Andreas Knorr

Technische Universität Berlin, Berlín, Berlin, Germany

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Publications (337)839.37 Total impact

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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.
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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 · 3.66 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 · 20.60 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 · 10.74 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: 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.
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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 · 12.94 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 · 9.70 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 · 7.73 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
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    ABSTRACT: The entanglement of photons from a biexciton cascade is strongly diminished by exciton fine-structure splitting. We demonstrate an optical feedback mechanism to counteract this loss and to control the photon entanglement.
    CLEO: QELS_Fundamental Science; 06/2014
  • Physical Review B 06/2014; 89(23). DOI:10.1103/PhysRevB.89.235314 · 3.66 Impact Factor
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    ABSTRACT: We propose an experiment to generate deterministic entanglement between separate nitrogen vacancy (NV) centers mediated by the mode of a photonic crystal cavity. Using numerical simulations the applicability and robustness of the entanglement operation to parameter regimes achievable with present technology is investigated. We find that even with moderate cavity Q-factors of $10^{4}$ a concurrence of $c>0.6$ can be achieved within a time of $t_{max}\approx150$~ns, while Q-factors of $10^{5}$ promise $c>0.8$. Most importantly, the investigated scheme is relative insensitive to spectral diffusion and differences between the optical transitions frequencies of the used NV centers.
    Physical Review A 05/2014; 89(6). DOI:10.1103/PhysRevA.89.060303 · 2.99 Impact Factor
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    ABSTRACT: A spectroscopic pump-probe method to determine the spatial shape of bound quantum states in semiconductor quantum dots is proposed. The method uses intraband transitions between the bound states and unbound continuum states in the host medium. The developed theoretical scheme is applicable to quantum dots embedded in bulk material and with some modification also for other nanostructures. In particular, the analyzed pump-probe spectra exhibit specific spectral signatures for different spatial extensions of the quantum dot confinement potential.
    Physical Review B 05/2014; 89(20-20). DOI:10.1103/PhysRevB.89.201414 · 3.66 Impact Factor
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    ABSTRACT: We present a microscopic explanation of the controversially discussed transient negative differential transmission observed in degenerate optical pump-probe measurements in graphene. Our approach is based on the density matrix formalism allowing a time- and momentum-resolved study of carrier-light, carrier-carrier, and carrier-phonon interaction on microscopic footing. We show that phonon-assisted optical intraband transitions give rise to transient absorption in the optically excited hot carrier system counteracting pure absorption bleaching of interband transitions. While interband transition bleaching is relevant in the first hundreds of fs after the excitation, intraband absorption sets in at later times. In particular, in the low excitation regime, these intraband absorption processes prevail over the absorption bleaching resulting in a zero-crossing of the differential transmission. Our findings are in good agreement with recent experimental pump-probe studies.
    Physical Review Letters 04/2014; 113(3). DOI:10.1103/PhysRevLett.113.035502 · 7.73 Impact Factor
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    ABSTRACT: We present recent experiments on the demonstration of fundamental quantum effects, like the quantum Zeno effect on a single NV center. Furthermore theoretical studies on the implementation of a fundamental cavity assisted two-qubit quantum gate are presented.
    Quantum Information and Measurement; 03/2014
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    ABSTRACT: We present pump-probe experiments on graphene, which reveal a pronounced dependence of the pump-induced transmission on the angle between pump and probe polarization. It reflects a strong anisotropy of the pump-induced occupation of photogenerated carriers in momentum space. Within 150 fs after excitation an isotropic carrier distribution is established. The experiments are well described by microscopic modelling, which identify carrier-phonon scattering to be the main relaxation mechanism giving rise to an isotropic carrier distribution.
    Nano Letters 02/2014; 14(3). DOI:10.1021/nl404730y · 12.94 Impact Factor
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    ABSTRACT: We develop a full quantum-optical approach for optical self-feedback of a microcavity laser. These miniaturized devices work in a regime between the quantum and classical limit and are test-beds for the differences between a quantized theory of optical self-feedback and the corresponding semiclassical theory. The light intensity and photon statistics are investigated with and without an external feedback: We show that in the low-gain limit, where relaxation oscillations do not appear, the recently observed photon bunching in a quantum dot microcavity laser with optical feedback can be accounted for only by the fully quantized model. By providing a description of laser devices with feedback in the quantum limit we reveal novel insights into the origin of bunching in quantized and semiclassical models.
    Physical Review A 12/2013; 89(4). DOI:10.1103/PhysRevA.89.041801 · 2.99 Impact Factor
  • Florian Wendler, Andreas Knorr, Ermin Malic
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    ABSTRACT: We present a microscopic study on the phonon-induced inter-Landau level carrier relaxation in graphene. Our approach is based on the density matrix formalism giving access to time-resolved relaxation dynamics in graphene under Landau quantization. We find a drastically enhanced scattering via optical phonons at specific magnetic fields predicting clear still unobserved signatures in experimental differential transmission spectra.
    Applied Physics Letters 12/2013; 103(25):253117-253117-3. DOI:10.1063/1.4852635 · 3.52 Impact Factor
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    ABSTRACT: We calculated Coulomb scattering rates from quantum dots (QDs) coupled to a two-dimensional (2D) carrier reservoir and QDs coupled to a three-dimensional (3D) reservoir. For this purpose we used a microscopic theory in the limit of Born-Markov approximation, in which the numerical evaluation of high dimensional integrals is done via a quasi-Monte Carlo method. Via a comparison of the so determined scattering rates, we investigated the question whether scattering from 2D is generally more efficient than scattering from 3D. In agreement with experimental findings, we did not observe a significant reduction of the scattering efficiency of a QD directly coupled to a 3D reservoir. In turn, we found that 3D scattering benefits from its additional degree of freedom in the momentum space.
    Physical Review B 11/2013; 88(23). DOI:10.1103/PhysRevB.88.235421 · 3.66 Impact Factor

Publication Stats

4k Citations
839.37 Total Impact Points

Institutions

  • 2001–2014
    • Technische Universität Berlin
      • Department of Theoretical Physics
      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