arXiv subjects

S. Hoefling

Publications and source records attributed to S. Hoefling.

11 recordsLinked to original sources

arXiv2026

Electrically tunable orbital coupling and quantum light emission from O-band quantum dot molecules

We present the observation of electrically tunable quantum coupling of orbital states in individual InAs/InGaAs quantum dot molecules emitting in the telecom O-band (~1300 nm). By tuning the static electric field along the growth axis of the QD-molecule, we observe pronounced anticrossings between excitonic transitions and determine the dependence of the interdot electron tunnel coupling on the interdot separation. As the electric field applied along the growth axis of the QD-molecules increases, positively charged exciton complexes sequentially emerge in the time-integrated emission spectra due to electron escape from the system while holes remain trapped. Moreover, for strong pumping, biexciton emission from the O-band molecules is identified. We demonstrate single-photon emission from the InAs/InGaAs QD-molecule emitting around 1300 nm with a g(2)(0) = 0.017(2) and explore the impact of tuning orbital coupling on the second-order correlation function.

cond-mat.mes-hall↗

arXiv2021

Quantum interference between independent solid-state single-photon sources separated by 300 km fiber

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.

quant-ph↗

arXiv2021

A Two-Kind-Boson Mixture Honeycomb Hamiltonian of Bloch Exciton-Polaritons

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.

cond-mat.mes-hall↗

arXiv2020

Accurate photon echo timing by optical freezing of exciton dephasing and rephasing in quantum dots

Semiconductor quantum dots are excellent candidates for ultrafast coherent manipulation of qubits by laser pulses on picosecond timescales or even faster. In inhomogeneous ensembles a macroscopic optical polarization decays rapidly due to dephasing, which, however, is reversible in photon echoes carrying complete information about the coherent ensemble dynamics. Control of the echo emission time is mandatory for applications. Here, we propose a novel concept to reach this goal. In a two-pulse photon echo sequence, we apply an additional resonant control pulse with multiple of 2pi area. Depending on its arrival time, the control slows down dephasing or rephasing of the exciton ensemble during its action. We demonstrate for self-assembled (In,Ga)As quantum dots that the photon echo emission time can be retarded or advanced by 5 ps relative to its nominal appearance time without control. This versatile protocol may be used to obtain significantly longer temporal shifts for suitably tailored control pulses.

cond-mat.mes-hall↗

arXiv2018

Controlled ordering of topological charges in an exciton-polariton chain

We demonstrate, experimentally and theoretically, controlled loading of an exciton-polariton vortex chain into a 1D array of trapping potentials. Switching between two types of vortex chains, with topological charges of the same or alternating sign, is realised by means of appropriate shaping of an incoherent pump beam that drives the system to the regime of bosonic condensation. In analogy to spin chains, these vortex sequences realise either a "ferromagnetic" or an "anti-ferromagnetic" order, whereby the role of spin is played by the orbital angular momentum. The "ferromagnetic" ordering of vortices is associated with the formation of a persistent chiral current. Our results pave the way for controlled creation of nontrivial distributions of orbital angular momentum and topological order in a periodic exciton-polariton system.

cond-mat.quant-gas↗

arXiv2018

A platform for electrically pumped polariton simulators and topological lasers

Two-dimensional electronic materials such as graphene and transition metal dichalgenides feature unique electrical and optical properties due to the conspirative effect of band structure, orbital coupling, and crystal symmetry. Synthetic matter, as accomplished by artificial lattice arrangements of cold atoms, molecules, electron patterning, and optical cavities, has emerged to provide manifold intriguing frameworks to likewise realize such scenarios. Exciton-polaritons have recently been added to the list of promising candidates for the emulation of system Hamiltonians on a semiconductor platform, offering versatile tools to engineer the potential landscape and to access the non-linear electro-optical regime. In this work, we introduce an electronically driven square and honeycomb lattice of exciton-polaritons, paving the way towards real world devices based on polariton lattices for on-chip applications. Our platform exhibits laser-like emission from high-symmetry points under direct current injection, hinting at the prospect of electrically driven polariton lasers with possibly topologically non-trivial properties.

physics.optics↗

arXiv2017

Observation of the transition from lasing driven by a bosonic to a fermionic reservoir in a GaAs quantum well microcavity

We show that, by monitoring the free carrier reservoir in a GaAs-based quantum well microcavity under nonresonant pulsed optical pumping, lasing supported by a fermionic reservoir (photon lasing) can be distinguished from lasing supported by a reservoir of bosons (polariton lasing). Carrier densities are probed by measuring the photocurrent between lateral contacts deposited directly on the quantum wells of a microcavity that are partially exposed by wet chemical etching. We identify two clear thresholds in the input-output characteristic of the photoluminescence signal which can be attributed to polariton and photon lasing, respectively. The power dependence of the probed photocurrent shows a distinct kink at the threshold power for photon lasing due to an increased radiative recombination of free carriers as stimulated emission into the cavity mode sets in. At the polariton lasing threshold, on the other hand, the nonlinear increase of the luminescence is caused by stimulated scattering of exciton polaritons to the ground state which do not contribute directly to the photocurrent.

physics.optics↗

arXiv2015

Exciton-Polariton Trapping and Potential Landscape Engineering

Exciton-polaritons in semiconductor microcavities have advanced to become a model system for studying dynamical Bose-Einstein condensation, macroscopic coherence, many-body effects, nonclassical states of light and matter, and possibly quantum phase transitions in a solid state. Being low mass bosons, these light-matter quasiparticles can condense at comparably high temperatures up to 300K, while preserving fundamental properties such as coherence in space and time domain even when they are out of equilibrium with the environment. Although the presence of an in-plane polariton confinement potential is not strictly necessary in order to observe condensation of polaritons, engineering the polariton confinement is a key to controlling, shaping and directing the flow of polaritons. Prototype polariton-based optoelectronic devices rely on ultrafast photon-like velocities and strong nonlinearities, as well as on tailored confinement. Nanotechnology provides several pathways to achieving such a confinement, and the specific features and advantages of the different techniques are discussed in this paper. As hybrid exciton-photon quasiparticles, polaritons can be trapped via their excitonic as well as their photonic component, which leads to a wide choice of highly complementary techniques. Here we highlight the almost free choice of trapping geometries and depths of confinement that provides a powerful tool for control and manipulation of polariton systems both in semi-classical and quantum domain. Furthermore, the possibility to observe effects of polariton blockade, Mott insulator physics, and population of higher-order bands in sophisticated lattice potentials is discussed. The observation of such effects will signify the opportunity for the realization of novel polaritonic non-classical light sources and quantum simulators.

cond-mat.quant-gas↗

arXiv2014

Controlled lasing from active optomechanical resonators

Planar microcavities with distributed Bragg reflectors (DBRs) host, besides confined optical modes, also mechanical resonances due to stop bands in the phonon dispersion relation of the DBRs. These resonances have frequencies in the sub-terahertz (10E10-10E11 Hz) range with quality factors exceeding 1000. The interaction of photons and phonons in such optomechanical systems can be drastically enhanced, opening a new route toward manipulation of light. Here we implemented active semiconducting layers into the microcavity to obtain a vertical-cavity surface-emitting laser (VCSEL). Thereby three resonant excitations -photons, phonons, and electrons- can interact strongly with each other providing control of the VCSEL laser emission: a picosecond strain pulse injected into the VCSEL excites long-living mechanical resonances therein. As a result, modulation of the lasing intensity at frequencies up to 40 GHz is observed. From these findings prospective applications such as THz laser control and stimulated phonon emission may emerge.

cond-mat.mes-hall↗

arXiv2013

Phase diagrams of magnetopolariton gases

The magnetic field effect on phase transitions in electrically neutral bosonic systems is much less studied than those in fermionic systems, such as superconducting or ferromagnetic phase transitions. Nevertheless, composite bosons are strongly sensitive to magnetic fields: both their internal structure and motion as whole particles may be affected. A joint effort of ten laboratories has been focused on studies of polariton lasers, where non-equilibrium Bose-Einstein condensates of bosonic quasiparticles, exciton-polaritons, may appear or disappear under an effect of applied magnetic fields. Polariton lasers based on pillar or planar microcavities were excited both optically and electrically. In all cases a pronounced dependence of the onset to lasing on the magnetic field has been observed. For the sake of comparison, photon lasing (lasing by an electron-hole plasma) in the presence of a magnetic field has been studied on the same samples as polariton lasing. The threshold to photon lasing is essentially governed by the excitonic Mott transition which appears to be sensitive to magnetic fields too. All the observed experimental features are qualitatively described within a uniform model based on coupled diffusion equations for electrons, holes and excitons and the Gross-Pitaevskii equation for exciton-polariton condensates. Our research sheds more light on the physics of non-equilibrium Bose-Einstein condensates and the results manifest high potentiality of polariton lasers for spin-based quantum logic applications.

cond-mat.mes-hall↗

arXiv2010

Conditional phase shift from a quantum dot in a pillar microcavity

Large conditional phase shifts from coupled atom-cavity systems are a key requirement for building a spin photon interface. This in turn would allow the realisation of hybrid quantum information schemes using spin and photonic qubits. Here we perform high resolution reflection spectroscopy of a quantum dot resonantly coupled to a pillar microcavity. We show both the change in reflectivity as the quantum dot is tuned through the cavity resonance, and measure the conditional phase shift induced by the quantum dot using an ultra stable interferometer. These techniques could be extended to the study of charged quantum dots, where it would be possible to realise a spin photon interface.

quant-ph↗