Impact of ex-situ rapid thermal annealing on the magneto-optical properties and the oscillator strength of In(Ga)As quantum dots

The authors acknowledge financial support by the State of Bavaria and the German Ministry of Education and Research (BMBF) within the project Q.com-HL.We discuss the influence of a rapid thermal annealing step on the magneto-optical emission properties of In(Ga)As/GaAs quantum dots. We map out a strong influence of the growth- and anneling parameters on the quantum excitons' effective Land\'e g-factors and in particular on their diamagnetic coefficients, which we directly correlate with the modification of the emitters shape and material composition. In addition, we study the excitons' spontaneous emission lifetime as a function of the annealing temperature and the dot height, and observe a strong increase of the emission rate with the quantum dot volume. The corresponding increase in oscillator strenth yields fully consistent results with the analysis of the diamagenic behavior. In particular, we demonstrate that a rapid thermal annealing step of 850ºC can be employed to increase the oscillator strength of as-grown InAs/GaAs QDs by more than a factor of 2.PostprintPeer reviewe

Spatio-temporal coherence in vertically emitting GaAs-based electrically driven polariton lasers

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.PostprintPeer reviewe

Room temperature organic exciton-polariton condensate in a lattice

Funding: The Würzburg group acknowledges financial support from the state of Bavaria. We also thank the Würzburg–Dresden Cluster of Excellence ct.qmat for financial support.Interacting Bosons in artificial lattices have emerged as a modern platform to explore collective manybody phenomena and exotic phases of matter as well as to enable advanced on-chip simulators. On chip, exciton–polaritons emerged as a promising system to implement and study bosonic non-linear systems in lattices, demanding cryogenic temperatures. We discuss an experiment conducted on a polaritonic lattice at ambient conditions: We utilize fluorescent proteins providing ultra-stable Frenkel excitons. Their soft nature allows for mechanically shaping them in the photonic lattice. We demonstrate controlled loading of the coherent condensate in distinct orbital lattice modes of different symmetries. Finally, we explore the self-localization of the condensate in a gap-state, driven by the interplay of effective interaction and negative effective mass in our lattice. We believe that this work establishes organic polaritons as a serious contender to the well-established GaAs platform for a wide range of applications relying on coherent Bosons in lattices.Publisher PDFPeer reviewe

Observation of macroscopic valley-polarized monolayer exciton-polaritons at room temperature

Funding: the State of Bavaria and the ERC (unlimit-2D), the DFG via SFB689, GRK 1570 and KO3612/1-1.In this Rapid Communication, we address the chiral properties of valley exciton-polaritons in a monolayer of WS2 in the regime of strong light-matter coupling with a Tamm-plasmon resonance. We observe that the effect of valley polarization, which manifests in the circular polarization of the emitted photoluminescence as the sample is driven by a circularly polarized laser, is strongly enhanced in comparison to bare WS2 monolayers and can even be observed under strongly nonresonant excitation at ambient conditions. In order to explain this effect in more detail, we study the relaxation and decay dynamics of exciton-polaritons in our device, elaborate the role of the dark state, and present a microscopic model to explain the wave-vector-dependent valley depolarization by the linear polarization splitting inherent to the microcavity. We believe that our findings are crucial for designing novel polariton-valleytronic devices which can be operated at room temperature.PostprintPeer reviewe

Room temperature strong coupling in a semiconductor microcavity with embedded AlGaAs quantum wells designed for polariton lasing

This work was supported by the State of Bavaria.We report a systematic study of the temperature and excitation density behavior of an AlAs/AlGaAs, vertically emitting microcavity with embedded ternary Al0.20Ga0.80As/AlAs quantum wells in the strong coupling regime. Temperature-dependent photoluminescence measurements of the bare quantum wells indicate a crossover from the type-II indirect to the type-I direct transition. The resulting mixing of quantum well and barrier ground states in the conduction band leads to an estimated exciton binding energy systematically exceeding 25 meV. The formation of exciton-polaritons is evidenced in our quantum well microcavity via reflection measurements with Rabi splittings ranging from (13.93 ± 0.15) meV at low temperature (30 K) to (8.58 ± 0.40) meV at room temperature (300 K). Furthermore, the feasibility of polariton laser operation is demonstrated under non-resonant optical excitation conditions at 20 K and emission around 1.835 eV.PostprintPeer reviewe

Polarization-dependent light-matter coupling and highly indistinguishable resonant fluorescence photons from quantum dot-micropillar cavities with elliptical cross section

Funding: Project HYPER-U-P-Shas received funding from the QuantERA ERA-NET Cofund in Quantum Technologies implemented within the European Union’s Horizon 2020 Programme.We study the optical properties of coupled quantum dot-microcavity systems with elliptical cross section. First, we develop an analytic model that describes the spectrum of the cavity modes that are split due to the reduced symmetry of the resonator. By coupling the QD emission to the polarized fundamental cavity modes, we observe the vectorial nature of the Purcell enhancement, which depends on the intrinsic polarization of the quantum dot and its relative alignment with respect to the cavity axis. The variable interaction strength of the QD with the polarized cavity modes leads to the observation of strong and weak coupling. Finally, we demonstrate the capability of elliptical micropillars to emit single and highly indistinguishable photons (visibility of 87%).PostprintPeer reviewe

Intrinsic and environmental effects on the interference properties of a high-performance quantum dot single-photon source

We acknowledge support by the State of Bavaria and the German Ministry of Education and Research (BMBF) within the project Q.com. J.I.-S. and J.M. acknowledge support from the Danish Research Council (DFF-4181-00416) and Villum Fonden (NATEC Centre). This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 703193.We report a joint experimental and theoretical study of the interference properties of a single-photon source based on a In(Ga)As quantum dot embedded in a quasiplanar GaAs microcavity. Using resonant laser excitation with a pulse separation of 2 ns, we find near-perfect interference of the emitted photons, and a corresponding indistinguishability of I=(99.6^+0.4_−1.4)%. For larger pulse separations, quasiresonant excitation conditions, increasing pump power, or with increasing temperature, the interference contrast is progressively and notably reduced. We present a systematic study of the relevant dephasing mechanisms and explain our results in the framework of a microscopic model of our system. For strictly resonant excitation, we show that photon indistinguishability is independent of pump power, but strongly influenced by virtual phonon-assisted processes which are not evident in excitonic Rabi oscillations.Publisher PDFPeer reviewe

Valley polarized relaxation and upconversion luminescence from Tamm-plasmon trion-polaritons with a MoSe2 monolayer

This work has been supported by the State of Bavaria and the ERC (unlimit-2D) as well as the DFG via grants GRK 1570, KO3612/1-1 and SFB 689.Transition metal dichalcogenides represent an ideal testbed to study excitonic effects, spin-related phenomena and fundamental light-matter coupling in nanoscopic condensed matter systems. In particular, the valley degree of freedom, which is unique to such direct band gap monolayers with broken inversion symmetry, adds fundamental interest in these materials. Here, we implement a Tamm-plasmon structure with an embedded MoSe2 monolayer and study the formation of polaritonic quasi-particles. Strong coupling conditions between the Tamm-mode and the trion resonance of MoSe2 are established, yielding bright luminescence from the polaritonic ground state under non-resonant optical excitation. We demonstrate, that tailoring the electrodynamic environment of the monolayer results in a significantly increased valley polarization. This enhancement can be related to change in recombination dynamics shown in time-resolved photoluminescence measurements. We furthermore observe strong upconversion luminescence from resonantly excited polariton states in the lower polariton branch. This upconverted polariton luminescence is shown to preserve the valley polarization of the trion–polariton, which paves the way towards combining spin-valley physics and exciton scattering experiments.PostprintPeer reviewe