Neuromorphic photonics is emerging as a powerful platform for fast and efficient optical information processing and sensing. However, future brain-inspired photonic systems require compact and scalable light sources, capable of generating the neuro-mimetic optical signals needed for their operation. This work demonstrates a single-stack laser that delivers optical and electrical neural-like spiking emission under solitary operation. Termed the Neuron Surface-Emitting Laser (NeuronSEL), this compact, multi-junction Vertical-Cavity Surface Emitting Laser (VCSEL) exhibits non-linear Negative Differential Resistance (NDR), similar to that observed in memristive devices. Leveraging this NDR behaviour enables the novel demonstration of multiple neuronal features in the NeuronSEL including refractoriness and threshold-/integrate-and-fire dynamics. We demonstrate the NeuronSEL's behaviour as an optical spiking neuron and its ability to perform processing functions, such as coincidence detection and exclusive OR operations. Its scalability is illustrated by proposing a network based on an array of NeuronSELs, able to perform classification tasks. The NeuronSEL emerges as a strong candidate for practical and scalable neuromorphic photonic hardware, with potential impact across a range of applications in optical sensing, communications and computing technologies, whilst benefitting from the inherent advantages of VCSEL technology -low manufacturing cost, compactness, efficiency, vertical emission, and straightforward integration into large arrayed-structures and networks.

We report the Neuron Surface-Emitting Laser (NeuronSEL) whose unique electrical/optical properties enable its operation as a fast, efficient, coherent light-emitting spiking neuron, offering great prospects for novel uses in neuromorphic photonic computing and sensing technologies.

VCSEL thermal resistances are determined from power-current-voltage-wavelength measurements performed on nominally identical epitaxial structures grown on Ge and GaAs substrates. We show that the effective thermal conductivity of the VCSEL is increased when grown on Ge substrates.

This work presents an approach to assess the quality of etched laser facets, considering factors such as roughness, inclination, and non-uniform light emission. Broad area InP lasers, using plasma etched facets, operating at 1550 nm are manufactured with varying facet quality on five 100 mm wafers. Comparison of the threshold current density of lasers of different length was used to derive relative facet reflectivity and demonstrated the relationship between the reflectivity and the optical mode weighted facet roughness and facet inclination.

A vertical cavity surface emitting laser (VCSEL) quick fabrication (VQF) process is applied to epitaxial materials designed for miniature atomic clock applications (MACs). The process is used to assess material quality and uniformity of a full 100 mm (4 ‐ inch) wafer against the stringent target specification of VCSELs for MACs. Target specifications in optical power ( > 0.6 mW) and differential efficiencies ( < 0.5 W/A) are achieved over large portions of a wafer; however, the variation in the oxide aperture diameter is shown to limit the yield. The emission of the fundamental mode

We report on a study using VCSEL Quick Fabrication (VQF) devices for the rapid assessment of epitaxial structures designed for emission at 894nm grown on 100mm substrates. A comparison of measured VQF device results to the epitaxial design specification allows for the extraction of key variances across the wafer and the identification of their potential causes. We also demonstrate the applicability of this technique for the assessment of uniformity and reproducibility of 150mm VCSEL wafers for emission at 940nm, identifying the potential sources for observed variations in device performance that impact in specification device yield.

Emerging consumer applications of VCSEL arrays demand larger sizes and improved reliability. Significant wafer bow seen on a 150-mm GaAs-substrate wafer can impact fabrication, characterisation, and yields. It has been reported that Ge-substrates are drop-in replacements for GaAs, but also have additional benefits. We report on the spatial performance of identical 940 nm VCSELs, grown on both types of 200-mm substrate. Threshold current densities vary by 0.1μA/cm2 at the wafer centre, and a 0.78% and 0.59% decrease in centre-to-edge emission wavelengths for Ge and GaAs respectively. Results show a potential route to larger manufacturing volumes with lower costs per wafer.

The strain-induced wafer bow for VCSEL epitaxial structures grown on GaAs substrates is measured and compared to that of Ge substrates. We find that the ~ 160 μm height difference between the centre and edge of a GaAs wafer results in a significant temperature gradient and hence has a large effect on oxidation rate in the high-Al layer in the top DBR of the epi-structure. We measure a resultant centre-to-edge variation in oxidation length of ~ 3 μm for a GaAs wafer. We assess the contributions of wafer bow and epi-layer non-uniformity, as well as temperature variation in the furnace, and find that the effect of the bow dominates. We employ a Very Quick Fabrication (VQF) method to rapidly produce oxide confined VCSELs across a 150 mm GaAs substrate wafer to assess the impact on device performance. By measuring threshold current density between 20 and 70 ℃, we find ~ 25 ℃ variation in the temperature corresponding to the alignment of the spectral peak of gain with the cavity resonance wavelength. However, we still find that the threshold current density at zero detuning, is lower for edge devices, which we attribute to material variation. We disentangle the different contributions to device performance to isolate the effect of material variation. We compare this remaining spatial non-uniformity to that of VCSELs grown on Ge substrates.

Stripped-back representative VCSEL devices with a simple fabrication process that very closely approaches the performance of standard BCB-planarised devices have been produced. These VCSEL Quick Fabrication (VQF) devices achieve threshold currents only 0.3 mA higher than that of a standard device produced from the same material. The predictability of standard performance from VQF performance is also robustly assessed in terms of temperature effects to account for the observed disparities. These VQF devices are then processed across a 6-inch (152 mm) wafer and the resulting device-level characteristics are mapped. From this, it is apparent that there is an approximately radial decrease in oxide aperture diameter from centre to edge, found to be driven by the strain-induced wafer bow. After corrections, a residual spatial variation across the wafer remains, which, in conjunction with temperature dependent measurements, is shown to be a result of epi-material variation. By observation at 50 °C, that is, at a temperature closely resembling that of intended application, the residual centre-to-edge variation in threshold current density is found to be only 0.2 kA/cm2, compared to 1.3 kA/cm2 when observing the room temperature variation of devices of nominally equivalent active volumes.

A simplified fabrication process for VCSELs which employs oxidation-vias for definition of the laser aperture and bond pad is applied to a full 150mm wafer as a technique for material characterisation. This Quick Fabrication process produces representative VCSELs, with performance comparable to standard process VCSELs, with threshold currents for 8μm oxide-aperture devices measured between 0.8 and 1.3mA for both device types. The redshift of the lasing wavelength and threshold currents are used for rapid assessment of the VCSEL wafers.

Results from an experimental comparison of like-for-like VCSELs grown on GaAs and Ge substrates are presented. Comparable performance, in terms of threshold current, device efficiency, and thermal management, is demonstrated.

Development of a quick fabrication (QF) method for commercial wafer characterisation based on rapid feedback of VCSEL performance. We report on the design of the fabrication process including the systematic removal of time-consuming steps of planarization, oxidation and substrate lapping, and the associated impact on device performance and yield. We show comparable performance of the oxide-confined QF etched trench VCSELs and full process devices and we show that unoxidised devices behave as large aperture oxidised devices. Further, we demonstrate similar performance of substrate-lapped and -unlapped VCSELs between 1.0-1.2 Ith with a difference in current tuning typically 0.064nm/mA.

A systematic analysis of the performance of VCSELs, fabricated with a decreasing number of structural elements, is used to assess the complexity of fabrication (and therefore time) required to obtain sufficient information on epitaxial wafer suitability. Initially, sub-mA threshold current VCSEL devices are produced on AlGaAs-based material, designed for 940 nm emission, using processing methods widely employed in industry. From there, stripped-back Quick Fabrication (QF) devices, based on a bridge-mesa design, are fabricated and this negates the need for benzocyclcobutane (BCB) planarisation. Devices are produced with three variations on the QF design, to characterise the impact on laser performance from removing time-consuming process steps, including wet thermal oxidation and mechanical lapping used to reduce substrate thickness. An increase in threshold current of 1.5 mA for oxidised QF devices, relative to the standard VCSELs, and a further increase of 1.9 mA for unoxidised QF devices are observed, which is a result of leakage current. The tuning of the emission wavelength with current increases by ~0.1 nm/mA for a VCSEL with a 16 μm diameter mesa when the substrate is unlapped, which is ascribed to the increased thermal resistance. Generally, relative to the standard VCSELs, the QF methods employed do not significantly impact the threshold lasing wavelength and the differences in mean wavelengths of the device types that are observed are attributed to variation in cavity resonance with spatial position across the wafer, as determined by photovoltage spectroscopy measurements.

sub-mA threshold currents are achieved for VCSELs using a simplified fabrication process which employs etched oxidation-vias for definition of the VCSEL aperture, as well as ease in formation of a bond pad without the need for bisbenzocyclobutane planarisation.

We report direct measurements of the optical gain profile for a vertical cavity surface emitting laser (VCSEL) epitaxial structure, by characterising the transverse electric (TE) in-plane net modal gain using the segmented contact method.