Petr Klenovský

The deterministic realization of quantum light sources operating at telecom wavelengths is essential for long-distance fiber-based quantum communication and distributed quantum computing. In this work, we demonstrate that telecom O-band emission can be achieved from site-controlled InGaAs/GaAs quantum dots (QDs). Our concept utilizes a buried AlAs/AlOx stressor layer with the unique feature of inducing a well-defined and controllable tensile strain field at the growth surface, enabling both site-selective nucleation at the mesa’s centers and a redshift of QD emission to the ∼1.3 μm wavelength range. This concept not only eliminates the need for strain-reducing layers (SRLs), which are known to degrade optical coherence, but also provides spatial control and spectral tunability. The grown telecom QDs show single-photon emission with g(2)(0) = (5.0 ± 1.0) × 10–2 at 4 K and (2.8 ± 0.3) × 10–1 at 77 K, demonstrating the quantum nature and thermal stability of the emitters. The emission characteristics of complex excitonic states are analyzed using 8-band k·p and configuration-interaction modeling, which quantitatively reproduces the experimental observations. Finally, we present a theory-supported strategy to further redshift the emission toward the center of the O-band and beyond by employing a multiburied stressor approach. This combined framework of experiment and theory establishes the buried stressor concept as a scalable route toward highly coherent, position-controlled O-band quantum emitters compatible with scalable photonic integration.

We compute the electronic and emission properties of Coulomb–correlated multiparticle states ( X 0 , X ± , XX ) in weakly confining GaAs/AlGaAs quantum dots using an eight-band k · p model coupled to continuum elasticity and configuration interaction (CI). We evaluate polarization-resolved oscillator strengths and radiative rates both in the dipole approximation and in a quasielectrostatic beyond-dipole (BDA) longitudinal formulation implemented via a Poisson reformulation exactly equivalent to the dyadic Green-tensor kernel. For the dots studied, BDA yields lifetimes in quantitative agreement with experiments, e.g., τ X = 0.279 ns vs 0.267 ns (exp.) and τ X X = 0.101 ns vs 0.115 ns (exp.). The framework also reproduces electric-field tuning of the multiparticle electronic structure and emission—including the indistinguishability inferred from P = τ X / ( τ X + τ X X ) —and we assess sensitivity to CI-basis size and to electron–electron and hole–hole exchange.

We report on the monolithic, two-step epitaxial growth of site-controlled InGaAs quantum dots via the buried stressor method with local quantum dot density variation. As a result of high fabrication accuracy, we achieve low lateral displacements of the individual buried stressor apertures of $ 17^{+19}_{-17}$~nm from mesa centers. We provide extensive micro-photoluminescence and cathodoluminescence characterization of the site-controlled quantum dots and give theoretical calculations, explaining the effect of the stressor aperture on the quantum dot emission properties, positioning, and density. We show reproducibility of the nucleation process for apertures of the same size and achieve precisely-positioned, low- and high-density quantum dot nucleation within one active layer growth step. The results presented in this work demonstrate the significant potential of the buried stressor concept in fabricating single photonic chips, simultaneously combining single-photon sources and microlasers featuring different local densities of site-controlled quantum dots, paving the way for highly functional source modules with applications in photonic quantum technology.

Semiconductor quantum dots (QDs) are high-quality nanocrystals that provide three-dimensional carrier confinement on the scale of the de Broglie wavelength. This makes them ideal candidates as light emitters, especially in the emerging field of photonic quantum technologies, where they can act as quantum light sources. However, their self-assembled epitaxial growth leads to randomness in position and emission wavelength, which hinders their scalable integration into photonic quantum devices. This review summarizes and highlights advances in the site-controlled growth of high-quality epitaxial QDs, with a particular focus on the buried stressor concept. Compared to other QD positioning techniques based for instance on nanohole arrays, nanowire arrays, and arrays of inverted pyramids as dot nucleation centers, the buried stressor growth method is distinguished by its ability to achieve not only spatial accuracy and precision, but also control of the local QD density in combination in an industry-compatible process flow. Therefore, the buried stressor growth technique is highly suitable for the development of both QD-based quantum light sources and microlasers. The buried stressor site-controlled QD growth technique involves the sub-surface embedding of a nano-engineered stressor material, which generates localized strain fields at the growth surface that control the nucleation of QDs. We provide an in-depth review of the underlying mechanisms and technological implementations, and discuss the differences and comparative advantages of the buried stressor method over other techniques for site-controlled growth of QDs. We also address persistent challenges, such as scalability and integration with existing semiconductor technologies, and outline potential future research directions.

The knowledge of the energy spectrum completely defines the dynamics of a quantum system for a given initial state. This makes spectroscopy a key characterization technique when studying or designing qubits and complex quantum systems. In semiconductor quantum dots, the electronic quantum states can be probed through charge transport spectroscopy, but the electric current itself disrupts the fragile quantum system, and the technique is practically limited to gate-defined quantum dots. Epitaxial quantum dots benefit from excellent optical properties, but are usually incompatible with charge transport, while alternative spectroscopy techniques provide only limited information. Here we demonstrate a spectroscopy technique which utilizes nuclear spins as a non-invasive probe. By using spin currents instead of the charge currents we achieve near-equilibrium probing. Experiments are conducted on low-strain GaAs/AlGaAs epitaxial dots, revealing energy spectra for charge configurations with up to seven electrons and the subtle properties of the multi-electron states. The rich variety of observations includes long-lived spin-qubit states in s and p shells, ground-state phase transitions, strong spin-orbit coupling regimes, and anomalously fast nuclear spin diffusion. Experiments are backed up by good agreement with the first-principles configuration-interaction numerical modelling. Our work uncovers few-electron states as a new operating regime for optically active quantum dots. Accurate control and probing of many-body states offers a test-bed system for fundamental physics studies, while prospective technological applications include electron spin qubits with extended coherence and scalable electrical control.

We report on the epitaxial growth, theoretical modeling, and structural as well as optical investigation of multi-layer, site-controlled quantum dots fabricated using the buried stressor method. This deterministic growth technique utilizes the strain from a partially oxidized AlAs layer to induce site-selective nucleation of InGaAs quantum dots. By implementing strain-induced spectral nano-engineering, we achieve spectral control of emission and a local increase in the emitter density. Furthermore, we achieve a threefold increase in the optical intensity and reduce the inhomogeneous broadening of the ensemble emission by 20% via stacking three layers of site-controlled emitters, which is valuable for using the SCQDs as a gain medium in microlaser applications. Our optimization of site-controlled growth of quantum dots enables the development of high-β microlasers with increased confinement factor.

Despite the superiority in quantum properties, self-assembled semiconductor quantum dots face challenges in terms of scalable device integration because of their random growth positions, originating from the Stranski-Krastanov growth mode. Even with existing site-controlled growth techniques, for example, nanohole or buried stressor concepts, a further lithography and etching step with high spatial alignment requirements isnecessary to accurately integrate QDs into the nanophotonic devices. Here, we report on the fabrication and characterization of strain-induced site-controlled microcavities where site-controlled quantum dots are positioned at the antinode of the optical mode field in a self-aligned manner without the need of any further nano-processing. We show that the Q-factor, mode volume, height, and the ellipticity of site-controlled microcavities can be tailored by the size of an integrated AlAs/Al2O3 buried stressor, with an opening ranging from 1 to 4 $μ$m. Lasing signatures, including super-linear input-output response, linewidth narrowing near threshold, and gain competition above threshold, are observed for a 3.6-$μ$m self-aligned cavity with a Q-factor of 18000. Furthermore, by waiving the rather complex lateral nano-structuring usually performed during the fabrication process of micropillar lasers and vertical-cavity surface emitting lasers, quasi-planar site-controlled cavities exhibit no detrimental effects of excitation power induced heating and thermal rollover. Our straightforward deterministic nanofabrication concept of high-quality quantum dot microcavities integrates seamlessly with the industrial-matured manufacturing process and the buried-stressor techniques, paving the way for exceptional scalability and straightforward manufacturing of high-\b{eta} microlasers and bright quantum light sources.

We report on the epitaxial growth, theoretical modeling, and structural as well as optical investigation of multi-layer, site-controlled quantum dots fabricated using the buried stressor method. This advanced growth technique utilizes the strain from a partially oxidized AlAs layer to induce site-selective nucleation of InGaAs quantum dots. By implementing strain-induced spectral nano-engineering, we achieve separation in emission energy by about 150 meV of positioned and non-positioned quantum dots and a local increase of the emitter density in a single layer. Furthermore, we achieve a threefold increase of the optical intensity and reduce the inhomogeneous broadening of the ensemble emission by 20% via stacking three layers of site-controlled emitters, which is particularly valuable for using the SCQDs in microlaser applications. Moreover, we obtain direct control over emission properties by adjusting the growth and fabrication parameters. Our optimization of site-controlled growth of quantum dots enables the development of photonic devices with enhanced light-matter interaction and microlasers with increased confinement factor and spontaneous emission coupling efficiency.