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 µeV, 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 µeV for an 8 × 8 array and down to 118 µeV 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.

We report on the realization of a dense, large-scale array of 900 quantum dot micropillar cavities with high spectral homogeneity. We target applications in photonic information processing such as optical reservoir computing which can be implemented in large arrays of optically coupled microlasers. To achieve the required spectral homogeneity for the underlying optical injection locking, we calculate and set the diameter of each individual micropillar within the array during the fabrication process by taking the diameter-dependent emission wavelength of the microcavities into account. Using this kind of diameter adjustment, we improve the overall wavelength homogeneity in a 30 × 30 micropillar array by 64% and reduce the standard deviation of the resonance energy distribution by 26% from 352 μeV in the planar unprocessed sample to 262 μeV in the fabricated array. In addition, we present a detailed analysis of the device quality and the diameter control of the micropillar’s emission wavelength, which includes important information for the effective application of the developed fabrication method for the realization of highly homogeneous micropillar arrays in the future.

We report on the experimental demonstration of triggered single-photon emission at the telecom O-band from In(Ga)As/GaAs quantum dots (QDs) grown by metal-organic vapor-phase epitaxy. Micro-photoluminescence excitation experiments allowed us to identify the p-shell excitonic states in agreement with high excitation photoluminescence on the ensemble of QDs. Hereby we drive an O-band-emitting GaAs-based QD into the p-shell states to get a triggered single photon source of high purity. Applying pulsed p-shell resonant excitation results in strong suppression of multiphoton events evidenced by the as measured value of the second-order correlation function at zero delay of 0.03 (and ~0.005 after background correction).

The temperature dependence of the electron-beam sensitive resist CSAR 62 is investigated in its negative-tone regime. The writing temperatures span a wide range from 4 K to room temperature with the focus on the liquid helium temperature regime. The importance of low temperature studies is motivated by the application of CSAR 62 for deterministic nanophotonic device processing by means of in-situ electron-beam lithography. At low temperature, CSAR 62 exhibits a high contrast of 10.5 and a resolution of 49 nm. The etch stability is almost temperature independent and it is found that CSAR 62 does not suffer from peeling which limits the low temperature application of the standard electron-beam resist PMMA. As such, CSAR 62 is a very promising negative-tone resist for in-situ electron-beam lithography of high quality nanostructures at low temperature.

We report on a 3D electron beam lithography (EBL) technique using polymethyl methacrylate (PMMA) in the negative-tone regime as a resist. First, we briefly demonstrate 3D EBL at room temperature. Then we concentrate on cryogenic temperatures where PMMA exhibits a low contrast, which allows for straightforward patterning of 3D nano- and microstructures. However, conventional EBL patterning at cryogenic temperatures is found to cause severe damage to the microstructures. Through an extensive study of lithography parameters, exposure techniques, and processing steps we deduce a hypothesis for the cryogenic PMMA's structural evolution under electron beam irradiation that explains the damage. In accordance with this hypothesis, a two step lithography technique involving a wide-area pre-exposure dose slightly smaller than the onset dose is applied. It enables us to demonstrate a >95% process yield for the low-temperature fabrication of 3D microstructures.