Andreas Knorr

Technische Universität Berlin, Berlín, Berlin, Germany

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Publications (322)756.79 Total impact

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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. · 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
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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.
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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).
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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; · 13.03 Impact Factor
  • 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 01/2014; 5:3703. · 10.02 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.
    12/2013; 89(4).
  • 11/2013;
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    ABSTRACT: We present a combined experimental and theoretical study of two-photon emission from the biexciton cascade in single GaN quantum dots. By changing the biexciton binding energy, pump power, and temperature, the balance between the one-and two-photon decay processes is controlled in this four-level system, which drastically affects the photon statistics of the resulting emission. As the most pronounced feature of this interplay we observe a bunching phenomenon and a transition from sub-to super-Poissonian photon statistics, originating from the complex nature of the biexciton cascade. This work highlights how photon statistics can be steered between one-and two-photon processes towards an increased, bunched two-photon emission probability up to 50 K with the perspective for efficient photon pair generation in the UV spectral range.
    Physical Review B 06/2013; 87(24):245314. · 3.66 Impact Factor
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    ABSTRACT: We investigate the influence of the Förster interaction between semiconductor quantum dots on the quantum light emission in proximity to a metal nanoparticle. A fully quantized theory for the excitons in the quantum dots, the plasmons in metal nanoparticles, and their interaction is used. Using an operator equation approach, we derive the Rayleigh emission spectra and the corresponding quantum statistics of the emission. For both observables, we investigate the influence of the exciton-plasmon coupling and the Förster interaction between the semiconductor quantum dots. Surprisingly, the influence of the Förster interaction is barely seen in the Rayleigh spectra whereas the second-order correlation function is strongly affected. In particular, we show that the Förster interaction is capable to tune the emission statistics: depending on the system parameters, the Förster interaction between the semiconductor quantum dots induces strong bunching and antibunching of the emission, respectively. We analyze the quasiparticles formed in the coupled system to explain the observed features.
    Physical Review B 06/2013; 87(24). · 3.66 Impact Factor
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    ABSTRACT: We investigate the influence of the Förster interaction in hybrid systems consisting of a metal nanoparticle and two semiconductor quantum dots. We show that this interaction strongly influences the emission statistics of the hybrid system.
    CLEO: QELS_Fundamental Science; 06/2013
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    ABSTRACT: We study the collective emission of a few emitters (up to three) in a cavity. In addition to the radiation coupling responsible for sub- and superradiance, we investigate emitters additionally coupled through a joint carrier reservoir. For such emitters, typically embedded in a solid state environment, the carrier reservoir provides a continuous pumping mechanism for the steady state emission. We show that the statistical properties of the emitted light depend strongly on the interaction between the emitters and the reservoir. Unexpectedly, the presence of the reservoir enhances the coherence of the emitted light already for a few emitters. This results from the fact that the carrier reservoir introduces new many-body correlations to the electronic transition and in this way suppresses multiphoton processes.
    Physical Review Letters 03/2013; 110(11). · 7.73 Impact Factor
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    ABSTRACT: Coherent multidimensional spectroscopy allows us to inspect the energies and the coupling of quantum systems. Coupled quantum systems—such as a coupled semiconductor quantum dot or pigments in photosynthesis—form delocalized exciton and two-exciton states. A technique is presented to decompose these delocalized wave functions into the basis of individual quantum emitters. This quantum state tomography protocol is illustrated for three coupled InAs quantum dots. To achieve the decomposition of the wavefunction, we combine the double-quantum-coherence spectroscopy with spatiotemporal control, which allows us to localize optical excitations at a specific quantum dot. Recently, a protocol was proposed for single exciton states (Richter et al 2012 Phys. Rev. B 86 085308). In this paper, we extend the method presented by Richter et al with respect to: the reconstruction of two-exciton states, a detailed analysis process of reconstruction and the effect of filtering to enhance the quality of the reconstructed wave function.
    New Journal of Physics 02/2013; 15(2):025004. · 4.06 Impact Factor
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    ABSTRACT: Optical properties of single-walled semiconducting and metallic carbon nanotubes are significantly influenced by excitonic effects. The excitonic binding energy strongly depends on Coulomb screening. Here, we show – using a non-perturbative single-time equation of motion method – how the momentum-dependent dielectric function ε(q)ε(q) for carbon nanotubes can be consistently derived within a microscopic theory. We investigate the influence of the corresponding screening on the absorption spectra of semiconducting and metallic carbon nanotubes. We observe clearly smaller excitonic binding energies for metallic nanotubes arising from an efficient screening stemming from the crossing bands. The presented method can be applied in a straightforward way to calculate the Coulomb screening in other nanostructures, such as graphene and carbon nanoribbons.
    Chemical Physics 02/2013; 413:3–10. · 1.96 Impact Factor
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    ABSTRACT: We present the results of pump-probe experiments on multilayer graphene samples performed in a wide spectral range, namely from the near infrared (photon energy 1.5 eV) to the terahertz (photon energy 8 meV) spectral range. In the near infrared, exciting carriers and probing at higher photon energies provides direct evidence for a hot carrier distribution. Furthermore, spectroscopic signatures of the highly doped graphene layers at the interface to SiC are observed in the near-infrared range. In the mid-infrared range, the various relaxation mechanisms, in particular scattering via optical phonons and Auger-type processes, are identified by comparing the experimental results to microscopic modeling. Changes from induced transmission to induced absorption are attributed to probing above or below the Fermi edge of the graphene layers. This effect occurs for certain photon energies in the near-infrared range, where it is related to highly doped graphene layers at the interface to SiC, and in the far-infrared range for the quasi-intrinsic graphene layers. In addition to the relaxation dynamics, the saturation of pump-induced bleaching of graphene is studied. Here a quadratic dependence of the saturation fluence on the pump photon energy in the infrared spectral range is revealed.
    Journal of Physics Condensed Matter 01/2013; 25(5):054202. · 2.22 Impact Factor
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    ABSTRACT: We propose a scheme to control cavity quantum electrodynamics in the single photon limit by delayed feedback. In our approach a single emitter-cavity system, operating in the weak coupling limit, can be driven into the strong coupling-type regime by an external mirror: The external loop produces Rabi oscillations directly connected to the electron-photon coupling strength. As an expansion of typical cavity quantum electrodynamics, we treat the quantum correlation of external and internal light modes dynamically and demonstrate a possible way to implement a fully quantum mechanical time-delayed feedback. Our theoretical approach proposes a way to experimentally feedback control quantum correlations in the single photon limit.
    Physical Review Letters 01/2013; 110(1):013601. · 7.73 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 01/2013; 103(25):253117-253117-3. · 3.79 Impact Factor
  • physica status solidi (RRL) - Rapid Research Letters 12/2012; 6(12). · 2.39 Impact Factor

Publication Stats

3k Citations
756.79 Total Impact Points


  • 2001–2014
    • Technische Universität Berlin
      • • Department of Theoretical Physics
      • • Department of Optics and Atomic Physics
      Berlín, Berlin, Germany
  • 2012
    • The Optical Society
      Society Hill, New Jersey, United States
    • The International Society for Optics and Photonics
      International Falls, Minnesota, United States
  • 2011
    • University of California, Irvine
      • Department of Chemistry
      Irvine, CA, United States
  • 2010
    • Weierstrass Institute for Applied Analysis and Stochastics
      Berlín, Berlin, Germany
  • 2008–2010
    • Freie Universität Berlin
      • Department of Physics
      Berlin, Land Berlin, Germany
  • 2007
    • Lund University
      • Division of Mathematical Physics
      Lund, Skane, Sweden
  • 2006–2007
    • Massachusetts Institute of Technology
      • Department of Materials Science and Engineering
      Cambridge, MA, United States
  • 1997–2006
    • Sandia National Laboratories
      • Semiconductor Material and Device Sciences Department
      Albuquerque, NM, United States
  • 1992–2006
    • The University of Arizona
      • • College of Optical Sciences
      • • Department of Mathematics
      Tucson, Arizona, United States
  • 1970–2006
    • Philipps University of Marburg
      • Faculty of Physics
      Marburg, Hesse, Germany
  • 2005
    • Max Planck Institute for Solid State Research
      Stuttgart, Baden-Württemberg, 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
  • 1993–1994
    • Georg-August-Universität Göttingen
      Göttingen, Lower Saxony, Germany