Andreas Knorr

Technische Universität Berlin, Berlín, Berlin, Germany

Are you Andreas Knorr?

Claim your profile

Publications (360)876.12 Total impact

  • Source
    [Show abstract] [Hide abstract]
    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 consecutive photon emission events, reveals a decrease in TPI visibility on a nanosecond timescale, 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 decrease in TPI visibility from $(96\pm4)\,$\% at 10\,K to a vanishing visibility at 40\,K. In contrast to Michelson-type measurements, our experiments provide direct access to the time-dependent coherence of a quantum emitter at a nanosecond timescale.
    Full-text · Article · Jan 2016 · Physical Review Letters
  • [Show abstract] [Hide abstract]
    ABSTRACT: We present a Heisenberg operator based formulation of coherent quantum feedback and Pyragas control. This model is easy to implement and allows for an efficient and fast calculation of the dynamics of feedback-driven observables as the number of contributing correlations grows in systems with a fixed number of excitations only linearly in time. Furthermore, our model unravels the quantum kinetics of entanglement growth in the system by explicitly calculating non-Markovian multi-time correlations, e.g., how the emission of a photon is correlated with an absorption process in the past. Therefore, the time-delayed differential equations are expressed in terms of insightful physical quantities. Another considerate advantage of this method is its compatibility to typical approximation schemes, such as factorization techniques and the semi-classical treatment of coherent fields. This allows the application on a variety of setups, ranging from closed quantum systems in the few excitation regimes to open systems and Pyragas control in general.
    No preview · Article · Dec 2015
  • Hannah Funk · Andreas Knorr · Florian Wendler · Ermin Malic
    [Show abstract] [Hide abstract]
    ABSTRACT: Placing a two-dimensional sheet of graphene in an external magnetic field the continuous electronic band structure is discretized due to Landau quantization. The resulting optical transitions are subject to a broadening, which can lead to a significant overlap of Landau levels. We investigate the possible microscopic processes that could cause a broadening of the corresponding peaks in the absorption spectrum of Landau-quantized graphene: (i) radiative decay, (ii) Coulomb interaction, (iii) optical phonons, (iv) acoustic phonons, and (v) impurities. Since recent experiments have shown that independent of the magnetic field the resolvable number of Landau levels is constant, we put a special focus on the dependence of the broadening on the external magnetic field B and the Landau level index n. Our calculations reveal the impurities to be the crucial broadening mechanism, where different regimes of well separated and densely spaced Landau levels need to be taken into account. Furthermore, carrier-carrier and carrier-phonon scattering give rise to a very specific dependence on the Landau level index n that has not been observed yet.
    No preview · Article · Nov 2015 · Physical Review B
  • [Show abstract] [Hide abstract]
    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.
    No preview · Article · Sep 2015
  • Source
    [Show abstract] [Hide abstract]
    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.
    Full-text · Article · Sep 2015 · Nature Communications
  • Source
    Faris Kadi · Torben Winzer · Andreas Knorr · Ermin Malic
    [Show abstract] [Hide abstract]
    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
    Preview · Article · Aug 2015 · Scientific Reports
  • [Show abstract] [Hide abstract]
    ABSTRACT: We reveal the ultrafast intervalley dynamics in monolayer WS2 with a spectrallyresolved ultrafast pump-probe experiment and a microscopic theory. We find strong intervalley Coulomb coupling in the atomically thin semiconductor.
    No preview · Article · May 2015
  • [Show abstract] [Hide abstract]
    ABSTRACT: We combine ultrafast time-resolved THz spectroscopy and microscopic modeling to study the hot-carrier relaxation and cooling dynamics in graphene; we obtain quantitative agreement without the need to invoke disorder effects and demonstrate that the dynamics are the result of the intricate interplay between carrier-carrier and carrier-phonon interactions.
    No preview · Article · May 2015
  • [Show abstract] [Hide abstract]
    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.
    No preview · Article · May 2015 · Physical Review A
  • Judith F. Specht · Andreas Knorr · Marten Richter
    [Show abstract] [Hide abstract]
    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.
    No preview · Article · Apr 2015 · Physical Review B
  • Source
    [Show abstract] [Hide abstract]
    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.
    Full-text · Article · Mar 2015 · Physical Review A
  • [Show abstract] [Hide abstract]
    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.
    No preview · Article · Mar 2015 · Proceedings of SPIE - The International Society for Optical Engineering
  • [Show abstract] [Hide abstract]
    ABSTRACT: We present a theoretical discussion on the feasibility of spasing for a specific spaser setup. For this purpose we develop a numerically exact and feasible solution to the open system Tavis-Cummings model in the Born- Markov Lindblad formalism. The complexity of the solution scales with the third power in the number of two level systems- a considerable advance compared to the exponential scaling of the brute force solution. The question of spasing is answered negatively in agreement with the literature.
    No preview · Article · Mar 2015 · Proceedings of SPIE - The International Society for Optical Engineering
  • Florian Wendler · Andreas Knorr · Ermin Malic
    [Show abstract] [Hide abstract]
    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.
    No preview · Article · Jan 2015
  • Source
    [Show abstract] [Hide abstract]
    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.
    Preview · Article · Dec 2014 · Physical Review B
  • [Show abstract] [Hide abstract]
    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.
    No preview · Article · Nov 2014 · Nature Physics
  • Source
    G. T. Adamashvili · D. J. Kaup · A. Knorr
    [Show abstract] [Hide abstract]
    ABSTRACT: A theory of dispersive soliton of the self-induced transparency in a medium consisting of atoms or semiconductor quantum dots of two types is considered. A two-component medium is modeled by a set of two-level atoms of two types embedded into a conductive host material. These types of atoms correspond to passive atoms (attenuator atoms) and active atoms (amplifier atoms) with inverse population of the energetic levels. The complete solution is given of the Maxwell\char21{}Bloch equations for ensembles of two-type atoms with different parameters and different initial conditions by inverse scattering transform. The solutions of the Maxwell\char21{}Bloch equations for many-component atomic systems by inverse scattering transform are also discussed. The influence of the difference between dipole moments of atoms, the longitudinal and transverse relaxation times, pumping, and conductivity on the soliton is taken into account by means of perturbation theory. The memory effects are described in terms of generalized non-Markovian optical Bloch equations. The condition of a balance between the energy supplied and lost is obtained.
    Full-text · Article · Nov 2014 · Physical Review A
  • Source
    Florian Wendler · Andreas Knorr · Ermin Malic
    [Show abstract] [Hide abstract]
    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.
    Full-text · Article · Oct 2014 · Nature Communications
  • Source
    [Show abstract] [Hide abstract]
    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.
    Full-text · Article · Oct 2014
  • [Show abstract] [Hide abstract]
    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.
    No preview · Article · Oct 2014 · Physical Review A

Publication Stats

5k Citations
876.12 Total Impact Points


  • 2001-2015
    • Technische Universität Berlin
      • Department of Theoretical Physics
      Berlín, Berlin, Germany
    • University of Bonn
      • Institute for Applied Physics
      Bonn, North Rhine-Westphalia, Germany
  • 2012
    • Austrian Academy of Sciences
      • Institute for Quantum Optics and Quantum Information - IQOQI Innsbruck
      Wien, Vienna, Austria
  • 2007
    • Freie Universität Berlin
      • Institute of Chemistry and Biochemistry
      Berlín, Berlin, Germany
  • 1998-2004
    • Max Planck Institute for Intelligent Systems, Stuttgart
      Stuttgart, Baden-Württemberg, Germany
    • Phillips University
      Phillips, Wisconsin, United States
  • 1994-2001
    • Philipps-Universität Marburg
      • Faculty of Physics
      Marburg, Hesse, Germany
  • 1992-1998
    • The University of Arizona
      • College of Optical Sciences
      Tucson, Arizona, United States
  • 1993-1994
    • Georg-August-Universität Göttingen
      • Institute for Theoretical Physics
      Göttingen, Lower Saxony, Germany