Andreas Knorr

Technische Universität Berlin, Berlín, Berlin, Germany

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Publications (351)856.42 Total impact

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    ABSTRACT: We investigate a semiconductor quantum dot as a microscopic analog of a basic optomechanical setup. We show, that optomechanical features can be reproduced by the solid-state platform, arising from parallels of the underlying interaction processes, which in the optomechanical case is the radiation pressure coupling and in the semiconductor case the electron-phonon coupling. In optomechanics, phonons are typically induced via confined photons, acting on a movable mirror, while in the semiconductor system the phonons are emitted by the laser-driven electronic system. There are analogous effects present for both systems, featuring bistabilities, optically induced phonon lasing and enhanced phonon loss. Nonetheless, the different statistical nature of the optical cavity and the electronic system also leads to qualitative differences.
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    ABSTRACT: The band-edge optical response of transition metal dichalcogenides, an emerging class of atomically thin semiconductors, is dominated by tightly bound excitons localized at the corners of the Brillouin zone (valley excitons). A fundamental yet unknown property of valley excitons in these materials is the intrinsic homogeneous linewidth, which reflects irreversible quantum dissipation arising from system (exciton) and bath (vacuum and other quasiparticles) interactions and determines the timescale during which excitons can be coherently manipulated. Here we use optical two-dimensional Fourier transform spectroscopy to measure the exciton homogeneous linewidth in monolayer tungsten diselenide (WSe2). The homogeneous linewidth is found to be nearly two orders of magnitude narrower than the inhomogeneous width at low temperatures. We evaluate quantitatively the role of exciton-exciton and exciton-phonon interactions and population relaxation as linewidth broadening mechanisms. The key insights reported here-strong many-body effects and intrinsically rapid radiative recombination-are expected to be ubiquitous in atomically thin semiconductors.
    Nature Communications 09/2015; 6:8315. DOI:10.1038/ncomms9315 · 11.47 Impact Factor
  • Faris Kadi · Torben Winzer · Andreas Knorr · Ermin Malic
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    ABSTRACT: We present a microscopic study on the impact of doping on the carrier dynamics in graphene, in particular focusing on its influence on the technologically relevant carrier multiplication in realistic, doped graphene samples. Treating the time- and momentum-resolved carrier-light, carrier-carrier, and carrier-phonon interactions on the same microscopic footing, the appearance of Auger-induced carrier multiplication up to a Fermi level of 300 meV is revealed. Furthermore, we show that doping favors the so-called hot carrier multiplication occurring within one band. Our results are directly compared to recent time-resolved ARPES measurements and exhibit an excellent agreement on the temporal evolution of the hot carrier multiplication for n- and p-doped graphene. The gained insights shed light on the ultrafast carrier dynamics in realistic, doped graphene samples
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    ABSTRACT: We probe the indistinguishability of photons emitted by a semiconductor quantum dot (QD) via time- and temperature- dependent two-photon interference (TPI) experiments. An increase in temporal-separation between two consecutive photon emission events, reveals a decrease in TPI visibility on a time scale $(13.7\pm2.3)\,$ns, theoretically described by a non-Markovian noise process in agreement with fluctuating charge-traps in the QD's vicinity. Phonon-induced pure dephasing results in a detrimental decrease in TPI visibility from $(96\pm4)\,$ % at $10\,$K to a vanishing visibility at $40\,$K.
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    ABSTRACT: A crucial property of quantum networks is the entanglement between different network nodes. We demonstrate that entanglement in quantum networks can be created and controlled by introducing quantum-coherent time-delayed self-feedback at single nodes.
    Physical Review A 05/2015; 91(5). DOI:10.1103/PhysRevA.91.052321 · 2.81 Impact Factor
  • Judith F. Specht · Andreas Knorr · Marten Richter
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    ABSTRACT: The linear and two-dimensional coherent optical spectra of Coulomb-coupled quantum emitters are discussed with respect to the underlying coupling processes. We present a theoretical analysis of the two different resonance energy transfer mechanisms between coupled nanostructures: F\"orster and Dexter interaction. Our investigation shows that the features visible in optical spectra of coupled quantum dots can be traced back to the nature of the underlying coupling mechanism (F\"orster or Dexter). Therefore, we discuss how the excitation transfer pathways can be controlled by choosing particular laser polarizations and mutual orientations of the quantum emitters in coherent two-dimensional spectroscopy. In this context, we analyze to what extent the delocalized double-excitonic states are bound to the optical selection rules of the uncoupled system.
    Physical Review B 04/2015; 91(15). DOI:10.1103/PhysRevB.91.155313 · 3.74 Impact Factor
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    ABSTRACT: We present an analytical solution of the single photon quantum feedback in a cavity quantum electrodynamics system based on a half cavity set-up coupled to a structured continuum. The exact analytical expression we obtain allows us to discuss in detail under which conditions a single emitter-cavity system, which is initially in the weak coupling regime, can be driven into the strong coupling regime via the proposed quantum feedback mechanism [Carmele et al, Phys.Rev.Lett. 110, 013601]. Our results reveal that the feedback induced oscillations rely on a well-defined relationship between the delay time and the atom-light coupling strength of the emitter. At these specific values the leakage into the continuum is prevented by a destructive interference effect, which pushes the emitter to the strong coupling limit.
  • Florian Wendler · Andreas Knorr · Ermin Malic
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    ABSTRACT: In an external magnetic field, the energy of massless charge carriers in graphene is quantized into non-equidistant degenerate Landau levels including a zero-energy level. This extraordinary electronic dispersion gives rise to a fundamentally new dynamics of optically excited carriers. Here, we review the state of the art of the relaxation dynamics in Landau-quantized graphene focusing on microscopic insights into possible many-particle relaxation channels.We investigate optical excitation into a non equilibrium distribution followed by ultrafast carrier- carrier and carrier-phonon scattering processes. We reveal that surprisingly the Auger scattering dominates the relaxation dynamics in spite of the non-equidistant Landau quantization in graphene. Furthermore, we demonstrate how technologically relevant carrier multiplication can be achieved and discuss the possibility of optical gain in Landau-quantized graphene. The provided microscopic view on elementary many-particle processes can guide future experimental studies aiming at the design of novel graphene-based optoelectronic devices, such as highly efficient photodetectors, solar cells, and spectrally broad Landau level lasers.
    01/2015; DOI:10.1515/nanoph-2015-0018
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    ABSTRACT: We present an analytical expression for the differential transmission of a delta-shaped light field in Landau-quantized graphene. This enables a direct comparison of experimental spectra to theoretical calculations reflecting the carrier dynamics including all relevant scattering channels. In particular, the relation is used to provide evidence for strong Auger scattering in Landau-quantized graphene.
    Proceedings of SPIE - The International Society for Optical Engineering 01/2015; 9361. DOI:10.1117/12.2075458 · 0.20 Impact Factor
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    ABSTRACT: A numerically exact solution to the many emitter -- cavity problem as an open many body system is presented. The solution gives access to the full, nonperturbative density matrix and thus the full quantum statistics and quantum correlations. The numerical effort scales with the third power in the number of emitters. Notably the solution requires none of the common approximations like good/bad cavity limit. As a first application the recently discussed concept of coherent surface plasmon amplification -- spaser -- is addressed: A spaser consists of a plasmonic nanostructure that is driven by a set of quantum emitters. In the context of laser theory it is a laser in the (very) bad cavity limit with an extremely high light matter interaction strength. The method allows us to answer the question of spasing with a fully quantized theory.
    Physical Review B 12/2014; 91(3). DOI:10.1103/PhysRevB.91.035306 · 3.74 Impact Factor
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    ABSTRACT: The energy spectrum of common two-dimensional electron gases consists of a harmonic (that is, equidistant) ladder of Landau levels, thus preventing the possibility of optically addressing individual transitions. In graphene, however, owing to its non-harmonic spectrum, individual levels can be addressed selectively. Here, we report a time-resolved experiment directly pumping discrete Landau levels in graphene. Energetically degenerate Landau-level transitions from n = −1 to n = 0 and from n = 0 to n = 1 are distinguished by applying circularly polarized THz light. An analysis based on a microscopic theory shows that the zeroth Landau level is actually depleted by strong Auger scattering, even though it is optically pumped at the same time. The surprisingly strong electron–electron interaction responsible for this effect is directly evidenced through a sign reversal of the pump–probe signal.
    Nature Physics 11/2014; 11(1):75-81. DOI:10.1038/nphys3164 · 20.15 Impact Factor
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    Florian Wendler · Andreas Knorr · Ermin Malic
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    ABSTRACT: Carrier multiplication is a many-particle process giving rise to the generation of multiple electron-hole pairs. This process holds the potential to increase the power conversion efficiency of photovoltaic devices. In graphene, carrier multiplication has been theoretically predicted and recently experimentally observed. However, due to the absence of a bandgap and competing phonon-induced electron-hole recombination, the extraction of charge carriers remains a substantial challenge. Here we present a new strategy to benefit from the gained charge carriers by introducing a Landau quantization that offers a tunable bandgap. Based on microscopic calculations within the framework of the density matrix formalism, we report a significant carrier multiplication in graphene under Landau quantization. Our calculations reveal a high tunability of the effect via externally accessible pump fluence, temperature and the strength of the magnetic field.
    Nature Communications 10/2014; 5:3703. DOI:10.1038/ncomms4703 · 11.47 Impact Factor
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    ABSTRACT: Monolayer transition metal dichalcogenides feature Coulomb-bound electron-hole pairs (excitons) with exceptionally large binding energy and coupled spin and valley degrees of freedom. These unique attributes have been leveraged for electrical and optical control of excitons for atomically-thin optoelectronics and valleytronics. The development of such technologies relies on understanding and quantifying the fundamental properties of the exciton. A key parameter is the intrinsic exciton homogeneous linewidth, which reflects irreversible quantum dissipation arising from system (exciton) and bath (vacuum and other quasiparticles) interactions. Using optical coherent two-dimensional spectroscopy, we provide the first experimental determination of the exciton homogeneous linewidth in monolayer transition metal dichalcogenides, specifically tungsten diselenide (WSe2). The role of exciton-exciton and exciton-phonon interactions in quantum decoherence is revealed through excitation density and temperature dependent linewidth measurements. The residual homogeneous linewidth extrapolated to zero density and temperature is ~1.5 meV, placing a lower bound of approximately 0.2 ps on the exciton radiative lifetime. The exciton quantum decoherence mechanisms presented in this work are expected to be ubiquitous in atomically-thin semiconductors.
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    ABSTRACT: We demonstrate the stabilization of an unstable steady state in an optomechanical system for low and high pump rates. The optomechanical system consists of a pumped cavity with one movable mirror. The stabilization is achieved by applying Pyragas control, i.e., feeding back the cavity field by an external mirror. This way, a maximal cavity field in the low pumping regime can be achieved, which corresponds to a large displacement of the movable mirror.
    Physical Review A 10/2014; 90(4). DOI:10.1103/PhysRevA.90.043835 · 2.81 Impact Factor
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    ABSTRACT: In our work we combined experimental and theoretical investigations of the relaxation dynamics of the single wall carbon nanotubes (SW-CNTs) in solution samples with enriched chiralities of $(7,5)$ and $(7,6)$ species. In two-color pump-probe studies we observe three-exponential decay in the differential transmission spectra in the range of few picoseconds, tens of picoseconds, and hundreds of picoseconds. Decay curves are very similar for both SW-CNT chiralities under resonant excitation and probing of excited and ground state transition energies, respectively. Both types of tubes exhibit no changes in decay for the different excitation energies in the range $\ifmmode\pm\else\textpm\fi{}50\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ around the excited state. By tuning the probe pulse towards energies higher then ground state (up to $+350\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$) we observe acceleration of the first decay component from $5.8\phantom{\rule{0.16em}{0ex}}\mathrm{ps}$ down to $1.6\phantom{\rule{0.16em}{0ex}}\mathrm{ps}$. Our experimental results are supported by time resolved microscopic calculations based on carbon nanotube Bloch equations proving the fast decay component behavior being dominated through scattering with acoustic phonons.
    Physical Review B 10/2014; 90(15). DOI:10.1103/PhysRevB.90.155402 · 3.74 Impact Factor
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    Roland Jago · Torben Winzer · Andreas Knorr · Ermin Malic
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    ABSTRACT: In contrast to conventional structures, efficient non-radiative carrier recombination counteracts the appearance of optical gain in graphene. Based on a microscopic and fully quantum-mechanical study of the coupled carrier, phonon, and photon dynamics in graphene, we present a strategy to obtain a long-lived gain: Integrating graphene into a photonic crystal nanocavity and applying a high-dielectric substrate gives rise to pronounced coherent light emission suggesting the design of graphene-based laser devices covering a broad spectral range.
    Physical Review B 09/2014; 92(8). DOI:10.1103/PhysRevB.92.085407 · 3.74 Impact Factor
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    ABSTRACT: We report on the first direct experimental observation of carrier multiplication in graphene reaching a multiplication factor of up to 2 and persisting on a ps time scale. Exploiting multi-colour pump-probe measurement techniques, the excited non-equilibrium carrier distribution is retrieved on an ultrafast timescale. This provides access to the temporal evolution of the optically excited carrier density and thus allows quantitative conclusions on possible carrier multiplication. Microscopic time- and momentum-resolved calculations on the ultrafast relaxation dynamics of optically excited carriers confirm the observation of carrier multiplication under corresponding experimental conditions, suggesting graphene as a promising material for novel high-efficiency photodetection devices.
    Nano Letters 08/2014; 14(9):5371–5375. DOI:10.1021/nl502114w · 13.59 Impact Factor
  • Frank Milde · Carsten Weber · Andreas Knorr
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    ABSTRACT: The carrier dynamics under solar excitation in the third generation AlAs-Ga0.27In0.73As quantum well solar cell on an InP substrate is theoretically investigated. The quantum well shows a significantly higher carrier occupation compared with that of a pure bulk sample. Furthermore, the influence of carrier scattering via the interaction with a bath of longitudinal optical phonons stemming from the substrate is taken into account, showing strong intraband and interband relaxation of the carrier distribution toward the lowest subband states. Copyright © 2014 John Wiley & Sons, Ltd.
    Progress in Photovoltaics Research and Applications 08/2014; DOI:10.1002/pip.2537 · 7.58 Impact Factor
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    ABSTRACT: In a solid-state platform for quantum information science, the biexciton cascade is an important source of entangled photons. However, the entanglement is usually reduced considerably by the fine-structure splitting of the exciton levels. We show how to counteract this loss of entanglement by applying optical feedback. Substantial control and enhancement of photon entanglement can be achieved by coherently feeding back a part of the emitted signal, e.g., by a mirror, and by tuning the feedback phase and delay time. We present full quantum-mechanical calculations, which include the external photon mode continuum, and discuss the mechanisms leading to the above effects.
    Physical Review Letters 07/2014; 113(2):027401. DOI:10.1103/PhysRevLett.113.027401 · 7.51 Impact Factor
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    ABSTRACT: Using ultrafast optical-pump terahertz-probe spectroscopy, we study the THz dynamics and electronic cooling in few-layer epitaxial and CVD graphene; a microscopic theory of carrier-carrier and carrier-phonon interactions accounts quantitatively for the observed dynamics
    CLEO: QELS_Fundamental Science; 06/2014

Publication Stats

4k Citations
856.42 Total Impact Points


  • 2001–2014
    • Technische Universität Berlin
      • Department of Theoretical Physics
      Berlín, Berlin, Germany
  • 2007
    • Freie Universität Berlin
      • Institute of Chemistry and Biochemistry
      Berlín, Berlin, Germany
  • 2004
    • Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy
      Berlín, Berlin, Germany
  • 1998–2004
    • Max Planck Institute for Intelligent Systems, Stuttgart
      Stuttgart, Baden-Württemberg, Germany
    • Phillips University
      Phillips, Wisconsin, United States
  • 1994–2004
    • Philipps-Universität Marburg
      • Faculty of Physics
      Marburg, Hesse, Germany
  • 1992–1998
    • The University of Arizona
      • • College of Optical Sciences
      • • Department of Mathematics
      Tucson, Arizona, United States
  • 1993–1994
    • Georg-August-Universität Göttingen
      • Institute for Theoretical Physics
      Göttingen, Lower Saxony, Germany