Papers by Alexander Steinhoff
Physical Review Research, 2021
Ultrafast Phenomena and Nanophotonics XXIV, 2020
Atomically thin transition metal dichalcogenides (TMDs) and halide perovskites have rapidly grown... more Atomically thin transition metal dichalcogenides (TMDs) and halide perovskites have rapidly grown as promising materials for efficient optoelectronic devices. In both material classes, the dynamics of optical excitations and their interactions on ultrafast timescales are still debated. Using a newly developed theory, we discuss the interaction of TMD excitons and unbound charge carriers with optical phonons in the dielectric environment of the 2d layer. We find a significant reduction of exciton binding energies as well as linewidth broadening due to the dynamical coupling to environmental phonons. Moreover, we investigate near-band-edge optical transitions in CsPbBr3 single crystals at room temperature by combining ultrafast two-dimensional electronic spectroscopy and semiconductor Bloch equation calculations. An exciton binding energy of ~30 meV and remarkably short <30-fs carrier relaxation rates are extracted.
The first chapter presents from a theoretical perspective fundamentals and advances made in the f... more The first chapter presents from a theoretical perspective fundamentals and advances made in the field of quantum light sources and cavity-QED devices that are based on self-organized semiconductor quantum dots (QDs) as active material. We summarize key physical properties of QDs as embedded solid-state emitters and how to account for their semiconductor properties, such as carrier scattering, dephasing, and non-resonant coupling in microscopic theories. In combination with a quantization of the electromagnetic field, these models allow for a quantitative description of device properties and non-classical effects that render few-emitter microcavity systems so useful for applications in the quantum-information technologies.
Physical Review Letters, 2021
In van der Waals (vdW) heterostructures formed by stacking two monolayers of transition metal dic... more In van der Waals (vdW) heterostructures formed by stacking two monolayers of transition metal dichalcogenides, multiple exciton resonances with highly tunable properties are formed and subject to both vertical and lateral confinement. We investigate how a unique control knob, the twist angle between the two monolayers, can be used to control the exciton dynamics. We observe that the interlayer exciton lifetimes in MoSe2/WSe2 twisted bilayers (TBLs) change by one order of magnitude when the twist angle is varied from 1 • to 3.5 •. Using a low-energy continuum model, we theoretically separate two leading mechanisms that influence interlayer exciton radiative lifetimes. The shift to indirect transitions in the momentum space with an increasing twist angle and the energy modulation from the moiré potential both have a significant impact on interlayer exciton lifetimes. We further predict distinct temperature dependence of interlayer exciton lifetimes in TBLs with different twist angles, which is partially validated by experiments. While many recent studies have highlighted how the twist angle in a vdW TBL can be used to engineer the ground states and quantum phases due to many-body interaction, our studies explore its role in controlling the dynamics of optically excited states, thus, expanding the conceptual applications of "twistronics".
Laser & Photonics Reviews, 2021
2D semiconductors and their heterostructures have developed into a flourishing research field. A ... more 2D semiconductors and their heterostructures have developed into a flourishing research field. A more or less simultaneous start of experimental and theoretical developments has led to continuous advancements in both branches of physics that continue to benefit from one another in an exemplary manner. This article gives an overview of the theoretical advancements that have resulted from a decade of research on 2D van der Waals materials and reviews current theoretical methods, focussing on exciton-plasma balance, excited-state optics, and laser physics.
Nature Communications, 2020
In a seminal paper, Mahan predicted that excitonic bound states can still exist in a semiconducto... more In a seminal paper, Mahan predicted that excitonic bound states can still exist in a semiconductor at electron-hole densities above the insulator-to-metal Mott transition. However, no clear evidence for this exotic quasiparticle, dubbed Mahan exciton, exists to date at room temperature. In this work, we combine ultrafast broadband optical spectroscopy and advanced many-body calculations to reveal that organic-inorganic lead-bromide perovskites host Mahan excitons at room temperature. Persistence of the Wannier exciton peak and the enhancement of the above-bandgap absorption are observed at all achievable photoexcitation densities, well above the Mott density. This is supported by the solution of the semiconductor Bloch equations, which confirms that no sharp transition between the insulating and conductive phase occurs. Our results demonstrate the robustness of the bound states in a regime where exciton dissociation is otherwise expected, and offer promising perspectives in fundamen...
Physical Review B, 2020
Atomically thin materials are exceedingly susceptible to their dielectric environment. For transi... more Atomically thin materials are exceedingly susceptible to their dielectric environment. For transition metal dichalcogenides, sample placement on a substrate or encapsulation in hexagonal boron nitride (hBN) are frequently used. In this paper we show that the dielectric response due to optical phonons of adjacent materials influences excitons in 2d crystals. We provide an analytic model for the coupling of 2d charge carriers to optical substrate phonons, which causes polaron effects similar to that of intrinsic 2d phonons. We apply the model to hBN-encapsulated WSe2, finding a significant reduction of the exciton binding energies due to dynamical screening effects.
Physical Review B, 2018
Atomically thin layers of transition metal dichalcogenides (TMDCs) exhibit exceptionally strong C... more Atomically thin layers of transition metal dichalcogenides (TMDCs) exhibit exceptionally strong Coulomb interaction between charge carriers due to the two-dimensional carrier confinement in connection with weak dielectric screening. The van der Waals nature of interlayer coupling makes it easy to integrate TMDC layers into heterostructures with different dielectric or metallic substrates. This allows to tailor electronic and optical properties of these materials, as Coulomb interaction inside atomically thin layers is very susceptible to screening by the environment. Here we theoretically investigate dynamical screening effects in TMDCs due to bulk substrates doped with carriers over a large density range, thereby offering three-dimensional plasmons as tunable degree of freedom. We report a wide compensation of renormalization effects leading to a spectrally more stable exciton than predicted for static substrate screening, even if plasmons and excitons are in resonance. We also find a nontrivial dependence of the single-particle band gap on substrate doping density due to dynamical screening. Our investigation provides microscopic insight into the mechanisms that allow for manipulations of TMDC excitons by means of arbitrary plasmonic environments on the nanoscale.
Nano Letters, 2018
Nanolasers operate with a minimal amount of active material and low losses. In this regime, singl... more Nanolasers operate with a minimal amount of active material and low losses. In this regime, single layers of transition-metal dichalcogenides (TMDs) are being investigated as next generation gain materials due to their high quantum efficiency. We provide results from microscopic gain calculations of highly excited TMD monolayers and specify requirements to achieve lasing with four commonly used TMD semiconductors. Our approach includes band-structure renormalizations due to excited carriers that trigger a direct-to-indirect band-gap transition. As a consequence, we predict a rollover for the gain that limits the excitation regime where laser operation is possible. A parametrization of the peak gain is provided that is used in combination with a rate-equation theory to discuss consequences for experimentally accessible laser characteristics.
Nano Letters, 2017
We report a rare atom-like interaction between excitons in monolayer WS 2 , measured using ultraf... more We report a rare atom-like interaction between excitons in monolayer WS 2 , measured using ultrafast absorption spectroscopy. At increasing excitation density, the exciton resonance energy exhibits a pronounced redshift followed by an anomalous blueshift. Using both material-realistic computation and phenomenological modeling, we attribute this observation to plasma effects and an attraction-repulsion crossover of the exciton-exciton interaction that mimics the Lennard-Jones potential between atoms. Our experiment demonstrates a strong analogy between excitons and atoms with respect to inter-particle interaction, which holds promise to pursue the predicted liquid and crystalline phases of excitons in two-dimensional materials.
Nano Letters, 2017
We investigate the photoluminescence of interlayer excitons in heterostructures consisting of mon... more We investigate the photoluminescence of interlayer excitons in heterostructures consisting of monolayer MoSe2 and WSe2 at low temperatures. Surprisingly, we find a doublet structure for such interlayer excitons. Both peaks exhibit long photoluminescence lifetimes of several ten nanoseconds up to 100 ns at low temperatures, which verifies the interlayer nature of both. The peak energy and linewidth of both show unusual temperature and power dependences. In particular, we observe a blue-shift of their emission energy for increasing excitation powers. At a low excitation power and low temperatures, the energetically higher peak shows several spikes. We explain the findings by two sorts of interlayer excitons; one that is indirect in real space but direct in reciprocal space, and the other one being indirect in both spaces. Our results provide fundamental insights into long-lived interlayer states in van der Waals heterostructures with possible bosonic many-body interactions.
Applied Physics B, 2016
We study the homogeneous linewidth of transitions between many-particle states in semiconductor q... more We study the homogeneous linewidth of transitions between many-particle states in semiconductor quantum dots due to scattering processes. Carrier–density-dependent scattering rates due to carrier–carrier Coulomb interaction and carrier–LO-phonon interaction are obtained on a non-perturbative level and connected to a von Neumann–Lindblad equation for the quantum-dot many-particle configurations, allowing to identify the dephasing of transitions between many-particle states. For different dot geometries, we discuss implications of energetic quantum-dot wetting-layer separation on Coulomb and LO-phonon contributions.
Nano letters, Jan 2, 2015
We discuss the photoluminescence (PL) of semiconducting transition metal dichalcogenides on the b... more We discuss the photoluminescence (PL) of semiconducting transition metal dichalcogenides on the basis of experiments and a microscopic theory. The latter connects ab initio calculations of the single-particle states and Coulomb matrix elements with a many-body description of optical emission spectra. For monolayer MoS2, we study the PL efficiency at the excitonic A and B transitions in terms of carrier populations in the band structure and provide a quantitative comparison to an (In)GaAs quantum well-structure. Suppression and enhancement of PL under biaxial strain is quantified in terms of changes in the local extrema of the conduction and valence bands. The large exciton binding energy in MoS2 enables two distinctly different excitation methods: above-band gap excitation and quasi-resonant excitation of excitonic resonances below the single-particle band gap. The latter case creates a nonequilibrium distribution of carriers predominantly in the K-valleys, which leads to strong emi...
Physical Review B, 2013
Experimental results for the carrier capture and relaxation dynamics in self-organized semiconduc... more Experimental results for the carrier capture and relaxation dynamics in self-organized semiconductor quantum dots are analyzed using a microscopic theory. Time-resolved differential transmission changes of the quantum-dot transitions after ultrafast optical excitation of the barrier states are studied in a wide range of carrier temperatures and excitation densities. The measurements can be explained by quantum-dot polaron scattering and their excitation-dependent renormalization due to additional Coulomb scattering processes. Results of configurationpicture and single-particle-picture descriptions, both with nonperturbative transition rates, show good agreement with the experiments while Boltzmann scattering rates lead to a different excitation density and temperature dependence.
Physical Review B, 2012
As quantum dots (QD) can confine a small number of carriers in localized states with discrete ene... more As quantum dots (QD) can confine a small number of carriers in localized states with discrete energies, it is questionable to neglect correlations between the carriers when describing their dynamics. We analyze the influence of carrier correlations in a single QD on Coulomb scattering processes, which are due to the contact with a quasicontinuum of wetting-layer (WL) states. Results obtained from a Boltzmann equation are compared with the fully correlated dynamics governed by a von Neumann-Lindblad equation. In a first step, we take into account correlations generated by the exact treatment of Pauli blocking due to the contributing QD carrier configurations. Subsequently, we include correlations generated by energy renormalizations due to Coulomb interaction between the QD carriers. It is shown that at low WL carrier densities, neither Pauli correlations nor Coulomb correlations can be safely neglected, if the dynamics of single-particle states in the QD are to be predicted qualitatively and quantitatively. In the high-density regime, both types of correlations play a lesser role and thus a description of carrier dynamics by a Boltzmann equation becomes reliable. Furthermore, the efficiency of WL-assisted scattering processes as well as scattering-induced dephasing rates depending on the WL carrier density are discussed.
Physical Review B, 2014
Recent experiments have demonstrated that for a quantum dot in an optical resonator off-resonant ... more Recent experiments have demonstrated that for a quantum dot in an optical resonator off-resonant cavity mode emission can occur even for detunings of the order of 10 meV. We show that Coulomb mediated Auger processes based on additional carriers in delocalized states can facilitate this far off-resonant emission. Using a novel theoretical approach for a non-perturbative treatment of the Auger-assisted quantum-dot carrier recombination, we present numerical calculations of the far offresonant cavity feeding rate and cavity mean photon number confirming efficient coupling at higher densities of carriers in the delocalized states. In comparison to fast Auger-like intraband scattering processes, we find a reduced overall efficiency of Coulomb-mediated interband transitions due the required electron-hole correlations for the recombination processes.
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Papers by Alexander Steinhoff