resonant tunnelling diode (RTD), including fabrication, characterization and device modelling and spike circuit simulation. The fabrication process of nano sized RTDs has been developed and devices exhibit peak currents of up to 100 μA. The energy efficiency of the RTD spike generator can reach as low as 0.09 fJ per spike. An accurate small signal model of nano RTD has also been developed and is described. This nano-RTD technology could underpin the development of energy efficient neuromorphic computing in the very near future

Traveling-Wave Electroabsorption Modulated Laser Based on Identical Epitaxial Layer Scheme and HSQ Planarization

Electroabsorption modulated lasers (EMLs), comprising a distributed feedback (DFB) laser and electroabsorption modulator (EAM) monolithically integrated into the same chip, are attractive because of their compact size, low fabrication cost, and their capability to offer a high modulation speed with low drive voltage, low chirp, and high extinction ratio [1] , [2] . The modulation speed of the EML is limited by the RC constant of the EAM electrode, which is conventionally configured with either a lumped or travelling-wave (TW) electrode. The latter approach overcomes the RC limit by including the EAM in a microwave circuit matched to the source [3] . However, due to restrictions imposed by size and materials, TW EAMs have to date been integrated externally using a specifically designed material structure or monolithically using selective area growth

Electroabsorption Modulated Laser Based on Identical Epitaxial Layer and Transmission Line Technology

Low-cost solutions for delivering high communication bandwidths in both short- and long-haul systems are urgently required. Electroabsorption modulated lasers (EMLs), comprising a distributed feedback (DFB) laser and electroabsorption modulator (EAM), can address this. They are compact and offer a high modulation speed with low drive voltage, low chirp, and high extinction ratio [1] . To the best of our knowledge, the EAM in EMLs based on identical epitaxial layer technology has so far been configured with a lumped electrode. This can take the form of either a circular-pad, a rectangular-pad or the centre electrode of a ground-signal-ground (GSG) configuration. All result in a high capacitance, which in turn limits the modulation speed. Although the GSG choice has a similar configuration to that of a coplanar waveguide (CPW), it still behaves as a lumped electrode because of the lack of impedance matching. A planarized film of low- k material can be used to reduce the capacitance, however standard methods such as Benzocyclobutene or polyimide-based planarization are very difficult to implement as they are incompatible with many photonic integration steps [2]

Photonic-electronic spiking neuron with multi-modal and multi-wavelength excitatory and inhibitory operation for high-speed neuromorphic sensing and computing

We report a multi-modal spiking neuron that allows optical and electronic input and control, and wavelength-multiplexing operation, for use in novel high-speed neuromorphic sensing and computing functionalities. The photonic-electronic neuron is built with a micro-scale, nanostructure resonant tunnelling diode (RTD) with photodetection (PD) capability. Leveraging the advantageous intrinsic properties of this RTD-PD system, namely highly nonlinear characteristics, photo-sensitivity, light-induced I-V curve shift, and the ability to deliver excitable responses under electrical and optical inputs, we successfully achieve flexible neuromorphic spike activation and inhibition regimes through photonic-electrical control. We also demonstrate the ability of this RTD-PD spiking sensing-processing neuron to operate under the simultaneous arrival of multiple wavelength-multiplexed optical signals, due to its large photodetection spectral window (covering the 1310 and 1550 nm telecom wavelength bands). Our results highlight the potential of RTD photonic-electronic neurons to reproduce multiple key excitatory and inhibitory spiking regimes, at high speed (ns-rate spiking responses, with faster sub-ns regimes theoretically predicted) and low energy (requiring only ~10 mV and ~150 microW, electrical and optical input amplitudes, respectively), similar in nature to those commonly found in the biological neurons of the visual system and the brain. This work offers a highly promising approach for the realisation of high-speed, energy-efficient photonic-electronic spiking neurons and spiking neural networks, enabling multi-modal and multi-wavelength operation for sensing and information processing tasks. This work therefore paves the way for innovative high-speed, photonic-electronic, and spike-based neuromorphic sensing and computing systems and artificial intelligence hardware.Comment: 12 pages, 9 figure

EML based on identical epitaxial layer, side-wall grating and HSQ planarization

We present an electroabsorption modulated laser based on an identical epitaxial scheme, side-wall grating, on-chip microwave probe interface, and a new low-permittivity planarization method. The modulation speed is significantly increased by reducing the electrode capacitance by planarizing with a 5-μm-thick HSQ layer. Furthermore, implementing the electrode with a direct ground-signal-ground probe interface provides a straightforward interconnection that obviates the need for an external circuit and bonding wires. The device operates at 1565 nm wavelength with stable single-mode lasing, no mode-hopping, and a side mode suppression ratio above 35 dB. An extinction ratio of 19.5 dB was recorded at the maximum modulator bias of –4 V. The electrical to optical power response of the modulated signal at –3-dBo demonstrated a 19 GHz bandwidth at an extinction ratio of 7 dB, which supports error-free data transmission up to 27 Gbit/s