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We report on the linear polarization of polariton condensates in a codirectional coupler that allows evanescent coupling between adjacent waveguides. During the condensate's formation, polaritons usually acquire a randomly oriented polarization, however, our results reveal a preferential orientation of the linear polarization along the waveguide propagation path. Furthermore, we observe polarization-dependent intensity oscillations in the output terminal of the coupler, and we identify the mode beating between the linear-polarized eigenmodes as the origin of these oscillations. Our findings provide an insight into the control of the polarization of polariton condensates, paving the way for the development of spin-based polaritonic architectures where condensates propagate over macroscopic distances, This work has been partly supported by the Spanish MINECO Grant Nos. MAT2017-83722-R and PID2020-113445GB-I00. A.Y. and I.A.S. were financially supported by the Ministry of Science and Higher Education of the Russian Federation through Megagrant Number 14.Y26.31.0015 and Goszadanie No. 2019-1246. I.A.S. acknowledges also the support from theIcelandic research fund, Grant No. 163082-051. The Würzburgand Jena group acknowledges financial support within the DFGProject Nos. PE 523/18-1 and KL3124/2-1. The Würzburggroup acknowledges financial support by the German ResearchFoundation (DFG) under Germany’s Excellence Strategy−EXC2147 “ct.qmat” (Project No. 390858490) and is grateful forsupport by the state of Bavaria

We demonstrate a Purcell-enhanced single-photon source realized by the fabrication of a hybrid III–V/dielectric circular Bragg grating cavity directly bonded onto a piezoelectric actuator. Such a k...

Interacting Bosons, loaded in artificial lattices, have emerged as a modern platform to explore collective manybody phenomena, quantum phase transitions and exotic phases of matter as well as to enable advanced on chip simulators. Such experiments strongly rely on well-defined shaping the potential landscape of the Bosons, respectively Bosonic quasi-particles, and have been restricted to cryogenic, or even ultra-cold temperatures. On chip, the GaAs-based exciton-polariton platform emerged as a promising system to implement and study bosonic non-linear systems in lattices, yet demanding cryogenic temperatures. In our work, we discuss the first experiment conducted on a polaritonic lattice at ambient conditions: We utilize fluorescent proteins as an excitonic gain material, providing ultra-stable Frenkel excitons. We directly take advantage of their soft nature by mechanically shaping them in the photonic one-dimensional lattice. We demonstrate controlled loading of the condensate in distinct orbital lattice modes of different symmetries, and finally explore, as an illustrative example, the formation of a gap solitonic mode, driven by the interplay of effective interaction and negative effective mass in our lattice. The observed phenomena in our open dissipative system are comprehensively scrutinized by a nonequilibrium model of polariton condensation. We believe, that this work is establishing the organic polariton platform as a serious contender to the well-established GaAs platform for a wide range of applications relying on coherent Bosons in lattices, given its unprecedented flexibility, cost effectiveness and operation temperature., 19 pages, 4 figures

Funding: The Wuerzburg group acknowledges financial support from the state of Bavaria and the DFG through the Wuerzburg-Dresden Cluster of Excellence on Complexity and Topology in Quantum Matter − ct.qmat (EXC 2147, project-id 39085490). Strong light-matter coupling of a photon mode to tightly bound Frenkel excitons in organic materials has emerged as a versatile, room-temperature platform to study nonlinear many-particle physics and bosonic condensation. However, various aspects of the optical response of Frenkel excitons in this regime remained largely unexplored. Here, a hemispheric optical cavity filled with the fluorescent protein mCherry is utilized to address two important questions. First, combining the high quality factor of the microcavity with a well-defined mode structure allows to address whether temporal coherence in such systems can be competitive with their low-temperature counterparts. To this end, a coherence time greater than 150 ps is evidenced via interferometry, which exceeds the polariton lifetime by two orders of magnitude. Second, the narrow linewidth of the device allows to reliably trace the emission energy of the condensate with increasing particle density and thus to establish a fundamental picture which quantitatively explains the core nonlinear processes. It is found that the blueshift of the Frenkel exciton-polaritons is largely dominated by the reduction of the Rabi splitting due to phase space filling effects, which is influenced by the redistribution of polaritons in the system. The highly coherent emission at ambient conditions establishes organics as promising room temperature polariton lasers and the detailed insights on the non-linearity are of great benefit towards implementing nonlinear polaritonic devices, optical switches and lattices based on exciton-polaritons at room temperature. Postprint

Topologically locked for emission The output power from a laser system can be increased by forming an array of lasers; however, because the individual lasers are independent, the resultant output may not be coherent. Dikopoltsev et al . report on the realization of a topological vertical-cavity surface-emitting laser (VCSEL) array. The topological nature of the array-based laser emission was achieved through a combination of topological in-plane propagation of evanescent light linking the vertical cavity surface-emitting lasers of the array. The topological features of the array force injection locking, making all emitters (30 in this case) act as a single coherent laser. This development will be important for realizing large-scale coherent laser arrays. —ISO

In the quest to realize a scalable quantum network, semiconductor quantum dots (QDs) offer distinct advantages including high single-photon efficiency and indistinguishability, high repetition rate (tens of GHz with Purcell enhancement), interconnectivity with spin qubits, and a scalable on-chip platform. However, in the past two decades, the visibility of quantum interference between independent QDs rarely went beyond the classical limit of 50$\%$ and the distances were limited from a few meters to kilometers. Here, we report quantum interference between two single photons from independent QDs separated by 302 km optical fiber. The single photons are generated from resonantly driven single QDs deterministically coupled to microcavities. Quantum frequency conversions are used to eliminate the QD inhomogeneity and shift the emission wavelength to the telecommunication band. The observed interference visibility is 0.67$\pm$0.02 (0.93$\pm$0.04) without (with) temporal filtering. Feasible improvements can further extend the distance to 600 km. Our work represents a key step to long-distance solid-state quantum networks., 14 pages, 5 figures

Solid-state quantum emitters with manipulable spin-qubits are promising platforms for quantum communication applications. Although such light-matter interfaces could be realized in many systems only a few allow for light emission in the telecom bands necessary for long-distance quantum networks. Here, we propose and implement an optically active solid-state spin-qubit based on a hole confined in a single InAs/GaAs quantum dot grown on an InGaAs metamorphic buffer layer emitting photons in the C-band. We lift the hole spin-degeneracy using an external magnetic field and demonstrate hole injection, initialization, read-out and complete coherent control using picosecond optical pulses. These results showcase a solid-state spin-qubit platform compatible with preexisting optical fiber networks.

Single self-assembled quantum dots are established emitters of single photons and entangled photon pairs. The entanglement of photons generated by quantum dots has been shown in polarization [1] , in time bin [2] , and also as hyperentanglement [3] . However, the achievable degree of entanglement and readiness of the source for use in quantum communication protocols, depend on several additional functionalities such as high collection efficiency, coherence of the emitted photon pairs, and fabrication scalability.

Semiconducting monolayer crystals have emerged as a new platform for studies of tightly bound excitons and many-body excitations in ultimately thin materials. Their giant dipole coupling to optical fields makes them very appealing for (nano-) photonic devices, and for fundamental investigations in the framework of cavity quantum electrodynamics [1] .

We demonstrate ultrafast control of a strongly coupled light–matter system via a giant ac Stark effect in a specially designed strongly coupled microcavity using ultrafast pump–probe spectroscopy. ...

In this letter, we present evanescently coupled semiconductor laser arrays designed with a distributed Bragg reflector section for spectrally narrow emission and a combined waveguide section for high optical output powers. These devices were designed and fabricated in the AlGaAs material system with an InGaAsP quantum well providing emission in the 760–770 nm wavelength range. Measurements of the light-current characteristics reveal high optical output powers of 600 mW at 20 °C and 700 mW at 5 °C limited by thermal rollover without any signs of a catastrophic optical mirror damage or rapid power degradation. Spectrally narrow emission of 0.2 nm at −3 dB and a high wavelength stability with a small current tuning coefficient of 0.7 pm/mA were found. Far-field investigations reveal pre-dominant coupling of lowest order in-phase supermode emission without any sections for adjusting the phase of the electrical field vector for adjacent elements. Postprint

Single-molecule localization microscopy (SMLM) aims for maximized precision and a high signal-to-noise ratio1. Both features can be provided by placing the emitter in front of a metal-dielectric nanocoating that acts as a tuned mirror2–4. Here, we demonstrate that a higher photon yield at a lower background on biocompatible metal-dielectric nanocoatings substantially improves SMLM performance and increases the localization precision by up to a factor of two. The resolution improvement relies solely on easy-to-fabricate nanocoatings on standard glass coverslips and is spectrally and spatially tunable by the layer design and wavelength, as experimentally demonstrated for dual-color SMLM in cells.

The authors apply photon number resolving detectors to explore the photon statistics of bimodal microlasers.

Following the ultrafast optical excitation of an inhomogeneously broadened ensemble, the macroscopic optical polarization decays rapidly due to dephasing. This destructive interference is, however, reversible in photon echo experiments. Here, we propose a concept in which a control pulse slows down either the dephasing or the rephasing of the exciton ensemble during its presence. We analyze and visualize this optical freezing process by showing and discussing results for different single and multiple sequences of control pulses using a simple model of inhomogeneously broadened two-level systems. This idea has been realized in experiments performed on self-assembled (In,Ga)As quantum dots where it was possible to retard or advance the photon echo emission time by several picoseconds. The measurements are in very good agreement with numerical simulations for a more realistic model which, in particular, takes the spatial shape of the laser pulses into account.

We have demonstrated that both ring quantum cascade (QCLs) and interband cascade lasers (ICLs) are excellent platforms for vertical light emission. Of these two lasers ICLs typically show lower power consumption and lasing threshold, qualifying them especially for miniaturized and battery-powered applications. With our work on ring ICLs we are aiming to build a compact portable sensing device, employing interferometric cavity-assisted photothermal spectroscopy. Here, we present our current work on interband cascade devices, as well as an overview of previous studies on ring QCLs. These devices rely on the light outcoupling via a second-order distributed-feedback grating from a ring cavity.

We report on a process for the fiber-coupling of electrically driven cavity-enhanced quantum dot light emitting devices. The developed technique allows for the direct and permanent coupling of p-i-n-doped quantum dot micropillar cavities to single-mode optical fibers. The coupling process, fully carried out at room temperature, involves a spatial scanning technique, where the fiber facet is positioned relative to a device with a diameter of 2 $\mu$m using the fiber-coupled electroluminescence of the cavity emission as a feedback parameter. Subsequent gluing and UV curing enables a rigid and permanent coupling between micropillar and fiber core. Comparing our experimental results with finite element method simulations indicates a cavity-to-fiber mode-coupling efficiency of ~46%. Furthermore, we demonstrate pulsed current injection at a repetition rate exceeding 200 MHz as well as low-temperature operation down to 77 K of the fiber-coupled micropillar device. The technique presented in this work is an important step in the quest for efficient and practical quantum light sources for applications in quantum information., Comment: 12 pages, 4 figures, revised including additional low-temperature data, to be published in APL

We demonstrate a deterministic Purcell-enhanced single-photon source realized by integrating an atomically thin WSe$_{2}$ layer with a circular Bragg grating cavity. The cavity significantly enhances the photoluminescence from the atomically thin layer, and supports single-photon generation with $g^{(2)}(0), 11 pages, 3 figures in the main text, 6 figures in the supplementary material

We investigated emission properties of photonic structures with InAs/InGaAlAs/InP quantum dashes grown by molecular beam epitaxy on a distributed Bragg reflector. In high-spatial-resolution photoluminescence experiment, well-resolved sharp spectral lines are observed and single-photon emission is detected in the third telecommunication window characterized by very low multiphoton events probabilities. The photoluminescence spectra measured on simple photonic structures in the form of cylindrical mesas reveal significant intensity enhancement by a factor of 4 when compared to a planar sample. These results are supported by simulations of the electromagnetic field distribution, which show emission extraction efficiencies even above 18% for optimized designs. When combined with relatively simple and undemanding fabrication approach, it makes this kind of structures competitive with the existing solutions in that spectral range and prospective in the context of efficient and practical single-photon sources for fiber-based quantum networks applications.

━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ DATA DESCRIPTION FOR "Difference-frequency generation in an AlGaAs Bragg-reflection waveguide using an on-chip electrically-pumped quantum dot laser" Alexander Schlager ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ also available as readme.txt 2021-06-16 Table of Contents ───────────────── 1) Figure 2 - Lasing Threshold 2) Figure 3 - Difference Frequency Generation 3) Table 1 - Conversion Efficiency 4) Figure A3 - Temperature Dependency 5) Table A3 - Further Conversion Efficiencies Each filename pre-pended with the figure/table number of the main and supplementary text that it contributes to. All files relevant to each figure/table are included. All numeric data is presented in tab separated variable (in a '*.txt' file) which can be opened with standard spreadsheet or text editing programs. Aside from the JSI ('FIG3c-JSI.txt') and the temperature dependency of the internal laser spectrum ('FIGA3a-laser-temp.txt') all the plots are directly generated with a Latex package called `pgfplots'. For the two exceptions a high resolution image ('*.png') has been generated with Wolfram Mathematica and imported via 'pgfplots'. The code used to process this data and calculate e.g. the conversion efficiencies is not contained in this repository, but can be requested from the authors if required. 1) Figure 2 - Lasing Threshold ══════════════════════════════ 'FIG2-threshold-data.txt' contains the raw data of the lasing threshold measurement. Column 1 and 2 contain the current and its standard deviation in Ampere and column 3 and 4 the optical power and its standard deviation in micro Watt after a measurement time of 30 seconds. 'FIG2-inset-540mA.txt' and 'FIG2-inset-700mA.txt' contain the data representing the spectra of the internal laser below and above lasing threshold. Column 1 is the wavelength in nanometers and column 2 the signal strength in kCounts/s and MCounts/s for a current of 540mA and 700mA, respectively. 2) Figure 3 - Difference Frequency Generation ═════════════════════════════════════════════ 'FIG3b-example-data.txt' contains a background subtracted dataset utilizing the internal laser and a seed wavelength of 1614nm. Column 1 is the wavelength in nanometers and column 2 the signal strength in Counts per second (Hz). This data has been averaged for an acquisition time of 300 seconds. 'FIG3c-JSI.txt' contains processed data for the joint spectral intensity. The DFG peaks collected in the measurements (such as in 'FIG3b-example-data.txt') have been fitted with a Gaussian function and these functions have been utilized to generate the data in this file. This allows us to properly show the Full-Width-Half-Maximum of the measured spectrum and get rid of residual background light from e.g. the telecom seed laser near the degeneracy. This data has been corrected for the bandpass filters utilized in the experiment and the detection efficiency of our detector at different wavelengths. It has then be renormalized to ease the creation of the plot. The raw data for the individual wavelength settings are not included in this repository, but can be requested from the authors if required. The data for Fig. 3d) is included 3) Table 1 - Conversion Efficiency (next section). 3) Table 1 - Conversion Efficiency ══════════════════════════════════ 'TAB1-DFG-power-int-1626nm.txt', 'TAB1-DFG-power-ext-1619nm.txt' and 'TAB1-DFG-power-pas-1510nm.txt' include the data to get the slope for the conversion efficiency for the internal laser, the external laser and the passive sample, respectively. Column 1 and 2 describe the optical power and its standard deviation of the seed laser within the waveguide in milli Watt (for correction, see supplementary materials) for a measurement time of 30 seconds. Column 3 and 4 contain the signal strength of the DFG light in counts/s (Hz) and its poissonian standard error of mean for an acquisition time of 300 seconds (internal), 10 seconds (external) and 2 seconds (passive). To extract the signal strength, we simple take sum of the maximum of the corresponding peak in the raw data (such as in 'FIG3b-example-data.txt') and its nearest neighbors, as this corresponds to the peak width of about 1-2 nm. The raw data are not included in this repository, but can be requested from the authors if required. 4) Figure A3 - Temperature Dependency ═════════════════════════════════════ 'FIGA3a-laser-temp.txt' contains the spectra of the internal laser for different temperature settings as shown in the supplementary materials. Column 1 is the temperature in °C, column 2 the wavelength in nanometers and column 3 the normalized signal strength. Note, that the picture used in Fig. A3a) is only a small cutout of the spectra, but contains all the important information, as there is only background below 788nm and above 794nm. 'FIGA3b-shg-type0.txt', 'FIGA3b-shg-type1.txt' and 'FIGA3b-shg-type2.txt' contain the spectral information of the type-0, type-1 and type-2 second harmonic generation process, respectively. This data has been processed already, as it contains the peak maximum position (column 3) and half-width-half-maximum (column 4) in nanometers extracted from Gaussian fits for the individual SHG spectra at different temperatures in °C (column 1) with an accuracy of 0.05°C (column 2). The maximum peak position is used to create the thick line and the half-width-half-maximum is used to create the slightly transparent bands in Fig. A3b). The raw data are not included in this repository, but can be requested from the authors if required. 'FIGA3c-spectrum-693mA.txt' and 'FIGA3c-spectrum-700mA.txt' include the raw internal laser spectrum at a temperature of 3.5°C and 693mA and 700mA of current, respectively. Column 1 is the wavelength in nanometers and column 2 the normalized laser power. 'FIGA3d-shg-spectrum.txt' shows the second harmonic generation spectrum of the type-2 process in our waveguide at a temperature of 3.5°C. Column 1 is the wavelength in nanometers and column 2 the normalized SHG power. This data has been generated by measuring the maximum of the SHG spectrum for different pump laser wavelengths. This way, we can extract the SHG spectrum at a much higher resolution by slowly varying the laser wavelength in steps of 0.01nm. The raw data are not included in this repository, but can be requested from the authors if required. 5) Table A3 - Further Conversion Efficiencies ══════════════════════════════════════════════ All files starting with 'TABA3-DFG-power-*.txt' include the data to get the slope for the conversion efficiency for the internal laser, the external laser and the passive sample shown in the supplementary materials, respectively. Aside from additional wavelength settings for the telecom seed laser, the description is exactly the same as in 3) Table 1 - Conversion efficiency.

We present the first experimental demonstration of a topological insulator VCSEL array. Our laser consists of 30 emitters, emitting in a single frequency, displaying interference proving that the emitters act as a single laser.

The electronic bandstructure of a solid is a collection of allowed bands separated by forbidden bands, revealing the geometric symmetry of the crystal structures. Comprehensive knowledge of the bandstructure with band parameters explains intrinsic physical, chemical and mechanical properties of the solid. Here we report the artificial polaritonic bandstructures of two-dimensional honeycomb lattices for microcavity exciton-polaritons using GaAs semiconductors in the wide-range detuning values, from cavity-photon-like (red-detuned) to exciton-like (blue-detuned) regimes. In order to understand the experimental bandstructures and their band parameters, such as gap energies, bandwidths, hopping integrals and density of states, we originally establish a polariton band theory within an augmented plane wave method with two-kind-bosons, cavity photons trapped at the lattice sites and freely moving excitons. In particular, this two-kind-band theory is absolutely essential to elucidate the exciton effect in the bandstructures of blue-detuned exciton-polaritons, where the flattened exciton-like dispersion appears at larger in-plane momentum values captured in our experimental access window. We reach an excellent agreement between theory and experiments in all detuning values., Comment: 11 pages, 7 figures

Microcavity exciton polariton systems can have a wide range of macroscopic quantum effects that may be turned into better photonic technologies. Polariton Bose-Einstein condensation and photon lasing have been widely accepted in the limits of low and high carrier densities, but identification of the expected Bardeen-Cooper-Schrieffer (BCS) state at intermediate densities remains elusive, as the optical-gain mechanism cannot be directly inferred from existing experiments. Here, using a microcavity with strong polarization selectivity, we gain direct experimental access to the reservoir absorption in the presence of polariton condensation and lasing, which reveals a fermionic gain mechanism underlying the polariton laser. A microscopic many-particle theory shows that this polariton lasing state is consistent with an open-dissipative-pumped system analog of a polaritonic BCS state.

The generation of photon pairs in single quantum dots is based on a process that is, in its nature, deterministic. However, an efficient extraction of these photon pairs from a high-index semiconductor host material requires engineering of the photonic environment. We report on a micropillar-based device featuring an extraction efficiency of 69.4(10)$\%$ that is achieved by harnessing a broadband operation suitable for extraction of photon pairs emitted from a single quantum dot. Opposing the approaches that rely solely on Purcell enhancement to realize the enhancement of the extraction efficiency, our solution exploits a suppression of the emission into the modes other than the cavity mode. Our technological implementation requires modest fabrication effort enabling higher device yields that can be scaled up to meet the growing needs of quantum technologies. Furthermore, the design of the device can be further optimized to allow for an extraction efficiency of 85$\%$.

We investigate the efficient generation of photons in broadband deterministic quantum dot-micropillar cavities. The quantum-dot cavity coupling leads to Purcell factors above 2 for both exciton and biexciton photons.

We perform extended numerical studies to maximize the overall photon coupling efficiency of fiber-coupled quantum dot single-photon sources emitting in the near-infrared and O-band and C-band. Using the finite element method, we optimize the photon extraction and fiber-coupling efficiency of quantum dot single-photon sources based on micromesas, microlenses, circular Bragg grating cavities and micropillars. The numerical simulations which consider the entire system consisting of the quantum dot source itself, the coupling lens, and the single-mode fiber, yield overall photon coupling efficiencies of up to 83%. Our work provides objectified comparability of different fiber-coupled single-photon sources and proposes optimized geometries for the realization of practical and highly efficient quantum dot single-photon sources.

Funding: J.H., S.K. and H.D. acknowledge financial support from the US Air Force Office of Scientific Research under grant FA9550-15-1-0240 and the US National Science Foundation (NSF) under grant DMR 1150593. The Würzburg group gratefully acknowledges support by the state of Bavaria. Vector vortex beams are a class of optical beams with singularities in their space-variant polarization. Vector vortex beam lasers have applications in many areas including imaging and communication, where vertical-cavity lasers emitting Gaussian beams have been most widely used so far. Generation of vector vortex beams from vertical-cavity lasers has required external control or modulation. Here, by utilizing a polarization-selective subwavelength grating as one of the reflectors in a vertical semiconductor microcavity, we design the spin textures of the polariton mode and demonstrate polariton lasing in a single-mode, radially polarized vector vortex beam. Polarization and phase distributions of the emission are characterized by polarization-resolved imaging and interferometry. This method of vector vortex laser beam generation allows low threshold power, stable single-mode operation, scalability, and on-chip integration, all of which are important for applications in imaging and communication. Postprint

The emergence of two-dimensional crystals has revolutionized modern solid-state physics. From a fundamental point of view, the enhancement of charge carrier correlations has sparked enormous research activities in the transport- and quantum optics communities. One of the most intriguing effects, in this regard, is the bosonic condensation and spontaneous coherence of many-particle complexes. Here, we find compelling evidence of bosonic condensation of exciton-polaritons emerging from an atomically thin crystal of MoSe2 embedded in a dielectric microcavity under optical pumping. The formation of the condensate manifests itself in a sudden increase of luminescence intensity in a threshold-like manner, and a significant spin-polarizability in an externally applied magnetic field. Spatial coherence is mapped out via highly resolved real-space interferometry, revealing a spatially extended condensate. Our device represents a decisive step towards the implementation of coherent light-sources based on atomically thin crystals, as well as non-linear, valleytronic coherent devices., Main text: 11 pages, 5 figures, Supplementary Materials: 4 pages, 5 figures

In this paper, we introduce a novel quantum photonic device, which we term photonic Bier-Glass cavity. We discuss its fabrication and functionality, which is based on the coupling of integrated In(Ga)As quantum dots to a broadband photonic cavity resonance. By design, the device architecture uniquely combines the Purcell enhancement of a photonic micropillar structure with broadband photonic mode shaping of a vertical, tapered waveguide, making it an interesting candidate to enable the efficient extraction of entangled photon pairs. We detail the epitaxial growth of the heterostructure and the necessary lithography steps to approach a GaAs-based photonic device with a height exceeding 15 $\mu$m, supported on a pedestal that can be as thin as 20 nm. We present an optical characterization, which confirms the presence of broadband optical resonances, in conjunction with amplified spontaneous emission of single photons., Comment: 14 pages, 9 figures

We present recent progress on novel mid-infrared (MIR) light emitters and detectors. Optimized heterostructure and high-quality crystal growth allow for room temperature operation of interband cascade lasers (ICLs) with lasing wavelengths 𝜆 ≥ 6 μm. They employ asymmetric W-shaped optical quantum wells comprising highly strained layers of InAs/GaInSb/InAs with broken bandgap alignment. Furthermore, we discuss novel interband cascade detectors (ICDs) and resonant tunneling diode photodetectors (RTD-PDs) for MIR light detection. Different superlattice (SL) absorber design strategies for ICD cut-off wavelengths exceeding 𝜆 ≥ 7.0 μm are presented. SL absorbers ranging from standard InAs/GaSb SL to M-/W-shaped SL absorbers employing ternary barriers are compared.

Funding: Ł.D.acknowledges the financial support from the Alexander von Humboldt Foundation. S.-H. K. acknowledges the financial support from the National Research Foundation of Korea through the Korean Government Grant NRF-2019R1A2C4069587. We are furthermore grateful for the support by the State of Bavaria. Integrated photonic circuits provide a versatile toolbox of functionalities for advanced quantum optics applications. Here, we demonstrate an essential component of such a system in the form of a Purcell-enhanced single-photon source based on a quantum dot coupled to a robust on-chip integrated resonator. For that, we develop GaAs monolithic ring cavities based on distributed Bragg reflector ridge waveguides. Under resonant excitation conditions, we observe an over 2-fold spontaneous emission rate enhancement using Purcell effect and gain a full coherent optical control of a QD-two-level system via Rabi oscillations. Furthermore, we demonstrate an on-demand single-photon generation with strongly suppressed multiphoton emission probability as low as 1% and two-photon interference with visibility up to 95%. This integrated single-photon source can be readily scaled up, promising a realistic pathway for scalable on-chip linear optical quantum simulation, quantum computation, and quantum networks. Postprint

We study the magneto-transport and magneto-electroluminescence properties of purely n-doped GaAs/Al$_{0.6}$Ga$_{0.4}$As resonant tunneling diodes with an In$_{0.15}$Ga$_{0.85}$As quantum well and emitter prewell. Before the resonant current condition, magneto-transport measurements reveal charge carrier densities comparable for diodes with and without the emitter prewell. The Landau level splitting is observed in the electroluminescence emission from the emitter prewell, enabling the determination of the charge carrier build-up. Our findings show that magneto-electroluminescence spectroscopy techniques provide useful insights on the charge carrier dynamics in resonant tunneling diodes and is a versatile tool to complement magneto-transport techniques. This approach will drive the way for developing potentially more efficient opto-electronic resonant tunneling devices, by e.g., monitoring voltage dependent charge accumulation for improving built-in fields and hence to maximize photodetector efficiency and/or minimize optical losses., 9 pages, 7 figures

Publisher's version (útgefin grein), Polariton condensates' propagation is strongly dependent on the particular energy landscape the particles are moving upon, in which the geometry of the pathway laid for their movement plays a crucial role. Bends in the circuit's trajectories affect the condensates' speed and oblique geometries introduce an additional discretization of the polaritons' momenta due to the mixing of short and long axis wavevectors on the propagating eigenvalues. In this work, the nature of the propagation of condensates along the arms of a polariton coupler is studied by a combination of time-resolved micro-tomography measurements and a theoretical model based on a mean field approximation where condensed polaritons are described by an equation for the slow varying amplitude of the polariton field coupled to an equation for the density of incoherent excitons., E.R. and J.B. contributed equally to this work. This work has been partly supported by the Spanish MINECO Grant No. MAT2017‐83722‐R. E.R. acknowledges financial support from the FPI Scholarship No. BES‐2015‐074708. The Würzburg and Jena group acknowledge financial support within the DFG project PE 523/18‐1, KL3124/2‐1 and SCHN1376 3‐1. The Würzburg group is grateful for support from the state of Bavaria and within the Würzburg‐Dresden Cluster of Excellence ct.qmat. A.Y. and I.A.S. thank the Russian Science Foundation for financial support, Project No. 18‐72‐10110.

D.W. acknowledges financial support by the Polish National Agency for Academic Exchange (NAWA) within the ULAM program (No. PPN/ULM/2019/1/00064). The Würzburg team acknowledges the support by the State of Bavaria and the Deutsche Forschungsgemeinschaft (DFG) within Project No. SCHN1376 5.1 / PR1749 1.1. The Jaynes-Cummings (JC) model represents one of the simplest ways in which single qubits can interact with single photon modes, leading to profound quantum phenomena like superpositions of light and matter states. One system, that can be described with the JC model, is a single quantum dot embedded in a micropillar cavity. In this joint experimental and theoretical study we investigate such a system using four-wave mixing (FWM) micro-spectroscopy. Special emphasis is laid on the dependence of the FWM signals on the number of photons injected into the microcavity. By comparing simulation and experiment, which are in excellent agreement with each other, we infer that up to ∼20 photons take part in the observed FWM dynamics. Thus we verify the validity of the JC model for the system under consideration in this non-trivial regime. We find that the inevitable coupling between the quantum dot exciton and longitudinal acoustic phonons of the host lattice influences the real time FWM dynamics and has to be taken into account for a sufficient description of the quantum dot-microcavity system. Performing additional simulations in an idealized dissipation-less regime, we observe that the FWM signal exhibits quasi-periodic dynamics, analog to the collapse and revival phenomenon of the JC model. In these simulations we also see that the FWM spectrum has a triplet structure, if a large number of photons is injected into the cavity. Postprint

Fast and efficient single-photon detectors are of paramount importance for a broad range of applications, e.g. in optical quantum information technologies. A variety of novel and refined photon counting devices has resulted from the need of ever better performance. One such device is the resonant tunneling diode (RTD) single-photon detector. Compared to conventional single-photon detectors, RTDs provide several advantageous characteristics. Since in RTDs a tunneling current is gated by photogenerated charge carriers, they are inherently photon-number resolving, and the RTD currentvoltage characteristic’s negative differential conductance region allows for higher functionality, e.g. the operation as an optically triggered bistable switch. Here, we present an overview on RTD single-photon detectors with a focus on the stateof- the-art, its underlying physical mechanisms as well the device limitations, and recent developments. We compare different RTD device geometries and operation schemes for enhanced detection efficiencies and operation frequencies.

Authors gratefully acknowledge the financial support by the state of Bavaria, the DFG within the projects Schn1376-3.1 as well as KL3124/2-1 and the Wurzburg-Dresden Cluster of Excellence on Complexity and Topology in Quantum Matter - ct.qmat. S.H. is grateful for funding received within the EPSRC Hybrid Polaritonics programme grant (EP/M025330/1). We report on the implementation of a GaAs-based, vertically emitting electrically pumped polariton laser operated at cryogenic temperatures. The structure consists of a high quality factor AlGaAs/AlAs microcavity (Q=15 000) with two stacks of four GaAs quantum wells and features a Rabi splitting of 11 meV. Polariton lasing manifests by a clear threshold in the input–output characteristics of our device with a sharp drop in the emission linewidth and a continuous blueshift of 0.7 meV above threshold with increasing injection current. We measure spatial and temporal coherence of our device in the condensed phase by utilizing interference spectroscopy. Our results clearly demonstrate that electrically driven polariton lasers have promise as monolithic polaritonic sources of coherent light. Postprint

We report on novel exciton-polariton routing devices created to study and purposely guide light-matter particles in their condensate phase. In a co-directional coupling device, two waveguides are connected by a partially etched section which facilitates tunable coupling of the adjacent channels. This evanescent coupling of the two macroscopic wavefunctions in each waveguide reveals itself in real space oscillations of the condensate. This Josephson-like oscillation has only been observed in coupled polariton traps so far. Here, we report on a similar coupling behavior in a controllable, propagative waveguide-based design. By controlling the gap width, channel length or the propagation energy, the exit port of the polariton flow can be chosen. This co-directional polariton device is a passive and scalable coupler element that can serve in compact, next generation logic architectures., 6 pages, 5 figures

Epitaxially grown self-assembled quantum dots (QDs) are promising candidates for an efficient single-photon generation. In order to maximize the number of photons extracted from the device, QDs are frequently embedded into photonic structures such as micro-cavities. Once the QD is positioned inside an optical cavity, the Purcell effect ensures that light is emitted predominantly into the cavity mode. In this contribution, we demonstrate results of emission tuning of QDs inserted in micro-cavities. A sample containing an InAs/GaAs QDs embedded in a planar cavity based on Bragg reflectors has been integrated onto the PMN-PT piezo crystal. Subsequently, micro-cavities have been fabricated by electron-beam lithography and reactive ion etching. The application of external stress produces linear shifts of QDs emission which could be tuned into the resonance with fundamental cavity mode and allow enhancement of QD emission due to the Purcell effect.

Vortices are topological objects representing the circular motion of a fluid. With their additional degree of freedom, the vorticity, they have been widely investigated in many physical systems and different materials for fundamental interest and for applications in data storage and information processing. Vortices have also been observed in non-equilibrium exciton-polariton condensates in planar semiconductor microcavities. There they appear spontaneously or can be created and pinned in space using ring-shaped optical excitation profiles. However, using the vortex state for information processing not only requires creation of a vortex but also efficient control over the vortex after its creation. Here we demonstrate a simple approach to control and switch a localized polariton vortex between opposite states. In our scheme, both the optical control of vorticity and its detection through the orbital angular momentum of the emitted light are implemented in a robust and practical manner., Vortices can be generated in many platforms and are useful due to manipulation of their vorticity. Here the authors show generation and optical switching of a vortex in exciton-polariton condensates using GaAs microcavities.

In recent years, novel two-dimensional materials such as graphene, bismuthene and transition-metal dichalcogenides have attracted considerable interest due to their unique physical properties. A range of physical effects can be transferred to the realms of photonics by creating artificial photonic lattices emulating these two-dimensional materials. Here, exciton-polaritons in semiconductor microcavities offer an exciting opportunity to study a part-light, part-matter quantum fluid of light in a complex lattice potential. In this paper, we study exciton-polaritons in a two-dimensional Lieb lattice of buried optical traps. The $S$ and $P_{xy}$ photonic orbitals of such a Lieb lattice give rise to the formation of two flatbands which are of greatest interest for the distortion-free storage of compact localized states. By using a well controlled etch-and-overgrowth technique, we manage to control the trapping as well as the site couplings with great precision. This allows us to spectroscopically monitor the flatness of the flatbands across the full Brillouin zone. Furthermore, we demonstrate experimentally that these flatbands can be directly populated by condensation under non-resonant laser excitation. Finally, using this advanced device approach we demonstrate resonant and deterministic excitation of flatband modes in transmission geometry. Our findings establish the exciton-polariton systems as a highly controllable, optical many-body system to study flatband effects and for distortion-free storage of compact localized states., 6 pages, 4 figures

When an electron-hole pair is optically excited in a semiconductor quantum dot the host crystal lattice needs to adapt to the presence of the generated charge distribution. Therefore the coupled exciton-phonon system has to establish a new equilibrium, which is reached in the form of a quasiparticle called polaron. Especially, when the exciton is abruptly generated on a timescale faster than the typical lattice dynamics, the lattice displacement cannot follow adiabatically. Consequently, a rich dynamics on the picosecond timescale of the coupled system is expected. In this study we combine simulations and measurements of the ultrafast, coherent, nonlinear optical response, obtained by four-wave mixing spectroscopy, to resolve the formation of this polaron. By detecting and investigating the phonon sidebands in the four-wave mixing spectra for varying pulse delays and different temperatures we have access to the influence of phonon emission and absorption processes which finally result in the emission of an acoustic wave packet out from the quantum dot., Comment: accepted in Optics Letters

The authors acknowledge financial support by the Deutsche Forschungsgemeinschaft (DFG) through the international collaborative research center TRR142 (grant No. 231447078, project A04) and Heisenberg program (grant No. 270619725) and by the Paderborn Center for Parallel Computing. X.M. further acknowledges support from the NSFC (No. 11804064). The Wurzburg group acknowledges support by the state of Bavaria. We study the spontaneous formation of exciton polariton vortices in an all-optical non-resonantly excited annular trap, which is formed by ring-shaped subpicosecond laser pulses. Since the light emitted by these vortices carries orbital angular momentum (OAM) corresponding to their topological charge, we apply a dedicated OAM spectroscopy technique to detect the OAM of the measurement signal with picosecond time resolution. This allows us to identify the formation of OAM modes and investigate the dynamics of the vortex formation process. We also study the power dependence of this process and how the ring diameter influences the formation of OAM modes. Postprint

We observe experimentally collective lasing of a topological mode in a vertical-cavity surface-emitting laser (VCSEL) array. The array is comprised of pillar-shaped VCSELs in a crystalline model geometry and is optically pumped.

Two-photon absorption (TPA) allows accessing “dark” states of matter that are otherwise inaccessible to light, which serve as important building blocks for quantum information processing. In a semiconductor microcavity, TPA-driven condensation of strongly coupled light-matter exciton–polaritons can enable new solid-state quantum simulations of “dark” state-condensate interactions and was predicted to stimulate THz emission. Here, we report the first observation of two-photon-pumped polariton condensation, demonstrated by angle-resolved photoluminescence in a GaAs-based microcavity. TPA is evidenced in the quadratic emission dependence on pump power below and above the condensation threshold, and second-harmonic generation is ruled out by both this threshold behavior and by the emission peak energy showing no dependence on pump photon energy. Our results pave the way toward novel polariton-based sources and solid-state coherent control of collective quantum states with individual two-level systems.

Topological concepts have been applied to a wide range of fields in order to successfully describe the emergence of robust edge modes that are unaffected by scattering or disorder. In photonics, indications of lasing from topologically protected modes with improved overall laser characteristics were observed. Here, we study exciton-polariton microcavity traps that are arranged in a one-dimensional Su-Schrieffer-Heeger lattice and form a topological defect mode from which we unequivocally observe highly coherent polariton lasing. Additionally, we confirm the excitonic contribution to the polariton lasing by applying an external magnetic field. These systematic experimental findings of robust lasing and high temporal coherence are meticulously reproduced by a combination of a generalized Gross-Pitaevskii model and a Lindblad master equation model. Thus, by using the comparatively simple SSH geometry, we are able to describe and control the exciton-polariton topological lasing, allowing for a deeper understanding of topological effects on microlasers., Comment: 25 pages, 6 figures

Strain tuning emerged as an appealing tool for tuning of fundamental optical properties of solid-state quantum emitters. In particular, the wavelength and fine structure of quantum dot states can be tuned using hybrid semiconductor-piezoelectric devices. Here, we show how an applied external stress can directly impact the polarization properties of coupled InAs quantum dot-micropillar cavity systems. In our experiment, we find that we can reversibly tune the anisotropic polarization splitting of the fundamental microcavity mode by approximately 60 μeV. We discuss the origin of this tuning mechanism, which arises from an interplay between elastic deformation and the photoelastic effect in our micropillar. Finally, we exploit this effect to tune the quantum dot polarization optomechanically via the polarization-anisotropic Purcell effect. Our work paves the way for optomechanical and reversible tuning of the polarization and spin properties of light-matter-coupled solid-state systems., We acknowledge funding by the DFG within the Projects No. SCHN1376-5.1 and No. PR1749/1-1. Further, we acknowledge financial support by the State of Bavaria and the German Ministry of Education and Research (BMBF) within the project Q.Link.X (FKZ 16KIS0871). Project HYPER-U-P-S has received funding from the QuantERA ERA-NET Cofund in Quantum Technologies implemented within the European Union’s Horizon 2020 Programme. A.P. would like to thank the Swedish Research Council and Carl Tryggers Stiftelse. J.M.-S. acknowledges financial support from the Ramón y Cajal Program from the Government of Spain (RYC2018-026196-I) and the ClarínProgramme from the Government of the Principality of Asturias and a Marie Curie-COFUND Grant (PA-18-ACB17-29).

Kagome lattices supporting Dirac cone and flatband dispersions are well known as a highly frustrated, two-dimensional lattice system. Particularly the flatbands therein are attracting continuous interest based on their link to topological order, correlations and frustration. In this work, we realize coupled microcavity implementations of Kagome lattices hosting exciton-polariton quantum fluids of light. We demonstrate precise control over the dispersiveness of the flatband as well as selective condensation of exciton-polaritons into the flatband. Subsequently, we focus on the spatial and temporal coherence properties of the laser-like emission from these polariton condensates that are closely connected to the flatband nature of the system. Notably, we find a drastic increase in coherence time due to the localization of flatband condensates. Our work illustrates the outstanding suitability of the exciton-polariton system for detailed studies of flatband states as a platform for microlaser arrays in compact localized states, including strong interactions, topology and non-linearity., Comment: 19 pages, 5 figures

The large oscillator strength of excitons in transition metal dichalcogenide layers facilitates the formation of exciton-polariton resonances for monolayers and van-der-Waals heterostructures embedded in optical microcavities. Here, we show, that locally changing the number of layers in a WSe2/hBN/WSe2 van-der-Waals heterostructure embedded in a monolithic, high-quality-factor cavity gives rise to a local variation of the coupling strength. This effect yields a polaritonic stair case potential, which we demonstrate at room temperature. Our result paves the way towards engineering local polaritonic potentials at length scales down to atomically sharp interfaces, based on purely modifying its real part contribution via the coherent light-matter coupling strength g.

Interacting bosonic particles in artificial lattices have proven to be a powerful tool for the investigation of exotic phases of matter as well as phenomena resulting from non-trivial topology. Exciton-polaritons, bosonic quasi-particles of light and matter, have shown to combine the on-chip benefits of optical systems with strong interactions, inherited form their matter character. Technologically significant semiconductor platforms, however, strictly require cryogenic temperatures for operability. In this paper, we demonstrate exciton-polariton lasing for topological defects emerging form the imprinted lattice structure at room temperature. We utilize a monomeric red fluorescent protein derived from DsRed of Discosoma sea anemones, hosting highly stable Frenkel excitons. Using a patterned mirror cavity, we tune the lattice potential landscape of a linear Su-Schrieffer-Heeger chain to design topological defects at domain boundaries and at the edge. In spectroscopic experiments, we unequivocally demonstrate polariton lasing from these topological defects. This progress promises to be a paradigm shift, paving the road to interacting Boson many-body physics at ambient conditions., Comment: Main: 15 pages, 4 figures ; Suppl.: 2 pages, 2 figures

We present the optical characterization of GaAs-based InAs quantum dots (QDs) grown by molecular beam epitaxy on a digitally alloyed InGaAs metamorphic buffer layer (MBL) with gradual composition ensuring a redshift of the QD emission up to the second telecom window. Based on the photoluminescence (PL) measurements and numerical calculations, we analyzed the factors influencing the energies of optical transitions in QDs, among which the QD height seems to be dominating. In addition, polarization anisotropy of the QD emission was observed, which is a fingerprint of significant valence states mixing enhanced by the QD confinement potential asymmetry, driven by the decreased strain with increasing In content in the MBL. The barrier-related transitions were probed by photoreflectance, which combined with photoluminescence data and the PL temperature dependence, allowed for the determination of the carrier activation energies and the main channels of carrier loss, identified as the carrier escape to the MBL barrier. Eventually, the zero-dimensional character of the emission was confirmed by detecting the photoluminescence from single QDs with identified features of the confined neutral exciton and biexciton complexes via the excitation power and polarization dependences.

Photon echo from trions and excitons in (In,Ga)As/GaAs quantum dots was studied theoretically and experimentally. Theoretical analysis allowed us to distinguish between photon echo signals from excitons and trions measured in the same range of wavelength using different polarization configurations of laser excitation. The theoretical predictions are in good agreement with the experimental data.