Hauptinhalt
Veröffentlichungen 2017
Inhalt ausklappen Inhalt einklappen 90. T. Winzer, M. Mittendorff, S. Winnerl, H. Mittenzwey, R. Jago, M. Helm, E. Malic and A. Knorr, "Unconventional double-bended saturation of optical transmission in graphene due to many-particle interaction", Nature Communications 8, 15042 (2017)90. T. Winzer, M. Mittendorff, S. Winnerl, H. Mittenzwey, R. Jago, M. Helm, E. Malic and A. Knorr, "Unconventional double-bended saturation of optical transmission in graphene due to many-particle interaction", Nature Communications 8, 15042 (2017)
Saturation of carrier occupation in optically excited materials is a well-established phenomenon. However, so far, the observed saturation effects have always occurred in the strong-excitation regime and have been explained by Pauli blocking of the optically filled quantum states. On the basis of microscopic theory combined with ultrafast pump-probe experiments, we reveal a new low-intensity saturation regime in graphene that is purely based on many-particle scattering and not Pauli blocking. This results in an unconventional double-bended saturation behaviour: both bendings separately follow the standard saturation model exhibiting two saturation fluences; however, the corresponding fluences differ by three orders of magnitude and have different physical origin. Our results demonstrate that this new and unexpected behaviour can be ascribed to an interplay between time-dependent many-particle scattering and phase-space filling effects.
Inhalt ausklappen Inhalt einklappen 89. M. Feierabend, G. Berghaeuser, A. Knorr and E. Malic, "Proposal for dark exciton based chemical sensors", Nature Communications 8, 14776 (2017)89. M. Feierabend, G. Berghaeuser, A. Knorr and E. Malic, "Proposal for dark exciton based chemical sensors", Nature Communications 8, 14776 (2017)
The rapidly increasing use of sensors throughout different research disciplines and the demand for more efficient devices with less power consumption depends critically on the emergence of new sensor materials and novel sensor concepts. Atomically thin transition metal dichalcogenides have a huge potential for sensor development within a wide range of applications. Their optimal surface-to-volume ratio combined with strong light–matter interaction results in a high sensitivity to changes in their surroundings. Here, we present a highly efficient sensing mechanism to detect molecules based on dark excitons in these materials. We show that the presence of molecules with a dipole moment transforms dark states into bright excitons, resulting in an additional pronounced peak in easy accessible optical spectra. This effect exhibits a huge potential for sensor applications, since it offers an unambiguous optical fingerprint for the detection of molecules—in contrast to common sensing schemes relying on small peak shifts and intensity changes.
Inhalt ausklappen Inhalt einklappen 88. D. Christiansen, M. Selig, G. Berghaeuser, R. Schmidt, I. Niehues, R. Schneider, A. Arora, S. Michaelis de Vasconcellos, R. Bratschitsch, E. Malic and A. Knorr, "Phonon Sidebands in Monolayer Transition Metal Dichalcogenides", Phys. Rev. Lett. 119, 187402 (2017) 88. D. Christiansen, M. Selig, G. Berghaeuser, R. Schmidt, I. Niehues, R. Schneider, A. Arora, S. Michaelis de Vasconcellos, R. Bratschitsch, E. Malic and A. Knorr, "Phonon Sidebands in Monolayer Transition Metal Dichalcogenides", Phys. Rev. Lett. 119, 187402 (2017)
Excitons dominate the optical properties of monolayer transition metal dichalcogenides (TMDs). Besides optically accessible bright exciton states, TMDs exhibit also a multitude of optically forbidden dark excitons. Here, we show that efficient exciton-phonon scattering couples bright and dark states and gives rise to an asymmetric excitonic line shape. The observed asymmetry can be traced back to phonon-induced sidebands that are accompanied by a polaron redshift. We present a joint theory-experiment study investigating the microscopic origin of these sidebands in different TMD materials taking into account intra- and intervalley scattering channels opened by optical and acoustic phonons. The gained insights contribute to a better understanding of the optical fingerprint of these technologically promising nanomaterials.
Phys. Rev. Lett. 119, 187402 (2017)Inhalt ausklappen Inhalt einklappen 87. F. Wendler, M. Mittendorff, J. König-Otto, S. Brem, C. Berger, W. A. de Heer, R. Böttger, H. Schneider, M. Helm, S. Winnerl and E. Malic87. F. Wendler, M. Mittendorff, J. König-Otto, S. Brem, C. Berger, W. A. de Heer, R. Böttger, H. Schneider, M. Helm, S. Winnerl and E. Malic"Symmetry-breaking supercollisions in Landau-quantized graphene", Phys. Rev. Lett. 119, 067405 (2017)
Recent pump-probe experiments performed on graphene in a perpendicular magnetic field have revealedcarrier relaxation times ranging from picoseconds to nanoseconds depending on the quality of the sample.To explain this surprising behavior, we propose a novel symmetry-breaking defect-assisted relaxationchannel. This enables scattering of electrons with single out-of-plane phonons, which drastically acceleratethe carrier scattering time in low-quality samples. The gained insights provide a strategy for tuning thecarrier relaxation time in graphene and related materials by orders of magnitude.
Inhalt ausklappen Inhalt einklappen 86. R. Jago, F. Wendler, E. Malic, "Microscopic understanding of the photoconduction effect in graphene", Phys. Rev. B 96, 085431 (2017) 86. R. Jago, F. Wendler, E. Malic, "Microscopic understanding of the photoconduction effect in graphene", Phys. Rev. B 96, 085431 (2017)
We investigate the photoresponse of intrinsic graphene in an in-plane electric field. Toward that end, we employ a microscopic approach that allows us to determine the time- and momentum-resolved charge-carrier distributions as a result of the interplay between the field-induced acceleration of optically excited carriers and Coulomb- and phonon-driven carrier scattering. Calculating the generated photocurrent that is determined by the asymmetry of the carrier distribution, we reveal the microscopic foundation of the photoconduction effect in graphene. In particular, we discuss the possibility of tuning the photocurrent via externally accessible knobs, such as electric field, temperature, and substrate. Furthermore, we study the impact of Auger-induced carrier multiplication on the photocurrent in graphene.
Phys. Rev. B 96, 085431 (2017)Inhalt ausklappen Inhalt einklappen 85. S. Brem, F. Wendler, E. Malic, "Microscopic modeling of tunable graphene-based terahertz Landau-level lasers", Phys. Rev. B 96, 045427 (2017)85. S. Brem, F. Wendler, E. Malic, "Microscopic modeling of tunable graphene-based terahertz Landau-level lasers", Phys. Rev. B 96, 045427 (2017)
In the presence of strong magnetic fields the electronic band structure of graphene drastically changes. The Dirac cone collapses into discrete nonequidistant Landau levels, which can be externally tuned by changing the magnetic field. In contrast to conventional materials, specific Landau levels are selectively addressable using circularly polarized light. Exploiting these unique properties, we propose the design of a tunable laser operating in the technologically promising terahertz spectral range. To uncover the many-particle physics behind the emission of light, we perform a fully quantum mechanical investigation of the nonequilibrium dynamics of electrons, phonons, and photons in optically pumped Landau-quantized graphene embedded in a high-quality optical cavity. The microscopic insights gained allow us to predict optimal experimental conditions to realize a technologically promising terahertz laser.
Inhalt ausklappen Inhalt einklappen 84. M. Feierabend, A. Morlet, G. Berghaeuser, E. Malic, "Impact of strain on the optical fingerprint of monolayer transition metal dichalcogenides", Phys. Rev. B 96, 045425 (2017)84. M. Feierabend, A. Morlet, G. Berghaeuser, E. Malic, "Impact of strain on the optical fingerprint of monolayer transition metal dichalcogenides", Phys. Rev. B 96, 045425 (2017)
Strain presents a straightforward tool to tune electronic properties of atomically thin nanomaterials that are highly sensitive to lattice deformations. While the influence of strain on the electronic band structure has been intensively studied, there are only a few works on its impact on optical properties of monolayer transition-metal dichalcogenides (TMDs). Combining microscopic theory based on Wannier and Bloch equations with nearest-neighbor tight-binding approximation, we present an analytical view on how uni- and biaxial strain influences the optical fingerprint of TMDs, including their excitonic binding energy, oscillator strength, optical selection rules, and the radiative broadening of excitonic resonances. We show that the impact of strain can be reduced to changes in the lattice structure (geometric effect) and in the orbital functions (overlap effect). In particular, we demonstrate that the valley-selective optical selection rule is softened in the case of uniaxial strain due to the introduced asymmetry in the lattice structure. Furthermore, we reveal a considerable increase of the radiative dephasing due to strain-induced changes in the optical matrix element and the excitonic wave functions.
Inhalt ausklappen Inhalt einklappen 83. S. Aeschlimann, R. Krause, M. Chávez-Cervantes, H. Bromberger, R. Jago, E. Malic, A. Al-Temimy, C. Coletti, A. Cavalleri and I. Gierz, "Ultrafast momentum imaging of pseudospin-flip excitations in graphene", Phys. Rev. 96, 020301(R) (2017)83. S. Aeschlimann, R. Krause, M. Chávez-Cervantes, H. Bromberger, R. Jago, E. Malic, A. Al-Temimy, C. Coletti, A. Cavalleri and I. Gierz, "Ultrafast momentum imaging of pseudospin-flip excitations in graphene", Phys. Rev. 96, 020301(R) (2017)
The pseudospin of Dirac electrons in graphene manifests itself in a peculiar momentum anisotropy for photoexcited electron-hole pairs. These interband excitations are in fact forbidden along the direction of the light polarization and are maximum perpendicular to it. Here, we use time- and angle-resolved photoemission spectroscopy to investigate the resulting unconventional hot carrier dynamics, sampling carrier distributions as a function of energy, and in-plane momentum. We first show that the rapidly-established quasithermal electron distribution initially exhibits an azimuth-dependent temperature, consistent with relaxation through collinear electron-electron scattering. Azimuthal thermalization is found to occur only at longer time delays, at a rate that depends on the substrate and the static doping level. Further, we observe pronounced differences in the electron and hole dynamics in n-doped samples. By simulating the Coulomb- and phonon-mediated carrier dynamics we are able to disentangle the influence of excitation fluence, screening, and doping, and develop a microscopic picture of the carrier dynamics in photoexcited graphene. Our results clarify new aspects of hot carrier dynamics that are unique to Dirac materials, with relevance for photocontrol experiments and optoelectronic device applications.
Inhalt ausklappen Inhalt einklappen 82. M. Feierabend, E. Malic, A. Knorr, and G. Berghaeuser, "Optical fingerprint of non-covalently functionalized transition metal dichalcogenides", J. Phys. Condens. Matter 29, 384003 (2017)82. M. Feierabend, E. Malic, A. Knorr, and G. Berghaeuser, "Optical fingerprint of non-covalently functionalized transition metal dichalcogenides", J. Phys. Condens. Matter 29, 384003 (2017)
Atomically thin transition metal dichalcogenides (TMDs) hold promising potential for applications in opto-electronics. Due to their direct band gap and the extraordinarily strong Coulomb interaction, TMDs exhibitefficient light-matter coupling and tightly bound excitons. Moreover, large spin orbit coupling in combinationwith circular dichroism allows for spin and valley selective optical excitation. As atomically thin materials,they are very sensitive to changes in the surrounding environment. This motivates a functionalization approach,where external molecules are adsorbed to the materials surface to tailor its optical properties. Here, we applythe density matrix theory to investigate the potential of non-covalently functionalized TMDs. Considering ex-emplary spiropyran molecules with a strong dipole moment, we predict spectral redshifts and the appearance ofan additional side peak in the absorption spectrum of functionalized TMDs. We show that the molecular char-acteristics, e.g. coverage, orientation and dipole moment, crucially influence the optical properties of TMDs,leaving a unique optical fingerprint in the absorption spectrum. Furthermore, we find that the molecular dipolemoments open a channel for coherent intervalley coupling between the high-symmetry K and K’ points whichmay open new possibilities for spin-valleytronics application.
Inhalt ausklappen Inhalt einklappen 81. S. Winnerl, M. Mittendorff, J. König-Otto, H. Schneider, M. Helm, T. Winzer, A. Knorr and E. Malic, "Ultrafast processes in graphene: from fundamental manybody interactions to device applications", Annalen der Physik 1700022 (2017) 81. S. Winnerl, M. Mittendorff, J. König-Otto, H. Schneider, M. Helm, T. Winzer, A. Knorr and E. Malic, "Ultrafast processes in graphene: from fundamental manybody interactions to device applications", Annalen der Physik 1700022 (2017)
A joint experiment‐theory investigation of the carrier dynamics in graphene, in particular in the energetic vicinity of the Dirac point, is reviewed. Radiation of low photon energy is employed in order to match the intrinsic energy scales of the material, i.e. the optical phonon energy (∼200 meV) and the Fermi energy (10‐20 meV), respectively. Significant slower carrier cooling is predicted and observed for photon energies below the optical phonon energy. Furthermore, a strongly anisotropic distribution of electrons in k‐space upon excitation with linearly polarized radiation is discussed. Depending on photon energy, the anisotropic distribution decays either rapidly via optical phonon emission, or slowly via non‐collinear Coulomb scattering. Finally, a room temperature operated ultra‐broadband hot‐electron bolometer is demonstrated. It covers the spectral range from the THz to visible region with a single detector element featuring a response time of 40 ps.
Annalen der Physik 1700022 (2017)Inhalt ausklappen Inhalt einklappen 80. E. Malic, G. Berghaeuser, M. Feierabend and A. Knorr, "Optical response from functionalized atomically thin nanomaterials", Annalen der Physik 1700097 (2017)80. E. Malic, G. Berghaeuser, M. Feierabend and A. Knorr, "Optical response from functionalized atomically thin nanomaterials", Annalen der Physik 1700097 (2017)
Chemical functionalization of atomically thin nanostructures presents a promising strategy to create new hybrid nanomaterials with remarkable and externally controllable properties. Here, we review our research in the field of theoretical modeling of carbon nanotubes, graphene, and transition metal dichalcogenides located in molecular dipole fields. In particular, we provide a microscopic view on the change of the optical response of these technologically promising nanomaterials due to the presence of photo‐active spiropyran molecules. The feature article presents a review of recent theoretical work providing microscopic view on the optical response of chemically functionalized carbon nanotubes, graphene, and monolayered transition metal dichalcogenides. In particular, we propose a novel sensor mechanism based on the molecule‐induced activation of dark excitons. This results in a pronounced additional peak presenting an unambiguous optical fingerprint for the attached molecules.
Annalen der Physik 1700097 (2017)Inhalt ausklappen Inhalt einklappen 79. E. Malic, T. Winzer, F. Wendler, S. Brem, R. Jago, A. Knorr, M. Mittendorff, J. König-Otto, T. Plötzing, D. Neumaier, H. Schneider, M. Helm and S. Winnerl, "Carrier dynamics in graphene: ultrafast many-particle phenomena", Annalen der Physik, 1700038 (2017)79. E. Malic, T. Winzer, F. Wendler, S. Brem, R. Jago, A. Knorr, M. Mittendorff, J. König-Otto, T. Plötzing, D. Neumaier, H. Schneider, M. Helm and S. Winnerl, "Carrier dynamics in graphene: ultrafast many-particle phenomena", Annalen der Physik, 1700038 (2017)
Graphene is an ideal material to study fundamental Coulomb‐ and phonon‐induced carrier scattering processes. Its remarkable gapless and linear band structure opens up new carrier relaxation channels. In particular, Auger scattering bridging the valence and the conduction band changes the number of charge carriers and gives rise to a significant carrier multiplication ‐ an ultrafast many‐particle phenomenon that is promising for the design of highly efficient photodetectors. Furthermore, the vanishing density of states at the Dirac point combined with ultrafast phonon‐induced intraband scattering results in an accumulation of carriers and a population inversion suggesting the design of graphene‐based terahertz lasers. Here, we review our work on the ultrafast carrier dynamics in graphene and Landau‐quantized graphene is presented providing a microscopic view on the appearance of carrier multiplication and population inversion.
Annalen der Physik, 1700038 (2017)Inhalt ausklappen Inhalt einklappen 78. R. Jago, F. Wendler, E. Malic, "Current enhancement due to field-induced dark carrier multiplication in graphene", 2D Mater. 4, 021031 (2017)78. R. Jago, F. Wendler, E. Malic, "Current enhancement due to field-induced dark carrier multiplication in graphene", 2D Mater. 4, 021031 (2017)
We present a microscopic study on current generation in graphene in response to an electric field. While scattering is generally considered to reduce the current, we reveal that in graphene Auger processes give rise to a current enhancement via a phenomenon we denote dark carrier multiplication. Based on a microscopic approach, we show that, if other scattering channels are absent, this prevents the carrier distribution to reach a stationary value. Taking into account scattering with phonons a finite current is restored, however its value exceeds the stationary current without scattering.
2D Mater. 4, 021031 (2017)Inhalt ausklappen Inhalt einklappen 77. G. Berghäuser, A. Knorr and E. Malic, "Optical fingerprint of dark 2p-states in transition metal dichalcogenides", 2D Mater. 4, 015029 (2017)77. G. Berghäuser, A. Knorr and E. Malic, "Optical fingerprint of dark 2p-states in transition metal dichalcogenides", 2D Mater. 4, 015029 (2017)
Atomically thin transition metal dichalcogenides exhibit a remarkably strong Coulomb interaction. This results in a fascinating many-particle physics including a variety of bright and dark excitonic states that determine optical and electronic properties of these materials. So far, the impact of dark states has remained literally in the dark to a large extent, since a measurement of these optically forbidden states is very challenging. Here we demonstrate a strategy to measure a direct fingerprint of dark states even in standard linear absorption spectroscopy. We present a microscopic study on bright and dark higher excitonic states in the presence of disorder for the exemplary material of tungsten disulfide (WS2). We show that the geometric phase cancels the degeneration of 2s and 2p states and that a significant disorder-induced coupling of these bright and dark states offers a strategy to circumvent optical selection rules. As a proof, we show a clear fingerprint of dark 2p states in the absorption spectrum of WS2. The predicted softening of optical selection rules through exciton-disorder coupling is of general nature and therefore applicable to related two-dimensional semiconductors.
2D Mater. 4, 015029 (2017)