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# Publications

The ongoing miniaturization of semiconductor lasers has enabled ultra-low threshold devices and even provided a path to approach thresholdless lasing with linear input–output characteristics. Such nanoscale lasers have initiated a discourse on the origin of the physical mechanisms involved and their boundaries, such as the required photon number, the importance of optimized light confinement in a resonator, and mode-density enhancement. Here, high-β metal-clad coaxial nanolasers, which facilitate thresholdless lasing are investigated. Both the conventional lasing characteristics, as well as the photon statistics of the emitted light at 10 K under continuous wave excitation are experimentally and theoretically investigated. While the former lacks adequate information to determine the threshold to coherent radiation, the latter reveals a finite threshold pump power. The work confirms an important aspect of high-β lasers, namely that a thresholdless laser does have a finite threshold pump power and must not be confused with a hypothetical zero-threshold laser. Moreover, the results reveal an excitation power dependent β-factor which needs to be taken into account to correctly describe the experimental data. S. Kreinberg et al., Thresholdless Transition to Coherent Emission at Telecom Wavelengths from Coaxial Nanolasers with Excitation Power Dependent β-Factors, Laser & Photonics Reviews, 2000065 (2020) |

Chiral light-matter interaction can lead to directional emission of two-level light emitters in waveguides. This interesting physics effect has raised considerable attention in recent years especially in terms of on-chip quantum systems. In this context, our work focuses on tailoring single semiconductor quantum dot – waveguide (QD-WG) systems to emit single photons with high directionality. We use low-temperature in-situ electron-beam lithography enabled by cathodoluminescence mapping to select suitable QDs and to integrate them deterministically into linear waveguide structures at specific chiral points determined by numerical calculations. We observe excitonic and biexcitonic emission from the fabricated QD-WG structure in a confocal µPL setup enabling the optical characterization in terms of directional emission of circularly polarized photons emitted by integrated QDs. Our results show a high degree of anisotropy on the level of 54% for directional QD emission and antibunching in autocorrelation experiment confirming the fabricated QD-WG system, which is a prerequisite for using this effect in advanced applications in integrated quantum circuits. P. Mrowiński et al., Directional single-photon emission from deterministic quantum dot waveguide structures, physica status solidi (RRL), Accepted Author Manuscript (2020) ArXiv e-prints, 2005.09322v1 (2020) |

A semiconductor quantum dot (QD) is an attractive resource to generate polarization-entangled photon pairs. We study the excitonic spin precession (flip-flop) in a family of QDs with different excitonic fine-structure splitting (FSS) and its impact on the entanglement of photons generated from the excitonic-biexcitonic radiative cascade. Our results reveal that coherent processes leave the time post-selected entanglement of QDs with finite FSS unaffected while changing the eigenstates of the system. The flip-flop's precession is observed via quantum tomography through anomalous oscillations of the coincidences in the rectilinear basis. A theoretical model is constructed with the inclusion of an excitonic flip-flop rate and is compared with a two-photon quantum tomography measurement on a QD exhibiting the spin flip-flop mechanism. A generalization of the theoretical model allows estimating the degree of entanglement as a function of the FSS and the spin-flip rate. For a finite temporal resolution, the negativity is found to be oscillating with respect to both the FSS and the spin-flip rate. This oscillatory behavior disappears for perfect temporal resolution and maximal entanglement is retrieved despite the flip-flop process. S. Bounouar et al., Entanglement robustness to excitonic spin precession in a quantum dot, ArXiv e-prints, 2001.11833 (2020) |

Mutual coupling and injection locking of semiconductor lasers is of great interest in non-linear dynamics and its applications for instance in secure data communication and photonic reservoir computing. Despite its importance, it has hardly been studied in microlasers operating at μW light levels. In this context, vertically emitting quantum dot micropillar lasers are of high interest. Usually, their light emission is bimodal, and the gain competition of the associated linearly polarized fundamental emission modes results in complex switching dynamics. We report on selective optical injection into either one of the two fundamental mode components of a bimodal micropillar laser. Both modes can lock to the master laser and influence the non-injected mode by reducing the available gain. We demonstrate that the switching dynamics can be tailored externally via optical injection in very good agreement with our theory based on semi-classical rate equations. E. Schlottmann et al., Stochastic polarization switching induced by optical injection in bimodal quantum-dot micropillar lasers, Opt. Express 27, 28816-28831 (2019) |

We report on the experimental study and numerical analysis of chiral light-matter coupling in deterministically fabricated quantum dot (QD) waveguide structures. We apply in-situ electron beam lithography to deterministically integrate single InGaAs/GaAs QDs into GaAs-DBR waveguides to systematically explore the dependence of chiral coupling on the position of the QD inside the waveguide. By a series of micro-photoluminescence measurements, we determine the directionality contrast of emission into left and right traveling waveguide modes revealing a maximum of 0.93 for highly off-center QDs and an oscillatory dependence of this contrast on the QD position. In numerical simulations we obtain insight into chiral light-matter coupling by computing the light field emitted by a circularly polarized source and its overlap with multiple guided modes of the structure, which enables us to calculate directional β-factors for the quantum emitters. The calculated dependence of the directionality on the off-center QD position is in good agreement with the experimental data. It confirms the control of chiral effects in deterministically fabricated QD-waveguide systems with high potential for future non-reciprocal on-chip systems required for quantum information processing. P. Mrowinski et al., Directional emission of a deterministically fabricated quantum dot - Bragg reflection multi-mode waveguide system, ACS Photonics ASAP (2019) ArXiv e-prints, 1902.01905 (2019) |

How small can a laser be made? What characteristic properties must a device possess to still be called a laser? How can one prove laser action in the limiting cases of single-emitter and thresholdless operation? What are the prospects and the application potential of semiconductor nanolasers, and where does the ongoing miniaturization lead to? This topical review focuses on the role that quantum-dot micro- and nanolasers have played in these developments and on the impressive progress that has been made over the last decade in this field, with an emphasis on quantum dot-micropillar lasers. We discuss the design, modeling, fabrication, and quantum-optical characterization of quantum-dot micropillar lasers, attempt to provide answers to the above questions, and give an outlook on the direction of development of the field and on exciting opportunities for future applications of micro- and nanoscale lasers in quantum nanophotonics. C. Gies and S. Reitzenstein, Quantum dot micropillar lasers, Semicond. Sci. Technol. 34, 073001 (2019) |

The ongoing miniaturization of semiconductor lasers has enabled ultra-low threshold devices and even provided a path to approach thresholdless lasing with linear input-output characteristics. Such nanoscale lasers have initiated a discourse on the origin of the physical mechanisms involved and their boundaries, such as the required photon number, the importance of optimized light confinement in a resonator and mode-density enhancement. Here, we investigate high-β metal-clad coaxial nanolasers, which facilitate thresholdless lasing. We experimentally and theoretically investigate both the conventional lasing characteristics, as well as the photon statistics of the emitted light. While the former lacks adequate information to determine the threshold to coherent radiation, the latter reveals a finite threshold pump power. Our work clearly highlights an important and often misunderstood aspect of high-β lasers, namely that a thresholdless laser does have a finite threshold pump power and must not be confused with a hypothetical zero threshold laser. S. Kreinberg et al., Thresholdless transition to coherent emission at telecom wavelengths from coaxial nanolasers, ArXiv e-prints, 1905.09924 (2019) |

We discuss phonon-induced non-Markovian and Markovian features in QD-based quantum nanooptics. We cover lineshapes in linear absorption experiments, phonon-induced incoherence in the Heitler regime, and memory correlations in two-photon coherences. To qualitatively and quantitatively understand the underlying physics, we present several theoretical models that capture the non-Markovian properties of the electron–phonon interaction accurately in different regimes. Examples are the Heisenberg equation of motion approach, the polaron master equation, and Liouville propagator techniques in the independent boson limit and beyond via the path integral method. Phenomenological modeling overestimates typically the dephasing due to the finite memory kernel of phonons and we give instructive examples of phonon-mediated coherence such as phonon-dressed anticrossings in Mollow physics, robust quantum state preparation, cavity feeding, and the stabilization of the collapse and revival phenomenon in the strong coupling limit of cavity quantum electrodynamics. A. Camele and S. Reitzenstein, Non-Markovian features in semiconductor quantum optics: quantifying the role of phonons in experiment and theory, Nanophotonics, 20180222, ISSN (Online) 2192-8614 (2019) |

Synchronization of coupled oscillators at the transition between classical physics and quantum physics has become an emerging research topic at the crossroads of nonlinear dynamics and nanophotonics. We study this unexplored field by using quantum dot microlasers as optical oscillators. Operating in the regime of cavity quantum electrodynamics (cQED) with an intracavity photon number on the order of 10 and output powers in the 100 nW range, these devices have high β-factors associated with enhanced spontaneous emission noise. We identify synchronization of mutually coupled microlasers via frequency locking associated with a sub-gigahertz locking range. A theoretical analysis of the coupling behavior reveals striking differences from optical synchronization in the classical domain with negligible spontaneous emission noise. Beyond that, additional self-feedback leads to zero-lag synchronization of coupled microlasers at ultra-low light levels. Our work has high potential to pave the way for future experiments in the quantum regime of synchronization. S. Kreinberg et al., Mutual coupling and synchronization of optically coupled quantum-dot micropillar lasers at ultra-low light levels, Nature Communications 10, 1539 (2019) ArXiv e-prints, 1808.01483 (2018) |

The development of ultimate microcavity lasers requires precise engineering of the gain medium. Of particular interest are microlasers based on discrete gain centers, which are aligned to the field maximum of the cavity mode to maximize the modal gain. Here, we report on micropillar lasers with a gain medium composed of site-controlled quantum dots (SCQDs). Adjusting the size of a buried stressor, we define the number of high-quality SCQDs located at the antinode of the fundamental cavity mode. Our deterministic nanoprocessing platform allows us to tightly control the emission properties of high-β microlasers operating in the few-QD regime. A. Kaganskiy et al., Micropillar lasers with site-controlled quantum dots as active medium, Optica, OSA, 2019, 6, 404-409 (2019) |

Resonant scattering of weak coherent laser pulses on a single two-level system realized in a semiconductor quantum dot is investigated with respect to a time delay between incoming and scattered light. This type of time delay was predicted by Wigner in 1955 for purely coherent scattering and was confirmed for an atomic system in 2013 [R. Bourgain et al., Opt. Lett. 38, 1963 (2013)]. In the presence of electron-phonon interaction, we observe deviations from Wigner’s theory related to incoherent and strongly non-Markovian scattering processes which are hard to quantify via a detuning-independent pure dephasing time. We observe detuning-dependent Wigner delays of up to 530 ps in our experiments which are supported quantitatively by microscopic theory allowing for pure dephasing times of up to 950 ps. M. Strauss et al., Wigner Time Delay Induced by a Single Quantum Dot, Phys. Rev. Lett., 122, 107401 (2019) ArXiv e-prints, 1805.06357 (2018) |

Mollow physics in the two-photon regime shows interesting features such as versatile generation of highly correlated photon pairs, timely ordered and energy tunable. Here, we calculate analytically the power spectral density S(ω) and the two-photon correlations g(2)(τ), which are essential to discuss and study such phenomena in the resonant-driven dressed-state regime. It is shown that there exist upper and lower bounds of the g(2)(τ) function for certain spectrally selected photon pairs. Recently reported experiments agree with the presented theory and thereby it is shown that the resonant-driven four-level system is an interesting source for steerable quantum light in quantum cascade setups. We furthermore discuss the unlikeliness of observing antibunching for the delay time τ=0 in the exciton-biexciton correlation functions in such experiments, since antibunching stems from a coherent and in-phase superposition of different photon-emission events. Due to the occurring laser photon scattering, this coherent superposition state is easily disturbed and leads to correlation functions of g(2)(0)=1. J. Schleibner et al., Suppressed antibunching via spectral filtering: An analytical study in the two-photon Mollow regime, Phys. Rev. A, 99, 23813 (2019) |

We investigate the mode-switching dynamics of an electrically driven bimodal quantum-dot micropillar laser when subject to delayed coherent optical feedback from a short external cavity. We experimentally characterize how the external cavity length, being on the same order than the microlaser’s coherence length, influences the spectral and dynamical properties of the micropillar laser. Moreover, we determine the relaxation oscillation frequency of the micropillar by superimposing optical pulse injection to a dc current. It is found that the optical pulse can be used to disturb the feedback-coupled laser within one roundtrip time in such a way that it reaches the same output power as if no feedback was present. Our results do not only expand the understanding of microlasers when subject to optical feedback from short external cavities, but pave the way towards tailoring the properties of this key nanophotonic system for studies in the quantum regime of self-feedback and its implementation to integrated photonic circuits. S. Holzinger et al., Quantum-dot micropillar lasers subject to coherent time-delayed optical feedback from a short external cavity, Scientific Reports 9, Article number: 631 (2019) |

The linewidth enhancement factor α is a key parameter determining the spectral and dynamical behavior of semiconductor lasers. Here, we propose and demonstrate a method for determining this parameter based on a direct measurement of variations in the laser gain and emission spectrum when subject to delayed optical feedback. We then use our approach to determine the pump current dependent linewidth enhancement factor of a high-β quantum dot micropillar laser. The validity of our approach is confirmed comparing it to two conventional methods, one based on the comparison of the linewidths above and below threshold and the other based on injection locking properties. Furthermore, the pump power dependence of α is quantitatively described by simulations based on a quantum-optical model. S. Holzinger et al., Determining the linewidth enhancement factor via optical feedback in quantum dot micropillar lasers, Opt. Express, OSA, 26, 31363-31371 (2018) |

This focused review discusses the increasing importance of quantum optics in the physics and engineering of optoelectronic components. Two influences relating to cavity quantum electrodynamics are presented. One involves the development of low threshold lasers, when the channeling of spontaneous emission into the lasing mode becomes so efficient that the concept of lasing needs revisiting. The second involves the quieting of photon statistics to produce single-photon sources for applications such as quantum information processing. An experimental platform, consisting of quantum-dot gain media inside micro- and nanocavities, is used to illustrate these influences of the quantum mechanical aspect of radiation. An overview is also given on cavity quantum electrodynamics models that may be applied to analyze experiments or design devices. W. W. Chow and S. Reitzenstein, Quantum-optical influences in optoelectronics—An introduction, Applied Physics Reviews 5, 041302 (2018) |

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. T. Heuser et al., Fabrication of dense diameter-tuned quantum dot micropillar arrays for applications in photonic information processing, APL Photonics 3, 116103 (2018) |

Two-level emitters constitute main building blocks of photonic quantum systems and are model systems for the exploration of quantum optics in the solid state. Most interesting is the strict-resonant excitation of such emitters to generate close to ideal quantum light and to control their occupation coherently. Up till now related experiments have been performed exclusively using bulky lasers which hinders the application of resonantly driven two-level emitters in photonic quantum systems. Here we perform quantum-optical spectroscopy of a two-level system using a compact high-β microlaser as excitation source. The two-level system is based on a semiconductor quantum dot (QD), which is excited resonantly by a fiber-coupled electrically driven micropillar laser. In this way we dress the excitonic state of the QD under continuous wave excitation and trigger the emission of single-photons with strong multi-photon suppression (g(2)(0)=0.02) and high photon indistinguishably (V=57 ± 9%) via pulsed resonant excitation at 156 MHz. S. Kreinberg et al., Quantum-optical spectroscopy of a two-level system using an electrically driven micropillar laser as a resonant excitation source, Light: Science & Applications 7, 41 (2018) ArXiv e-prints, 1711.09705 (2017) |

Microlasers are ideal candidates to bring the fascinating variety of nonlinear complex dynamics found in delay-coupled systems to the realm of quantum optics. Particularly attractive is the possibility of tailoring the devices' emission properties via non-invasive delayed optical coupling. However, until now scarce research has been done in this direction. Here, we experimentally and theoretically investigate the effects of delayed optical feedback on the mode-switching dynamics of an electrically driven bimodal quantum-dot micropillar laser, characterizing its impact on the micropillar's output power, optical spectrum and photon statistics. Feedback is found to influence the switching dynamics and its characteristics time scales. In addition, stochastic switching is reduced with the subsequent impact on the microlaser photon statistics. Our results contribute to the comprehension of feedback-induced phenomena in micropillar lasers and pave the way towards the external control and tailoring of the properties of these key systems for the nanophotonics community. S. Holzinger et al., Tailoring the mode-switching dynamics in quantum-dot micropillar lasers via time-delayed optical feedback, Opt. Express 26 (17), 22457--22470 (2018) |

We measure the full photon-number distribution emitted from a Bose condensate of microcavity exciton-polaritons confined in a micropillar cavity. The statistics are acquired by means of a photonnumber resolving transition edge sensor. We directly observe that the photon-number distribution evolves with the non-resonant optical excitation power from geometric to quasi-Poissonian statistics, which is canonical for a transition from a thermal to a coherent state. Moreover, the photon-number distribution allows evaluating the higher-order photon correlations, shedding further light on the coherence formation and phase transition of the polariton condensate. The experimental data is analyzed in terms of thermal coherent states which allows one to directly extract the thermal and coherent fraction from the measured distributions. These results pave the way for a full understanding of the contribution of interactions in light-matter condensates in the coherence buildup at threshold. M. Klaas et al., Photon number-resolved measurement of an exciton-polariton condensate, Phys. Rev. Lett. 121, 047401 (2018) ArXiv e-prints, 1805.02959 (2018) |

A photon-number-resolving transition edge sensor (TES) is used to measure the photon-number distribution of two microcavity lasers. The investigated devices are bimodal microlasers with similar emission intensity and photon statistics with respect to the photon autocorrelation. Both high-β microlasers show partly thermal and partly coherent emission around the lasing threshold. For higher pump powers, the strong mode of microlaser A emits Poissonian distributed photons, while the emission of the weak mode is thermal. By contrast, laser B shows a bistability resulting in overlayed thermal and Poissonian distributions. While a standard Hanbury Brown and Twiss experiment cannot distinguish between the simple thermal emission of laser A and the temporal mode switching of the bistable laser B, TESs allow us to measure the photon-number distribution, which provides important insight into the underlying emission processes. Indeed, our experimental data and their theoretical description by a master equation approach show that TESs are capable of revealing subtle effects like the mode switching of bimodal microlasers. As such, we clearly demonstrate the benefit and importance of investigating nanophotonic devices via photon-number-resolving transition edge sensors. E. Schlottmann et al.,Exploring the Photon-Number Distribution of Bimodal Microlasers, Phys. Rev. Applied 9, 064030 (2018) ArXiv eprints 1709.04312 (2017) |

The development of multi-node quantum optical circuits has attracted great attention in recent years. In particular, interfacing quantum-light sources, gates and detectors on a single chip is highly desirable for the realization of large networks. In this context, fabrication techniques that enable the deterministic integration of pre-selected quantum-light emitters into nanophotonic elements play a key role when moving forward to circuits containing multiple emitters. Here, we present the deterministic integration of an InAs quantum dot into a 50/50 multi-mode interference beamsplitter via in-situ electron beam lithography. We demonstrate the combined emitter-gate interface functionality by measuring triggered single-photon emission on-chip with g^{(2)}(0) = 0.13 ± 0.02. Due to its high patterning resolution as well as spectral and spatial control, in-situ electron beam lithography allows for integration of pre-selected quantum emitters into complex photonic systems. Being a scalable single-step approach, it paves the way towards multi-node, fully integrated quantum photonic chips. P. Schnauber et al., Deterministic integration of quantum dots into on-chip multi-mode interference beamsplitters using in-situ electron beam lithography, Nano Letters (2018) ArXiv eprints 1712.03837 (2017) |

We provide experimental and theoretical insight into single-emitter lasing effects in a quantum dot (QD)-microlaser under controlled variation of background gain provided by off-resonant discrete gain centers. For that purpose, we apply an advanced two-color excitation concept where the background gain contribution of off-resonant QDs can be continuously tuned by precisely balancing the relative excitation power of two lasers emitting at different wavelengths. In this way, by selectively exciting a single resonant QD and off-resonant QDs, we identify distinct single-QD signatures in the lasing characteristics and distinguish between gain contributions of a single resonant emitter and a countable number of off-resonant background emitters to the optical output of the microlaser. Our work addresses the important question whether single-QD lasing is feasible in experimentally accessible systems and shows that, for the investigated microlaser, the single-QD gain needs to be supported by the background gain contribution of off-resonant QDs to reach the transition to lasing. Interestingly, while a single QD cannot drive the investigated micropillar into lasing, its relative contribution to the emission can be as high as 70% and it dominates the statistics of emitted photons in the intermediate excitation regime below threshold. F. Gericke et al.,Controlling the gain contribution of background emitters in few-quantum-dot microlasers, New Journal of Physics 20, 023036 (2018) ArXiv eprints 1711.09235 (2017) |

We report on the realization of micropillars with site-controlled quantum dots (SCQDs) in the active layer. The SCQDs are grown via the buried stressor approach which allows for the positioned growth and device integration of a controllable number of QDs with high optical quality. This concept is very powerful as the number and the position of SCQDs in the cavity can be simultaneously controlled by the design of the buried stressor. The fabricated micropillars exhibit a high degree of position control for the QDs above the buried stressor and Q-factors of up to 12000 at an emission wavelength around 930 nm. We experimentally analyze and numerically model the cavity Q-factor, the mode volume, the Purcell factor and the photon-extraction efficiency as a function of the aperture diameter of the buried stressor. Exploiting these SCQD micropillars, we experimentally observe the Purcell enhancement in the single-QD regime with F_P = 4.3 ± 0.3. A. Kaganskiy et al., Micropillars with a controlled number of site-controlled quantum dots, Applied Physics Letters 112, 071101 (2018) ArXiv eprints 1711.09235 (2017) |

Exploring the limits of spontaneous emission coupling is not only one of the central goals in the development of nanolasers, it is also highly relevant regarding future large-scale photonic integration requiring energy-efficient coherent light sources with a small footprint. Recent studies in this field have triggered a vivid debate on how to prove and interpret lasing in the high-β regime. We investigate close-to-ideal spontaneous emission coupling in GaN nanobeam lasers grown on silicon. Such nanobeam cavities allow for efficient funneling of spontaneous emission from the quantum well gain material into the laser mode. By performing a comprehensive optical and quantum-optical characterization, supported by microscopic modeling of the nanolasers, we identify high-β lasing at room temperature and show a lasing transition in the absence of a threshold nonlinearity at 156 K. This peculiar characteristic is explained in terms of a temperature and excitation power-dependent interplay between zero-dimensional and two-dimensional gain contributions. S. Jagsch et al., A quantum optical study of thresholdless lasing features in high-β nitride nanobeam cavities, Nature Communications 9, 564 (2018) |

In this work, we present a stand-alone and fiber-coupled quantum-light source. The plug-and-play device is based on an optically driven quantum dot delivering single photons via an optical fiber. The quantum dot is deterministically integrated in a monolithic microlens which is precisely coupled to the core of an optical fiber via active optical alignment and epoxide adhesive bonding. The rigidly coupled fiber-emitter assembly is integrated in a compact Stirling cryocooler with a base temperature of 35 K. We benchmark our practical quantum device via photon auto-correlation measurements revealing g(2)(0) = 0.07 ± 0.05 under continuous-wave excitation and we demonstrate triggered non-classical light at a repetition rate of 80 MHz. The long-term stability of our quantum light source is evaluated by endurance tests showing that the fiber-coupled quantum dot emission is stable within 4% over several successive cool-down/warm-up cycles. Additionally, we demonstrate non-classical photon emission for a user-intervention-free 100-hour test run and stable single-photon count rates up to 11.7 kHz with a standard deviation of 4%. A. Schlehahn et al., A stand-alone fiber-coupled single-photon source, Scientific Reports 8, 1340 (2018) |

The dark exciton state in semiconductor quantum dots constitutes a long-lived solid-state qubit which has the potential to play an important role in implementations of solid-state based quantum information architectures. In this work, we exploit deterministically fabricated QD microlenses with enhanced photon extraction, to optically prepare and readout the dark exciton spin and observe its coherent precession. The optical access to the dark exciton is provided via spin-blockaded metastable biexciton states acting as heralding state, which are identified deploying polarization-sensitive spectroscopy as well as time-resolved photon cross-correlation experiments. Our experiments reveal a spin-precession period of the dark exciton of (0.82±0.01)ns corresponding to a fine-structure splitting of (5.0±0.7) μeV between its eigenstates |↑⇑±↓⇓⟩. By exploiting microlenses deterministically fabricated above pre-selected QDs, our work demonstrates the possibility to scale up implementations of quantum information processing schemes using the QD-confined dark exciton spin qubit, such as the generation of photonic cluster states or the realization of a solid-state-based quantum memory. T. Heindel et al., Accessing the dark exciton in deterministic quantum-dot microlenses, APL Photonics 2, 121303 (2017) featured headliner | Press Release |

Mollow physics in the two-photon regime shows interesting features such as path-controlled time-reordering of photon pairs without the need to delay them. Here, we calculate analytically the two-photon correlations g(2)(τ), essential to discuss and study such phenomena in the resonant-driven dressed-state regime. It is shown that there exists upper and lower bounds of the g(2)(τ)−function for certain spectrally-selected photon pairs. Recent reported experiments agree with the presented theory and thereby it is shown that the resonant-driven four-level system is an interesting source for steerable quantum light in quantum cascade setups. We furthermore discuss the unlikeliness to observe antibunching for the delay time τ=0 in the exciton-biexciton correlation functions in such experiments, since antibunching stems from a coherent and in-phase superposition of different photon emission events. Due to the occuring laser photon scattering, this coherent superposition state is easily disturbed and leads to correlation functions of g(2)(0) = 1 A. Carmele, S. Bounouar, M. Strauss, S. Reitzenstein and A. Knorr, Correlations of cascaded photons: An analytical study in the two-photon Mollow regime, ArXiv e-prints 1710.03031 (2017) |

We report on the realization of scalable single-photon sources (SPSs) based on a single site-controlled quantum dot (SCQD) and deterministically fabricated microlenses. The fabrication process comprises the buried-stressor growth technique complemented with low-temperature in-situ electron-beam lithography for the integration of SCQDs into microlens structures with high yield and high alignment accuracy. The microlens-approach leads to a broadband enhancement of the photon extraction efficiency of up to (21 ± 2 %) and a high suppression of multi-photon events with g(2)(τ = 0) < 0.06 without background subtraction. As such the combination of site-controlled growth of QDs and in-situ electron-beam lithography is relevant for arrays of efficient SPSs which can be applied in photonic quantum circuits and advanced quantum computation schemes. A. Kaganskiy et al., Enhancing the photon-extraction efficiency of site-controlled quantum dots by deterministically fabricated microlenses, Optics Communications, 418, 162-166 (2018) ArXiv e-prints, 1708.03512 (2017) |

Insight is given into the regime where strong light-matter coupling and lasing coincide in cavity- QED systems. An analytic expression is provided for the emission spectrum and for the condition marking the transition from strong to weak coupling that remains valid close to the laser threshold. In a direct comparison between theory and experiment, we demonstrate the applicability of our approach and give evidence for the coexistence of strong coupling and lasing in a high-beta quantum- dot microcavity laser. We address the limitations of present realizations of quantum-dot microlasers operating in the strong coupling regime and derive parameters under which true single emitter lasing can be achieved in the future. C. Gies et al., Strong light-matter coupling in the presence of lasing, Phys. Rev. A 96, 023806 (2017) ArXiv e-prints 1606.05591 (2016) |

We investigate the switching of the coherent emission mode of a bimodal microcavity device, occurring when the pump power is varied. We compare experimental data to theoretical results and identify the underlying mechanism based on the competition between the effective gain, on the one hand, and the intermode kinetics, on the other.When the pumping is ramped up, above a threshold, the mode with the largest effective gain starts to emit coherent light, corresponding to lasing. In contrast, in the limit of strong pumping, it is the intermode kinetics that determines which mode acquires a large occupation and shows coherent emission. We point out that this latter mechanism is akin to the equilibrium Bose-Einstein condensation of massive bosons. Thus, the mode switching in our microcavity device can be viewed as a minimal instance of Bose-Einstein condensation of photons. Moreover, we show that the switching from one cavity mode to the other always occurs via an intermediate phase where both modes are emitting coherent light and that it is associated with both superthermal intensity fluctuations and strong anticorrelations between both modes. H.A.M. Leymann et al., Pump-Power-Driven Mode Switching in a Microcavity Device and Its Relation to Bose-Einstein Condensation, Phys. Rev. X 7, 021045 (2017) ArXiv e-prints, 1612.04312 (2016) |

Integrated single-photon sources with high photonextraction efficiency are key building blocks for applications in the field of quantum communications. We report on a bright singlephoton source realized by on-chip integration of a deterministic quantum dot microlens with a 3D-printed multilens micro-objective. The device concept benefits from a sophisticated combination of in situ 3D electron-beam lithography to realize the quantum dot microlens and 3D femtosecond direct laser writing for creation of the micro-objective. In this way, we obtain a high-quality quantum device with broadband photon-extraction efficiency of (40 ± 4)% and high suppression of multiphoton emission events with g(2)(τ = 0) < 0.02. Our results highlight the opportunities that arise from tailoring the optical properties of quantum emitters using integrated optics with high potential for the further development of plug-and-play fiber-coupled single-photon sources. S. Fischbach et al.,Single Quantum Dot with Microlens and 3D-Printed Micro-objective as Integrated Bright Single-Photon Source, ACS Photonics 4, 6, 1327-1332 (2017) |

The two-photon dressing of a “three-level ladder” system, here the ground state, the exciton, and the biexciton of a semiconductor quantum dot, leads to new eigenstates and allows one to manipulate the time ordering of the paired photons without unitary postprocessing.We show that, after spectral postselection of the single dressed states, the time ordering of the cascaded photons can be removed or conserved. Our joint experimental and theoretical study demonstrates the high potential of a “ladder” system to be a versatile source of orthogonally polarized, bunched or antibunched pairs of photons. Physical Review Letters 118, 233601 (2017) doi: 10.1103/PhysRevLett.118.233601 ArXiv e-prints, 1610.08268 (2016) |

Measured and calculated results are presented on the emission properties of a new class of emitters operating in the cavity quantum electrodynamics regime. The structures are based on high-finesse GaAs/AlAs micropillar cavities, each with an active medium consisting of a layer of InGaAs quantum dots and distinguishing feature of having substantial fraction of spontaneous emission channeled into one cavity mode (high-beta factor). This paper shows that the usual criterion for lasing with a conventional (low-beta factor) cavity, a sharp nonlinearity in an input-output curve accompanied by noticeable linewidth narrowing, has to be reinforced by the equal-time second-order photon autocorrelation function for confirming lasing. It will also show that the equal-time second-order photon autocorrelation function is useful for recognizing superradiance, a manifestation of the correlations possible in high- microcavities operating with quantum dots. In terms of consolidating the collected data and identifying the physics underlying laser action, both theory and experiment suggest a sole dependence on intracavity photon number. Evidence for this comes from all our measured and calculated data on emission coherence and fluctuation, for devices ranging from LEDs and cavity-enhanced LEDs to lasers, lying on the same two curves: one for linewidth narrowing versus intracavity photon number and the other for g(2)(0) versus intracavity photon number. Light: Science & Applications (2017) 6, e17030; doi: 10.1038/lsa.2017.30. ArXiv e-prints, 1610.04129 (2016) |

Implementing time-delayed feedback in optoelectronic circuits allows one to uncover the rich physics and application potential of nonlinear dynamics. Important feedback effects are, for instance, the generation of broadband chaos, or laser self-pulsing. We explore the effect of optoelectronic feedback in an ultracompact microlaser–microdetector assembly operating in the regime of cavity quantum electrodynamics (cQED). This system is used to generate self-pulsing at MHz frequencies in the emission of a microlaser, which is qualitatively explained by a rate equation model taking cQED effects into account. The results show promise for exploring chaos in ultracompact nanophotonic systems and for technological approaches toward chaos-based secure communication, random number generation, and self-pulsed single photon sources on a highly integrated semiconductor platform. P. Munnelly et al., On-chip optoelectronic feedback in a micropillar laser-detector assembly , Optica 4, 303–306 (2017) See Press Release: Chaotische Lichtpulse in winzigen Quantenbauteilen |

We study experimentally and theoretically a coherently-driven strongly-coupled quantum dot-microcavity system. Our focus is on physics of the unexplored intermediate excitation regime where the resonant laser field dresses a strongly-coupled single exciton-photon (polariton) system resulting in a ladder of laser-dressed Jaynes-Cummings states. In that case both the coupling of the emitter to the confined light field of the microcavity and to the light field of the external laser are equally important, as proved by observation of injection pulling of the polariton branches by an external laser. This intermediate interaction regime is of particular interest since it connects the purely quantum mechanical Jaynes-Cummings ladder and the semi-classical Autler-Townes ladder. Exploring the driving strength-dependence of the mutually coupled system we establish the maximum in the resonance fluorescence signal to be a robust fingerprint of the intermediate regime and observe signatures indicating the laser-dressed Jaynes-Cummings ladder. In order to address the underlying physics we excite the coupled system via the matter component of fermionic nature undergoing saturation - in contrast to commonly used cavity-mediated excitation. C. Hopfmann et al., Transition from Jaynes-Cummings to Autler-Townes ladder in a quantum dot-microcavity system, Phys. Rev. B 95, 035302 (2017) arxiv.org/abs/1609.03462 |

Site-controlled growth of semiconductor quantum dots (QDs) represents a major advancement to achieve scalable quantum technology platforms. One immediate benefit is the deterministic integration of quantum emitters into optical microcavities. However, site-controlled growth of QDs is usually achieved at the cost of reduced optical quality. Here, we show that the buried-stressor growth technique enables the realization of high-quality site-controlled QDs with attractive optical and quantum optical properties. This is evidenced by performing excitation power dependent resonance fluorescence experiments at cryogenic temperatures showing QD emission linewidths down to 10 μeV. Resonant excitation leads to the observation of the Mollow triplet under CW excitation and enables coherent state preparation under pulsed excitation. Under resonant π-pulse excitation we observe clean single photon emission associated with g(2)(0)=0.12 limited by non-ideal laser suppression. M. Strauß et al., Resonance fuorescence of a site-controlled quantum dot realized by the buried-stressor growth technique, Appl. Phys. Lett. 110, 111101 (2017) arxiv.org/abs/1612.08063 |

We investigate the switching of the coherent emission mode of a bimodal microcavity device, occurring when the pump power is varied. We compare experimental data to theoretical results and identify the underlying mechanism to be based on the competition between the effective gain on the one hand and the intermode kinetics on the other. When the pumping is ramped up, above a threshold the mode with the largest effective gain starts to emit coherent light, corresponding to lasing. In contrast, in the limit of strong pumping it is the intermode kinetics that determines which mode acquires a large occupation and shows coherent emission. We point out that this latter mechanism is akin to the equilibrium Bose-Einstein condensation of massive bosons. Thus, the mode switching in our microcavity device can be viewed as a minimal instance of Bose-Einstein condensation of photons. We, moreover, show that the switching from one cavity mode to the other occurs always via an intermediate phase where both modes are emitting coherent light and that it is associated with both superthermal intensity fluctuations and strong anticorrelations between both modes. H. A. M. Leymann et al., Pump-power-driven mode switching in a microcavity device and its relation to Bose-Einstein condensation, ArXiv e-prints 1612.04312 (2016) |

We report on two-photon interference (TPI) experiments using remote deterministic single-photon sources. Employing 3D in-situ electron-beam lithography, we fabricate quantum-light sources at specific target wavelengths by integrating pre-selected semiconductor quantum dots within monolithic microlenses. The individual single-photon sources show TPI visibilities of 49% and 22%, respectively, under pulsed p-shell excitation at 80 MHz. For the mutual TPI of the remote sources, we observe an uncorrected visibility of 29%, in quantitative agreement with the pure dephasing of the individual sources. Due to its efficient photon extraction within a broad spectral range (> 20 nm), our microlens-based approach is predestinated for future entanglement swapping experiments utilizing entangled photon pairs emitted by distant biexciton-exciton radiative cascades. A. Thoma et al., Two-photon interference from remote deterministic quantum dot microlenses, Appl. Phys. Lett. 110, 011104 (2016) arxiv.org/abs/1611.06859 |

We experimentally and theoretically investigate injection locking of quantum dot (QD) microlasers in the regime of cavity quantum electrodynamics (cQED). We observe frequency locking and phase-locking where cavity enhanced spontaneous emission enables simultaneous stable oscillation at the master frequency and at the solitary frequency of the slave microlaser. Measurements of the second-order autocorrelation function prove this simultaneous presence of both master and slave-like emission, where the former has coherent character with g(2)(0)=1 while the latter one has thermal character with g(2)(0)=2. Semi-classical rate-equations explain this peculiar behavior by cavity enhanced spontaneous emission and a low number of photons in the laser mode. E. Schlottmann et al., Injection Locking of Quantum-Dot Microlasers Operating in the Few-Photon Regime, Phys. Rev. Applied 6, 044023 (2016) Physical Review Applied Editors' Suggestion (October 2016) arxiv.org/abs/1604.02817 |

The two-photon dressing of a "three-level ladder" system, here the ground state, the exciton and the biexciton of a semiconductor quantum dot, leads to new eigenstates and allows one to manipulate the time ordering of the paired photons without unitary post processing. We show that, after spectral post-selection of the single dressed states, the time ordering of the cascaded photons can be removed or conserved. Our joint experimental and theoretical study demonstrates the high potential of a "ladder" system to be a versatile source of orthogonally polarized, bunched or antibunched pairs of photons. S. Bounouar et al., Time reordering of paired photons in a dressed three-level cascade, ArXiv e-prints 1610.08268 (2016) |

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. A. Kaganskiy et al., CSAR 62 as negative-tone resist for high-contrast e-beam lithography at temperatures between 4 K and room temperature, J. Vac. Sci. Technol. B 34, 061603 (2016) arxiv.org/abs/1611.07266 |

We investigate the influence of the photon statistics on the excitation dynamics of a single two level system. A single semiconductor quantum dot represents the two level system and is resonantly excited either with coherent laser light, or excited with chaotic light, with photon statistics corresponding to that of thermal radiation. Experimentally, we observe a reduced absorption cross-section under chaotic excitation in the steady-state. In the transient regime, the Rabi oscillations observable under coherent excitation disappear under chaotic excitation. Likewise, in the emission spectrum the well-known Mollow triplet, which we observe under coherent drive, disappears under chaotic excitation. Our observations are fully consistent with theoretical predictions based on the semi-classical Bloch equation approach. M. Strauß et al., Photon-statistics excitation spectroscopy of a single two-level system, Phys. Rev. B 93, 241306(R) (2016) arxiv.org/abs/1601.05234 |

The super-thermal photon bunching in quantum-dot (QD) micropillar lasers is investigated both experimentally and theoretically via simulations driven by dynamic considerations. Using stochastic multi-mode rate equations we obtain very good agreement between experiment and theory in terms of intensity profiles and intensity-correlation properties of the examined QD micro-laser's emission. Further investigations of the time-dependent emission show that super-thermal photon bunching occurs due to irregular mode-switching events in the bimodal lasers. Our bifurcation analysis reveals that these switchings find their origin in an underlying bistability, such that spontaneous emission noise is able to effectively perturb the two competing modes in a small parameter region. We thus ascribe the observed high photon correlation to dynamical multistabilities rather than quantum mechanical correlations. Redlich et al., Mode-switching induced super-thermal bunching in quantum-dot microlasers, New J. Phys. 18, 063011 (2016) |

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