Integriertes Seminar zu aktuellen Problemen der Physik dimensionsreduzierter Festkörper

(Seminar)

Veranstaltungs-Nr. 114093

  • 27.04.2016

    Sven Borghardt

    Forschungszentrum Jülich

    Optical Characterization and Quantitative Electron Holography of Two-Dimensional WSe2 for the Development of New Quantum Light Sources

    An emission of single photons from WSe2 monolayers has been recently demonstrated but the origin of the emission is still not clear. The aim of our research is to understand its origin and then control the localized emission in this material to make use of the unique properties
    of the material.

    Samples prepared with different exfoliation techniques are measured using polarization resolved µ-photoluminescence. Our results show linearly polarized emission doublet lines with an energy splitting of up to a few meV. The samples show a high density of such lines close to the sample edges. We attribute the linear polarization of the localized states to a mixing of K- and K'-states. There is an evidence of an alignment of the polarization of emission with the crystal lattice. Photoluminescence from the localized states decays faster with the temperature than the one from the free exciton states.

    For atomistic study of the origin of the localized states, high resolution off-axis electron holography experiments are suggested. We present first experimental and theoretical results for pristine few-layer WSe2 as the starting point for these studies. In order to simulate the electron wave function accurately – as required for defect imaging – we use DFT to determine the electrostatic potential distribution in the film. Our simulations show that charge reorganisation due to atomic bonding has strong influence on the phase acquired by the probe electrons passing through few-layer TMD structures. Comparisons with the experimental data confirm this conclusion.

    Einladender: Bratschitsch

  • 11.05.2016

    Daniel Laumann

    Fachliche Strukturierung und Entwicklung multipler
    Repräsentationen zum Magnetismus für die Hochschule

  • 25.05.2016

    Rolf Binder

    College of Optical Sciences and Department of Physics,
    University of Arizona, Tuscon, USA

    Formation and optical control of polaritonic density and spin patterns

    Excitons in GaAs quantum wells and their interactions yield a wide variety of linear and nonlinear optical effects. In this talk, I will focus on the quantum fluid formed by exciton polaritons in a semiconductor microcavity. Here, four-wave mixing can lead to instabilities and the formation of spatial patterns in the polariton density. The control of the density patterns and of the far-field emission, which we believe can lead to new photonic devices, can be analyzed using the concept of non-equilibrium phase transitions. I will also discuss the optical control of spatial pseudo-spin textures, which arise as a consequence of pseudo-spin-orbit interaction (so-called optical spin Hall effect). As an aside, I will briefly discuss the interband dipole matrix element, which is an integral part of exciton physics, and which to this day causes much confusion in the semiconductor community.

    Einladender: Kuhn

  • 01.06.2016

    Lukas Eschmann

    Spinpolarisation von Photoelektronen in topologischen Isolatoren

  • 07.06.2016

    S O N D E R T E R M I N

    Dienstag, 07.06.2016, 14:15 Uhr

    Jacek Kasprzak

    Université Grenoble Alpes and CNRS, Institut Néel, Grenoble, France

    Coherence of individual emitters in photonic nanostructures

    I will highlight recent advances in retrieving and manipulating coherent nonlinear responses of semiconductor nanostructures. In particular, wave mixing spectroscopy is a well-established approach to infer coherence and population dynamics of exciton ensembles, for instance confined in quantum wells. I will start by revisiting this physics by demonstrating enhanced four-wave mixing (FWM) response in monolayers of transition metal dichalcogenides [1], in particular in MoSe2. I will discuss the impact of a local disorder and temperature on exciton dephasing and lifetime in this material. Performing FWM spectroscopy on individual transitions, like excitons in single quantum dots (QDs), is far more challenging due to an overwhelming ratio (typically 106…8 in the field) between the required resonant optical driving and the amount of the emitted FWM signal. In this talk, I will show that by embedding a QD in well-designed photonic devices - like planar GaAs [2,3] or CdTe microcavities, one-dimensional waveguides antennas [4] or deterministic microlenses [5] - one can suppress the background-signal ratio down to 102…4, abolishing a long-standing issue regarding a poor retrieval efficiency of coherent responses from single excitons. Such dramatically increased sensitivity of the measurements opens novel opportunities in this field. It enables to verify theoretical proposals, which until now have been considered as experimentally unfeasible. I will highlight two such examples. The first one is to monitor polaron formation and to reveal corresponding phonon-induced dephasing in a QD [5]. The second one involves implementation of novel, fast, coherent control schemes via multi-wave mixing [2]. As an outlook, I will focus on ongoing experiments, using stochastic and deterministic approaches, aiming to demonstrate long-range radiative coupling within a pair of distant excitons.

    1. T. Jakubczyk, V. Delmonte et al., Radiatively limited dephasing and exciton dynamics in MoSe2 monolayers, resubmitted to Nano Letters (2016)
    2. F. Fras, Q. Mermillod et al., Multi-wave coherent control of a solid state single emitter, Nature Photonics 10, 155 (2016)
    3. Q. Mermillod, D. Wigger et al., Dynamics of excitons in individual InAs quantum dots revealed in four-wave mixing spectroscopy, Optica 3, 377 (2016)
    4. Q. Mermillod, T. Jakubczyk et al., Harvesting, coupling, and control of single-exciton coherences in photonic waveguide antennas, Physical Review Letters 116, 163903 (2016).
    5. in preparation (2016), Impact of phonons on dephasing of individual excitons in deterministic quantum dot microlenses

    Einladender: Kuhn

  • 08.06.2016

    Henry Wortelen

    Spin-Orbit Effects in the Surface Electronic Structure of W(110) and Ta(110)

  • 14.06.2016

    S O N D E R T E R M I N

    Dienstag, 14.06.2016, 14:15 Uhr

    Andrey Akimov

    School of Physics and Astronomy, University of Nottingham, UK

    Electro-Phononics in semiconductor nanostructures

    A brief review of the experiments with coherent phonons in semiconductor nanostructures will be given. Traditionally, THz coherent phonons are generated and detected by femtosecond optical pulses from ultrafast lasers. The group of physicists in the University of Nottingham aims at the development of methods for the generation and detection of coherent phonons without ultrafast lasers basing on the transport properties of semiconductor nanostructures like Schottky and tunneling devices [1,2] and superlattices [3]. In more details coherent phonon optics experiment with superlattices and p-i-n devices will be described [4].

    1. D.M. Moss et al., PRL 106, 066602 (2011)
    2. E.S.K. Young et al., PRL 108, 226601 (2012)
    3. C. Poyser et al., New J. Phys. 17, 083064 (2015)
    4. C. Poyser et al., Sci. Rep. 5, 8279, (2015)

    Einladende: Reiter

  • 15.06.2016

    Christoph Angrick

    Spin-Bahn-Effekte an Oberflächen am Beispiel von Tl/Si(111)

  • 22.06.2016

    Philipp Marauhn

    Exzitonen in zweidimensionalen Materialien

  • 29.06.2016

    S O N D E R T E R M I N

    Mittwoch, 29.06.2016, 10:15 Uhr, SR 718

    Stefano Guazzotti

    Department of Physics, Imperial College London, UK

    Dynamical Calculation of Higher Harmonic Generation in Two-Dimensional Semiconductors

    The generation of ever shorter optical pulses enables a host of techniques for material characterization, allowing for the exploration and manipulation of electron dynamics on its intrinsic timescales. Semiconductor saturable absorber mirrors (SESAMs) with a semiconductor quantum well (QW) are commonly used for femtosecond pulse generation [1], however an adequate theoretical description of the optically induced non-linear phenomena in the semiconductor is quitechallenging. We combine a finite difference time domain (FDTD) description of the light field propagation that allows us to track the full light field dynamics, beyond the rotating wave and slowly varying envelope approximations, with wave-vector resolved electron dynamics in a semiconductor QW. We describe the QW in a two band model and include Coulomb interaction [2] to account for excitonic effects which are expected to play a crucial role in the non-linear physics. With this method we go beyond commonly used FDTD approaches which model the semiconductor as a simple few level model. We employ this approach to describe the process of third harmonic generation (THG) in a InGaAs QW that is illuminated with intense femtosecond pulses [3].

    When the excitation is well below the bandgap of the QW, i.e., no carriers are created in the semiconductor, we found that the THG intensity has a cubic dependence on the intensity of the exciting pulse. This is in agreement with a description based on an expansion in powers of the field with constant non-linear susceptibilities as coefficients. For an intense pulse resonant with the exciton energy, a substantial carrier density is created from absorption at the excitonic resonance. This affects the way in which the THG intensity depends on the intensity of the exciting pulse. We observe that the dependence stays a power law, where the characteristic exponent, decreases alongside the detuning between the pulse frequency and the excitonic resonance.

    The simultaneous spatio-temporal description of the light field and carrier dynamics provided by our model is also readily expandable to other 2-dimensional semiconductor systems like graphene or the newly found transition metal dichalcogenides.

    1. D. Jung et al., Appl. Phys. B 65, 137 (1997)
    2. K. Böhringer and O. Hess, Prog. Quantum Electron. 32, 159-246 (2008)
    3. S. Guazotti et al., submitted to PRB (2016)

  • 06.07.2016

    Marvin Nyenhuis

    Weyl-Halbmetalle

  • 13.07.2016

    Johannes Kirchner

    Ultraschnelle Dynamik in Graphen

  • 20.07.2016

    Phong-Minh Timmy Ly

    Linienbreiten und Stokes-Verschiebungen

  • Veranstalter

    • Bratschitsch, Rudolf, Prof. Dr.
    • Doltsinis, Nikos, Prof. Dr.
    • Donath, Markus, Prof. Dr.
    • Kohl, Helmut, Prof. Dr.
    • Krüger, Peter, apl. Prof. Dr.
    • Kuhn, Tilmann, Prof. Dr.
    • Rohlfing, Michael, Prof. Dr.
    • Zacharias, Helmut, Prof. Dr.
    • Michaelis De Vasconcellos, Steffen, Dr.
    • Reiter, Doris, Dr.
    • Schmidt, Anke, Dr.