Abstract
Neuromorphic computing has received considerable attention as promising alternatives to classical von Neumann computing architectures. An attractive concept in this field is reservoir computing which is based on coupled non-linear elements to enable for instance ultra-fast pattern recognition. We focus on the development of microlasers in a dense regular array for the implementation of photonic reservoir computing based on the diffractive coupling. The coupling relies on injection locking of microlasers and sets stringent requirements on the spectral homogeneity of the array, which needs to be on the order of the achievable locking range. We realize GaAs/AlGaAs micropillar arrays with InGaAs quantum dots as active medium. To achieve the high spectral homogeneity on the order of 100 μ eV, as determined by injection locking experiments, the emission energy of each individual micropillar is adjusted to compensate for local inhomogeneities of order ∼1.3 meV in the underlying microcavity structure. The realized micropillar arrays have a spectral inhomogeneity as low as 190 μ eV for an 8 × 8 array and down to 118 μ eV for a 5 × 5 sub-array. The arrays have high potential to enable the implementation of powerful photonic reservoir computing, which can be extended to a reservoir of hundreds of microlasers in the future.
Highlights
N EUROMORPHIC computing dates back to the pioneering work by C
We developed a nanoprocessing platform for the realization of highly homogeneous arrays of optically pumped quantum dot (QD) micropillar lasers for applications in photonic reservoir computing (RC)
This application is enabled by the epitaxial growth of QD-microcavity structures and the subsequent nanoprocessing of regular micropillar cavity arrays using electron beam lithography and plasma etching
Summary
N EUROMORPHIC computing dates back to the pioneering work by C. One interesting type of microlasers is based on high-quality quantum dot (QD) micropillar cavities, which have enabled numerous studies and advances in the field of cavity-enhanced lasers [22] Such QD-micropillar lasers with diameters typically in the 1 μm–5 μm range have comparatively small mode volumes and large cavity quality (Q) factors of several tens of thousands, which leads to high spontaneous emission coupling factors (β-factors) of up to about 60% and low threshold pump powers [23]. E.g., laterally emitting whispering gallery mode microlasers, QD-micropillar lasers are very advantageous for our envisioned application in photonic RC because of the very directional axial emission normal to the sample surface Due to their small diameters, pitches between neighboring devices of 10 μm and below can be achieved enabling the realization of dense microlaser arrays with 30 × 30 and more lasers within the available area of about 300 μm × 300 μm for diffractive photonic enabled RC [24]. Beyond that we refine the diameter-tuning concept for the fabrication of homogenous large-scale laser arrays by including local optical variations of the underlying wafer material to the reach record high spectral homogeneity with a deviation of 190 μeV for an 8 × 8 micropillar array and a value of 118 μeV for a 5 × 5 micropillar array which compares well with an extrapolated and measured locking range of about 200 μeV and 80 μeV respectively
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