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.

Surface Passivation of III-V GaAs Nanopillars by Low-Frequency Plasma Deposition of Silicon Nitride for Active Nanophotonic Devices.

Numerous efforts have been devoted to improve the electronic and optical properties of III-V compound materials via reduction of their nonradiative states, aiming at highly efficient III-V sub-micrometer active devices and circuits. Despite many advances, the poor reproducibility and short-term passivation effect of chemical treatments, such as sulfidation and nitridation, requires the use of protective encapsulation methods, not only to protect the surface, but also to provide electrical isolation for device manufacturing. There is still a controversial debate on which combination of chemical treatment and capping dielectric layer can best reproducibly protect the crystal surface of III-V materials while being compatible with readily available semiconductor-foundry plasma deposition methods. This work reports on a systematic experimental study on the role of sulfide ammonium chemical treatment followed by dielectric coating (either silicon oxide or nitride) in the passivation effect of GaAs/AlGaAs nanopillars. Our results conclusively show that, under ambient conditions, the best surface passivation is achieved using ammonium sulfide followed by encapsulation with a thin layer of silicon nitride by low-frequency plasma-enhanced chemical deposition. Here, the sulfurized GaAs surfaces, high level of hydrogen ions, and low-frequency (380 kHz) excitation plasma that enable intense bombardment of hydrogen, all seem to provide a combined active role in the passivation mechanism of the pillars by reducing the surface states. As a result, we observe up to a 29-fold increase of the photoluminescence (PL) integrated intensity for the best samples as compared to untreated nanopillars. X-ray photoelectron spectroscopy analysis confirms the best treatments show remarkable removal of gallium and arsenic native oxides. Time-resolved micro-PL measurements display nanosecond lifetimes resulting in a record-low surface recombination velocity of ∼1.1 × 104 cm s-1 for dry-etched GaAs nanopillars. We achieve robust, stable, and long-term passivated nanopillar surfaces, which creates expectations for remarkable high internal quantum efficiency (IQE > 0.5) in nanoscale light-emitting diodes. The enhanced performance paves the way to many other nanostructures and devices such as miniature resonators, lasers, photodetectors, and solar cells, opening remarkable prospects for GaAs active nanophotonic devices.

Spectral-temporal luminescence properties of Colloidal CdSe/ZnS Quantum Dots in relevant polymer matrices for integration in low turn-on voltage AC-driven LEDs.

This work employs spectral and spectral-temporal Photoluminescence (PL) spectroscopy techniques to study the radiative mechanisms in colloidal CdSe/ZnS Quantum Dot (QD) thin films without and with 1% PMMA polymer matrix embedding (QDPMMA). The observed bimodal transient-spectral PL distributions reveal bandgap transitions and radiative recombinations after interdot electron transfer. The PMMA polymer embedding protects the QDs during the plasma-sputtering of inorganic layers electroluminescent (EL) devices, with minimal impact on the charge transfer properties. Further, a novel TiO2-based, all-electron bandgap, AC-driven QLED architecture is fabricated, yielding a surprisingly low turn-on voltage, with PL-identical and narrow-band EL emission. The symmetric TiO2 bilayer architecture is a promising test platform for alternative optical active materials.

Two-photon polymerization simulation and fabrication of 3D microprinted suspended waveguides for on-chip optical interconnects.

Quantum and neuromorphic computational platforms in integrated photonic circuits require next-generation optical functionalities. Often, increasingly complex on-chip light-routing that allow superpositions not attainable by planar technologies are paramount e.g. for artificial neural networks. Versatile 3D waveguides are achievable via two-photon polymerization (TPP)-based microprinting. Here, a 3D morphology prediction tool which considers experimental TPP parameters, is presented, enabling on-chip 3D waveguide performance simulations. The simulations allow reducing the cost-intensive systematic experimental optimization process. Fabricated 3D waveguides show optical transmission properties in agreement with simulations, demonstrating that the developed morphology prediction methodology is beneficial for the development of versatile on-chip and potentially inter-chip photonic interconnect technology.

Efficient light extraction in subwavelength GaAs/AlGaAs nanopillars for nanoscale light-emitting devices.

This work reports on high extraction efficiency in subwavelength GaAs/AlGaAs semiconductor nanopillars. We achieve up to 37-fold enhancement of the photoluminescence (PL) intensity from sub-micrometer (sub-µm) pillars without requiring back reflectors, high-Q dielectric cavities, nor large 2D arrays or plasmonic effects. This is a result of a large extraction efficiency for nanopillars <500 nm width, estimated in the range of 33-57%, which is much larger than the typical low efficiency (∼2%) of micrometer pillars limited by total internal reflection. Time-resolved PL measurements allow us to estimate the nonradiative surface recombination of fabricated pillars. We conclusively show that vertical-emitting nanopillar-based LEDs, in the best case scenario of both reduced surface recombination and efficient light out-coupling, have the potential to achieve notable large external quantum efficiency (∼45%), whereas the efficiency of large µm-pillar planar LEDs, without further methods, saturates at ∼2%. These results offer a versatile method of light management in nanostructures with prospects to improve the performance of optoelectronic devices including nanoscale LEDs, nanolasers, single photon sources, photodetectors, and solar cells.

Delay dynamics of neuromorphic optoelectronic nanoscale resonators: Perspectives and applications.

With the recent exponential growth of applications using artificial intelligence (AI), the development of efficient and ultrafast brain-like (neuromorphic) systems is crucial for future information and communication technologies. While the implementation of AI systems using computer algorithms of neural networks is emerging rapidly, scientists are just taking the very first steps in the development of the hardware elements of an artificial brain, specifically neuromorphic microchips. In this review article, we present the current state of the art of neuromorphic photonic circuits based on solid-state optoelectronic oscillators formed by nanoscale double barrier quantum well resonant tunneling diodes. We address, both experimentally and theoretically, the key dynamic properties of recently developed artificial solid-state neuron microchips with delayed perturbations and describe their role in the study of neural activity and regenerative memory. This review covers our recent research work on excitable and delay dynamic characteristics of both single and autaptic (delayed) artificial neurons including all-or-none response, spike-based data encoding, storage, signal regeneration and signal healing. Furthermore, the neural responses of these neuromorphic microchips display all the signatures of extended spatio-temporal localized structures (LSs) of light, which are reviewed here in detail. By taking advantage of the dissipative nature of LSs, we demonstrate potential applications in optical data reconfiguration and clock and timing at high-speeds and with short transients. The results reviewed in this article are a key enabler for the development of high-performance optoelectronic devices in future high-speed brain-inspired optical memories and neuromorphic computing.

Excitability and optical pulse generation in semiconductor lasers driven by resonant tunneling diode photo-detectors.

We demonstrate, experimentally and theoretically, excitable nanosecond optical pulses in optoelectronic integrated circuits operating at telecommunication wavelengths (1550 nm) comprising a nanoscale double barrier quantum well resonant tunneling diode (RTD) photo-detector driving a laser diode (LD). When perturbed either electrically or optically by an input signal above a certain threshold, the optoelectronic circuit generates short electrical and optical excitable pulses mimicking the spiking behavior of biological neurons. Interestingly, the asymmetric nonlinear characteristic of the RTD-LD allows for two different regimes where one obtain either single pulses or a burst of multiple pulses. The high-speed excitable response capabilities are promising for neurally inspired information applications in photonics.

Photo-detectors integrated with resonant tunneling diodes.

We report on photo-detectors consisting of an optical waveguide that incorporates a resonant tunneling diode (RTD). Operating at wavelengths around 1.55 µm in the optical communications C band we achieve maximum sensitivities of around 0.29 A/W which is dependent on the bias voltage. This is due to the nature of RTD nonlinear current-voltage characteristic that has a negative differential resistance (NDR) region. The resonant tunneling diode photo-detector (RTD-PD) can be operated in either non-oscillating or oscillating regimes depending on the bias voltage quiescent point. The oscillating regime is apparent when the RTD-PD is biased in the NDR region giving rise to electrical gain and microwave self-sustained oscillations Taking advantage of the RTD's NDR distinctive characteristics, we demonstrate efficient detection of gigahertz (GHz) modulated optical carriers and optical control of a RTD GHz oscillator. RTD-PD based devices can have applications in generation and optical control of GHz low-phase noise oscillators, clock recovery systems, and fiber optic enabled radio frequency communication systems.

A Self-Synchronized Optoelectronic Oscillator based on an RTD Photo-Detector and a Laser Diode.

We propose and demonstrate a simple and stable low-phase noise optoelectronic oscillator (OEO) that uses a laser diode, an optical fiber delay line and a resonant tunneling diode (RTD) free-running oscillator that is monolithic integrated with a waveguide photo-detector. The RTD-OEO exhibits single-side band phase noise power below -100 dBc/Hz with more than 30 dB noise suppression at 10 kHz from the center free-running frequency for fiber loop lengths around 1.2 km. The oscillator power consumption is below 0.55 W, and can be controlled either by the injected optical power or the fiber delay line. The RTD-OEO stability is achieved without using other high-speed optical/optoelectronic components and amplification.