@article{4044, abstract = {{A simulation environment for metallic nanostructures based on the Discontinuous Galerkin Time Domain method is presented. The model is used to compute the linear and nonlinear optical response of split ring resonators and to study physical mechanisms that contribute to second harmonic generation.}}, author = {{Grynko, Yevgen and Förstner, Jens and Meier, Torsten}}, issn = {{1825-1242}}, journal = {{AAPP | Atti della Accademia Peloritana dei Pericolanti}}, keywords = {{tet_topic_numerics, tet_topic_shg, tet_topic_meta}}, number = {{1}}, title = {{{Application of the discontinous Galerkin time domain method to the optics of metallic nanostructures}}}, doi = {{10.1478/C1V89S1P041}}, volume = {{89}}, year = {{2011}}, } @article{4091, abstract = {{We present a nonequilibrium ab initio method for calculating nonlinear and nonlocal optical effects in metallic slabs with a thickness of several nanometers. The numerical analysis is based on the full solution of the time‐dependent Kohn–Sham equations for a jellium system and allows to study the optical response of metal electrons subject to arbitrarily shaped intense light pulses. We find a strong localization of the generated second‐harmonic current in the surface regions of the slabs. }}, author = {{Wand, Mathias and Schindlmayr, Arno and Meier, Torsten and Förstner, Jens}}, issn = {{1521-3951}}, journal = {{Physica Status Solidi B}}, keywords = {{tet_topic_shg}}, number = {{4}}, pages = {{887--891}}, publisher = {{Wiley-VCH}}, title = {{{Simulation of the ultrafast nonlinear optical response of metal slabs}}}, doi = {{10.1002/pssb.201001219}}, volume = {{248}}, year = {{2011}}, } @article{4543, abstract = {{The vibrational properties of the LiNbO3 (0001) surfaces have been investigated both from first principles and with Raman spectroscopy measurements. Firstly, the phonon modes of bulk and of the (0001) surface are calculated by means of the density functional theory. Our calculations reveal the existence of localised vibrational modes both at the positive and at the negative surface. The surface vibrations are found at energies above and within the bulk bands. Phonon modes localised at the positive and at the negative surface differ substantially. In a second step, the Raman spectra of LiNbO3 bulk and of the two surfaces have been measured. Raman spectroscopy is shown to be sensitive to differences between bulk and surface and between positive and negative surface. The calculated and measured frequencies are in agreement within the error of the method.}}, author = {{Sanna, S. and Berth, Gerhard and Hahn, W. and Widhalm, A. and Zrenner, Artur and Schmidt, Wolf Gero}}, issn = {{0015-0193}}, journal = {{Ferroelectrics}}, number = {{1}}, pages = {{1--8}}, publisher = {{Informa UK Limited}}, title = {{{Localised Phonon Modes at LiNbO3(0001) Surfaces}}}, doi = {{10.1080/00150193.2011.594396}}, volume = {{419}}, year = {{2011}}, } @inproceedings{4122, abstract = {{We experimentally and theoretically investigate injection currents generated by femtosecond single-color circularly-polarized laser pulses in (110)-oriented GaAs quantum wells. The current measurements are performed by detecting the emitted Terahertz radiation at room temperature. The microscopic theory is based on a 14 x 14 k • p band-structure calculation in combination with the multi-subband semiconductor Bloch equations. For symmetric GaAs quantum wells grown in (110) direction, an oscillatory dependence of the injection currents on the exciting photon energy is obtained. The results of the microscopic theory are in good agreement with the measurements. }}, author = {{Duc, H. T. and Pochwala, M. and Förstner, Jens and Meier, Torsten and Priyadarshi, S. and Racu, A. M. and Pierz, K. and Siegner, U. and Bieler, M.}}, booktitle = {{Ultrafast Phenomena in Semiconductors and Nanostructure Materials XV}}, editor = {{Tsen, Kong-Thon and Song, Jin-Joo and Betz, Markus and Elezzabi, Abdulhakem Y.}}, keywords = {{tet_topic_qw}}, publisher = {{SPIE}}, title = {{{Injection currents in (110)-oriented GaAs/AlGaAs quantum wells: recent progress in theory and experiment}}}, doi = {{10.1117/12.876972}}, volume = {{7937}}, year = {{2011}}, } @article{4046, abstract = {{We demonstrate by spin quantum beat spectroscopy that in undoped symmetric (110)-oriented GaAs/AlGaAs single quantum wells, even a symmetric spatial envelope wave function gives rise to an asymmetric in-plane electron Land´e g-factor. The anisotropy is neither a direct consequence of the asymmetric in-plane Dresselhaus splitting nor a direct consequence of the asymmetric Zeeman splitting of the hole bands, but rather it is a pure higher-order effect that exists as well for diamond-type lattices. The measurements for various well widths are very well described within 14 × 14 band k·p theory and illustrate that the electron spin is an excellent meter variable for mapping out the internal—otherwise hidden—symmetries in two-dimensional systems. Fourth-order perturbation theory yields an analytical expression for the strength of the g-factor anisotropy, providing a qualitative understanding of the observed effects.}}, author = {{Hübner, J. and Kunz, S. and Oertel, S. and Schuh, D. and Pochwała, M. and Duc, H. T. and Förstner, Jens and Meier, Torsten and Oestreich, M.}}, issn = {{1098-0121}}, journal = {{Physical Review B}}, keywords = {{tet_topic_qw}}, number = {{4}}, pages = {{041301(R)}}, publisher = {{American Physical Society (APS)}}, title = {{{Electron g-factor anisotropy in symmetric (110)-oriented GaAs quantum wells}}}, doi = {{10.1103/physrevb.84.041301}}, volume = {{84}}, year = {{2011}}, } @article{43202, abstract = {{For numerous applications, the computation and provision of exact derivative information plays an important role for optimizing the considered system. This paper introduces the technique of algorithmic differentiation, a method to compute derivatives of arbitrary order within working precision. This derivative information will be combined with a calculus-based optimization algorithm to optimize a non-trivially shaped laser pulse which coherently steers the electron dynamics in a semiconductor quantum wire. Numerical results illustrating the cost for the derivative computation and the optimization process are presented and discussed.}}, author = {{Meier, Torsten and Reichelt, Matthias and Walther, A.}}, journal = {{Photonics and Nanostructures - Fundamentals and Applications}}, number = {{4}}, pages = {{328--336}}, publisher = {{Elsevier}}, title = {{{Calculus-based optimization of the electron dynamics in nanostructures}}}, doi = {{10.1016/j.photonics.2011.03.006}}, volume = {{9}}, year = {{2011}}, } @inbook{43260, author = {{Güdde, J. and Rohleder, M. and Meier, Torsten and Koch, S.W. and Höfer, U.}}, booktitle = {{Dynamics at Solid State Surfaces and Interfaces}}, editor = {{Bovensiepen, U. and Petek, H. and Wolf, M.}}, pages = {{579--591}}, publisher = {{Wiley‐VCH Verlag}}, title = {{{Coherently controlled electrical currents at surfaces}}}, doi = {{10.1002/9783527633418.ch24}}, volume = {{1}}, year = {{2010}}, } @article{4127, abstract = {{The dynamics of charge and spin injection currents excited by circularly polarized, one-color laser beams in semiconductor quantum wells is analyzed. Our microscopic approach is based on a 14x14 k · p band-structure theory in combination with multisubband semiconductor Bloch equations which allows a detailed analysis of the photogenerated carrier distributions and coherences in k space. Charge and spin injection currents are numerically calculated for [110]- and [001]-grown GaAs quantum wells including dc population contributions and ac contributions that arise from intersubband coherences. The dependencies of the injection currents on the excitation conditions, in particular, the photon energy are computed and discussed.}}, author = {{Duc, Huynh Thanh and Förstner, Jens and Meier, Torsten}}, issn = {{1098-0121}}, journal = {{Physical Review B}}, keywords = {{tet_topic_qw}}, number = {{11}}, publisher = {{American Physical Society (APS)}}, title = {{{Microscopic analysis of charge and spin photocurrents injected by circularly polarized one-color laser pulses in GaAs quantum wells}}}, doi = {{10.1103/physrevb.82.115316}}, volume = {{82}}, year = {{2010}}, } @inproceedings{43261, abstract = {{We report the development of an experimental technique to measure the dynamics of electrical currents on the femtosecond timescale. The technique combines methods of coherent control with time- and angle-resolved photoelectron spectroscopy. Direct snapshots of the momentum distribution of the excited electrons as function of time are then determined by photoelectron spectroscopy. In this way we gain information on the generation and decay of ultrashort current pulses in unprecedented detail. In particular, this technique allows the observation of elastic electron scattering in terms of an incoherent population dynamics in momentum space. We have applied this optical current generation and detection scheme to electrons in so-called image-potential states which represent a prototype of two-dimensional electronic surface states. Electrons in these states are bound perpendicular to the metal surface by the Coulombic image potential whereas they can move almost freely parallel to the surface. For the (n=1) image-potential state of Cu(100) we find a decay time of 10 fs due to electron scattering with steps and surface defects.}}, author = {{Güdde, J. and Rohleder, M. and Meier, Torsten and Koch, S.W. and Höfer, U.}}, booktitle = {{Ultrafast Phenomena in Semiconductors and Nanostructure Materials XIV}}, editor = {{Song, Jin-Joo and Tsen, Kong-Thon and Betz, Markus and Y. Elezzabi, Abdulhakem}}, publisher = {{SPIE}}, title = {{{Ultrafast coherent control of electric currents at metal surfaces}}}, doi = {{10.1117/12.839672}}, volume = {{7600}}, year = {{2010}}, } @inproceedings{4176, abstract = {{A microscopic theory that describes injection currents in GaAs quantum wells is presented. 14 × 14 band k.p theory is used to compute the band structure including anisotropy and spin-orbit interaction. Transient injection currents are obtained via numerical solutions of the semiconductor Bloch equations. Depending on the growth direction of the considered quantum well system and the propagation and polarization directions of the incident light beam, it is possible to generate charge and/or spin photocurrents on ultrashort time scales. The dependence of the photocurrents on the excitation conditions is computed and discussed.}}, author = {{Duc, Huynh Thanh and Förstner, Jens and Meier, Torsten}}, booktitle = {{Ultrafast Phenomena in Semiconductors and Nanostructure Materials XIV}}, editor = {{Song, Jin-Joo and Tsen, Kong-Thon and Betz, Markus and Elezzabi, Abdulhakem Y.}}, keywords = {{tet_topic_qw}}, pages = {{76000S--76000S--9}}, publisher = {{SPIE}}, title = {{{Microscopic theoretical analysis of optically generated injection currents in semiconductor quantum wells}}}, doi = {{10.1117/12.840388}}, volume = {{7600}}, year = {{2010}}, } @article{44063, abstract = {{We present an analysis of the coupling between photonic crystal cavities in different geometries. Inline‐, side‐ and angle‐coupled L3‐geometries are investigated numerically in three dimensions. Asymmetric mode splitting of the fundamental mode for the L3‐cavity is shown for all geometrical setups evidencing for strong cavity‐cavity interactions. The coupling efficiency for the fundamental mode is shown to be best for a 30° angle between the cavity‐centers due to the direction of in‐plane leackage out of the cavites.}}, author = {{Meier, Torsten and Declair, S. and Förstner, J.}}, journal = {{AIP Conference Proceedings}}, number = {{46 }}, publisher = {{American Institute of Physics}}, title = {{{Numerical Analysis of Coupled Photonic Crystal Cavities}}}, doi = {{10.1063/1.3506125}}, volume = {{1291}}, year = {{2010}}, } @article{24980, abstract = {{We discuss transport and localization properties on the insulating side of the disorder dominated superconductor-insulator transition, described in terms of the dirty boson model. Analyzing the spectral properties of the interacting bosons in the absence of phonons, we argue that the Bose glass phase admits three distinct regimes. For strongest disorder the boson system is a fully localized, perfect insulator at any temperature. At smaller disorder, only the low temperature phase exhibits perfect insulation while delocalization takes place above a finite temperature. We argue that a third phase must intervene between these perfect insulators and the superconductor. This conducting Bose glass phase is characterized by a mobility edge in the many body spectrum, located at finite energy above the ground state. In this insulating regime purely electronically activated transport occurs, with a conductivity following an Arrhenius law at asymptotically low temperatures, while a tendency to superactivation is predicted at higher T. These predictions are in good agreement with recent transport experiments in highly disordered films of superconducting materials.}}, author = {{Gögh, N. and Thomas, P. and Kuznetsova, I. and Meier, Torsten and Varga, I.}}, issn = {{0003-3804}}, journal = {{Annalen der Physik}}, number = {{12}}, pages = {{905--909}}, title = {{{Localization of excitons in weakly disordered semiconductor structures: A model study}}}, doi = {{10.1002/andp.20095211219}}, volume = {{18}}, year = {{2010}}, } @article{23480, abstract = {{We discuss transport and localization properties on the insulating side of the disorder dominated superconductor-insulator transition, described in terms of the dirty boson model. Analyzing the spectral properties of the interacting bosons in the absence of phonons, we argue that the Bose glass phase admits three distinct regimes. For strongest disorder the boson system is a fully localized, perfect insulator at any temperature. At smaller disorder, only the low temperature phase exhibits perfect insulation while delocalization takes place above a finite temperature. We argue that a third phase must intervene between these perfect insulators and the superconductor. This conducting Bose glass phase is characterized by a mobility edge in the many body spectrum, located at finite energy above the ground state. In this insulating regime purely electronically activated transport occurs, with a conductivity following an Arrhenius law at asymptotically low temperatures, while a tendency to superactivation is predicted at higher T. These predictions are in good agreement with recent transport experiments in highly disordered films of superconducting materials.}}, author = {{Gögh, N. and Thomas, P. and Kuznetsova, I. and Meier, Torsten and Varga, I.}}, issn = {{0003-3804}}, journal = {{Annalen der Physik}}, number = {{12}}, pages = {{905--909}}, title = {{{Localization of excitons in weakly disordered semiconductor structures: A model study}}}, doi = {{10.1002/andp.200910382}}, year = {{2010}}, } @article{18558, abstract = {{We present an implementation of the GW approximation for the electronic self-energy within the full-potential linearized augmented-plane-wave (FLAPW) method. The algorithm uses an all-electron mixed product basis for the representation of response matrices and related quantities. This basis is derived from the FLAPW basis and is exact for wave-function products. The correlation part of the self-energy is calculated on the imaginary-frequency axis with a subsequent analytic continuation to the real axis. As an alternative we can perform the frequency convolution of the Green function G and the dynamically screened Coulomb interaction W explicitly by a contour integration. The singularity of the bare and screened interaction potentials gives rise to a numerically important self-energy contribution, which we treat analytically to achieve good convergence with respect to the k-point sampling. As numerical realizations of the GW approximation typically suffer from the high computational expense required for the evaluation of the nonlocal and frequency-dependent self-energy, we demonstrate how the algorithm can be made very efficient by exploiting spatial and time-reversal symmetry as well as by applying an optimization of the mixed product basis that retains only the numerically important contributions of the electron-electron interaction. This optimization step reduces the basis size without compromising the accuracy and accelerates the code considerably. Furthermore, we demonstrate that one can employ an extrapolar approximation for high-lying states to reduce the number of empty states that must be taken into account explicitly in the construction of the polarization function and the self-energy. We show convergence tests, CPU timings, and results for prototype semiconductors and insulators as well as ferromagnetic nickel.}}, author = {{Friedrich, Christoph and Blügel, Stefan and Schindlmayr, Arno}}, issn = {{1550-235X}}, journal = {{Physical Review B}}, number = {{12}}, publisher = {{American Physical Society}}, title = {{{Efficient implementation of the GW approximation within the all-electron FLAPW method}}}, doi = {{10.1103/PhysRevB.81.125102}}, volume = {{81}}, year = {{2010}}, } @inproceedings{44064, abstract = {{We compute photocurrents generated by femtosecond single-color laser pulses in non-centrosymmetric semiconductor quantum wells by combining a 14 x 14 k.p band structure theory with multi-band semiconductor Bloch equations. The transient photocurrents are investigated experimentally by measuring the associated Terahertz emission. The dependencies of the photocurrent and the Terahertz emission on the excitation conditions are discussed for (110)-oriented GaAs quantum wells. The comparison between theory and experiment shows a good agreement.}}, author = {{Meier, Torsten and Duc, Huynh Thanh and Foerstner, Jens and Priyadarshi, S. and Racu, Ana Maria and Pierz, Klaus and Siegner, Uwe and Bieler, Mark}}, booktitle = {{DPG Spring meeting 2010 }}, issn = {{0420-0195}}, location = {{Regensburg, Germany}}, number = {{3}}, title = {{{Experimental and theoretical investigations of photocurrents in non-centrosymmetric semiconductor quantum wells}}}, volume = {{45}}, year = {{2010}}, } @inproceedings{44067, abstract = {{Semiconductor nanostructures always contain a certain degree of disorder due to interface roughness and/or alloy disorder. The disorder has significant influence on the optical properties, e.g., excitonic absorption spectra. An adaptive wavelet approach for the solution of the excitonic Schroedinger equation, i.e., the semiconductor Bloch equation for the interband coherence linear in the external field, has been developed and applied to compute absorption spectra and/or wave functions. Results obtained for a thin GaAs semiconductor quantum wire considering a number of different model disorder potentials are presented and discussed.}}, author = {{Meier, Torsten and Mollet, Christian and Kunoth, Angela}}, booktitle = {{DPG Spring meeting 2010}}, issn = {{ 0420-0195}}, location = {{Regensburg, Germany}}, number = {{3}}, title = {{{A numerical adaptive wavelet approach to excitonic absorption spectra of disordered semiconductor nanostructures}}}, volume = {{45}}, year = {{2010}}, } @inproceedings{44065, abstract = {{The behavior of waveguide plasmon polaritons is studied employing ultrafast coherent control like schemes for a gold lattice coupled to a photonic waveguide for the structure. Different models to describe the third-harmonic generation are presented and the resulting equations are solved numerically. The calculations are compared to recent experimental data and show good agreement for the most prominent features in the time-integrated third order intensity.}}, author = {{Meier, Torsten and Podzimski, Reinold and Reichelt, Matthias and Utikal, Tobias and Giessen, Harald}}, booktitle = {{DPG Spring meeting 2010}}, issn = {{0420-0195}}, location = {{Regensburg, Germany}}, number = {{3}}, title = {{{Controlling the third-harmonic generation in a metallic photonic crystal coupled to a waveguide}}}, volume = {{45}}, year = {{2010}}, } @inproceedings{44066, abstract = {{We present a joint theoretical study of the optical properties of GaAs and AlAs bulk and heterostructures. Thereby, we compare the theoretical description of optical excitation on several levels of theory. Results obtained from kp theory are discussed along with data from ab initio calculations within the frameworks of independent-particle (DFT), independent quasiparticle (GW), or Coulomb-correlated quasiparticle (BSE) approximation.}}, author = {{Meier, Torsten and Landmann, Marc and Pochwala, Michal and Foerstner, Jens and Rauls, Eva and Schmidt, Wolf Gero}}, booktitle = {{DPG Spring meeting 2010}}, issn = {{0420-0195}}, location = {{Regensburg, Germany}}, number = {{3}}, title = {{{Theoretical description of optical properties in III-V semiconductor nanostructures}}}, volume = {{45}}, year = {{2010}}, } @article{13581, author = {{Wippermann, S. and Schmidt, Wolf Gero and Bechstedt, F. and Chandola, S. and Hinrichs, K. and Gensch, M. and Esser, N. and Fleischer, K. and McGilp, J. F.}}, issn = {{1862-6351}}, journal = {{physica status solidi (c)}}, number = {{2}}, pages = {{133--136}}, title = {{{Optical anisotropy of Si(111)-(4 × 1)/(8 × 2)-In nanowires calculated fromfirst-principles}}}, doi = {{10.1002/pssc.200982413}}, volume = {{7}}, year = {{2010}}, } @article{13573, abstract = {{Given the vast range of lithium niobate (LiNbO3) applications, the knowledge about its electronic and optical properties is surprisingly limited. The direct band gap of 3.7 eV for the ferroelectric phase – frequently cited in the literature – is concluded from optical experiments. Recent theoretical investigations show that the electronic band‐structure and optical properties are very sensitive to quasiparticle and electron‐hole attraction effects, which were included using the GW approximation for the electron self‐energy and the Bethe‐Salpeter equation respectively, both based on a model screening function. The calculated fundamental gap was found to be at least 1 eV larger than the experimental value. To resolve this discrepancy we performed first‐principles GW calculations for lithium niobate using the full‐potential linearized augmented plane‐wave (FLAPW) method. Thereby we use the parameter‐free random phase approximation for a realistic description of the nonlocal and energydependent screening. This leads to a band gap of about 4.7 (4.2) eV for ferro(para)‐electric lithium niobate.}}, author = {{Thierfelder, Christian and Sanna, Simone and Schindlmayr, Arno and Schmidt, Wolf Gero}}, issn = {{1610-1642}}, journal = {{Physica Status Solidi C}}, location = {{Weimar}}, number = {{2}}, pages = {{362--365}}, publisher = {{Wiley-VCH}}, title = {{{Do we know the band gap of lithium niobate?}}}, doi = {{10.1002/pssc.200982473}}, volume = {{7}}, year = {{2010}}, }