@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{4177, abstract = {{Excitonic spectra of weakly disordered semiconductor heterostructures are simulated on the basis of a one-dimensional tight-binding model. The influence of the length scale of weak disorder in quantum wells on the redshift of the excitonic peak and its linewidth is studied. By calculating two-dimensional Fouriertransform spectra we are able to determine the contribution of disorder to inhomogeneous and also to homogeneous broadenings separately. This disorder-induced dephasing is related to a Fano-type coupling and leads to contributions to the homogeneous linewidth that depends on energy within the inhomogeneously broadened line. The model includes heavy- and light-hole excitons and yields smaller inhomogeneous broadening for the light-hole exciton if compared to the heavy-hole exciton, which agrees qualitatively with the experiment.}}, author = {{Kuznetsova, I. and Gőgh, N. and Förstner, Jens and Meier, Torsten and Cundiff, S. T. and Varga, I. and Thomas, P.}}, issn = {{1098-0121}}, journal = {{Physical Review B}}, keywords = {{tet_topic_qw}}, number = {{7}}, publisher = {{American Physical Society (APS)}}, title = {{{Modeling excitonic line shapes in weakly disordered semiconductor nanostructures}}}, doi = {{10.1103/physrevb.81.075307}}, volume = {{81}}, year = {{2010}}, } @article{4125, abstract = {{We numerically investigate the behavior of Whispering Gallery Modes (WGMs) in circularly shaped resonators like microdisks, with diameters in the range of optical vacuum wavelengths. The microdisk is embedded in an uniaxial anisotropic dielectric environment. By changing the optical anisotropy, one obtains spectral tunability of the optical modes. The degree of tunability strongly depends on the radial (azimuthal) mode order M (N). As the modes approach each other spectrally, anticrossing is observed, leading to a rearrangement of the optical states.}}, author = {{Declair, S. and Meier, Cedrik and Meier, Torsten and Förstner, Jens}}, issn = {{1569-4410}}, journal = {{Photonics and Nanostructures - Fundamentals and Applications}}, keywords = {{tet_topic_microdisk}}, number = {{4}}, pages = {{273--277}}, publisher = {{Elsevier BV}}, title = {{{Anticrossing of Whispering Gallery Modes in microdisk resonators embedded in an anisotropic environment}}}, doi = {{10.1016/j.photonics.2010.03.002}}, volume = {{8}}, year = {{2010}}, } @article{4123, abstract = {{GaAs-based semiconductor microdisks with high quality whispering gallery modes (Q44000) have been fabricated.A layer of self-organized InAs quantumdots (QDs) served as a light source to feed the optical modes at room temperature. In order to achieve frequency tuning of the optical modes, the microdisk devices have been immersed in 4 – cyano – 4´-pentylbiphenyl (5CB), a liquid crystal(LC) with a nematic phase below the clearing temperature of TC≈34°C .We have studied the device performance in the temperature rangeof T=20-50°C, in order to investigate the influence of the nematic–isotropic phase transition on the optical modes. Moreover,we havea pplied an AC electric field to the device,which leads in the nematic phase to a reorientation of the anisotropic dielectric tensor of the liquid crystal.This electrical anisotropy can be used to achieve electrical tunability of the optical modes.Using the finite-difference time domain (FDTD) technique with an anisotropic material model, we are able to describe the influence of the liquid crystal qualitatively.}}, author = {{Piegdon, Karoline A. and Offer, Matthias and Lorke, Axel and Urbanski, Martin and Hoischen, Andreas and Kitzerow, Heinz-Siegfried and Declair, Stefan and Förstner, Jens and Meier, Torsten and Reuter, Dirk and Wieck, Andreas D. and Meier, Cedrik}}, issn = {{1386-9477}}, journal = {{Physica E: Low-dimensional Systems and Nanostructures}}, keywords = {{tet_topic_qd, tet_topic_microdisk}}, number = {{10}}, pages = {{2552--2555}}, publisher = {{Elsevier BV}}, title = {{{Self-assembled quantum dots in a liquid-crystal-tunable microdisk resonator}}}, doi = {{10.1016/j.physe.2009.12.051}}, volume = {{42}}, year = {{2010}}, } @article{4169, abstract = {{It is demonstrated that valence-band mixing in GaAs quantum wells tremendously modifies electronic transport. A coherent control scheme in which ultrafast currents are optically injected into undoped GaAs quantum wells upon excitation with femtosecond laser pulses is employed. An oscillatory dependence of the injection current amplitude and direction on the excitation photon energy is observed. A microscopic theoretical analysis shows that this current reversal is caused by the coupling of the light- and heavy-hole bands and that the hole currents dominate the overall current response. These surprising consequences of band mixing illuminate fundamental physics as they are unique for experiments which are able to monitor electronic transport resulting from carriers with relatively large momenta.}}, author = {{Priyadarshi, S. and Racu, A. M. and Pierz, K. and Siegner, U. and Bieler, M. and Duc, H. T. and Förstner, Jens and Meier, Torsten}}, issn = {{0031-9007}}, journal = {{Physical Review Letters}}, keywords = {{tet_topic_qw}}, number = {{21}}, publisher = {{American Physical Society (APS)}}, title = {{{Reversal of Coherently Controlled Ultrafast Photocurrents by Band Mixing in Undoped GaAs Quantum Wells}}}, doi = {{10.1103/physrevlett.104.217401}}, volume = {{104}}, year = {{2010}}, } @inproceedings{4167, abstract = {{The electromagnetic field in the vicinity of sharp edges needs a special treatment in numeric calculation whenever accurate, fast converging results are necessary. One of the fundamental works concerning field singularities has been proposed in 1972 [1] and states that the electromagnetic energy density must be integrable over any finite domain, even if this domain contains singularities. It is shown, that the magnetic field H(, ϕ) and electric field E(, ϕ) are proportional to ∝ (t−1) for  → 0. The variable  is the distance to the edge and t has to fulfill the integrability condition and thus is restricted to 0 < t < 1. This result is often used to reduce the error corresponding to the singularity without increasing the numerical effort [2 - 5]. For this purpose, a correction factor K is estimated by inserting the proportionality into the wave equation. It is shown, that this method improves the accuracy of the result significantly, however the order of convergence is often not studied. In [4] a method to modify the material parameters in order to use analytic results to improve the numeric calculation is presented. In this contribution we will - inspired by the scheme given in [4] - develop a new method to estimate a correction factor for perfect conducting materials (PEC) and demonstrate the improvement of the results compared to the standard edge correction. Therefore analytic results (comparable to [1]) are consequently merged with the scheme in [4]. The main goal of this work is the calculation of the second harmonic generation (SHG) in the wave response of so-called metamaterials [6]. Frequently these structures contain sharp metallic edges with field singularities at the interfaces which have a strong impact on the SHG signals. Thus, an accurate simulation of singularities is highly important. However, the following approach can also be applied to many other setups, and one of them is shown in the example below.}}, author = {{Classen, C and Förstner, Jens and Meier, Torsten and Schuhmann, R}}, booktitle = {{2010 IEEE Antennas and Propagation Society International Symposium}}, isbn = {{9781424449675}}, keywords = {{tet_topic_numerics}}, location = {{Toronto, ON, Canada}}, publisher = {{IEEE}}, title = {{{Enhanced FDTD edge correction for nonlinear effects calculation}}}, doi = {{10.1109/aps.2010.5562017}}, year = {{2010}}, } @article{4172, abstract = {{Microdisks made from GaAs with embedded InAs quantum dots are immersed in the liquid crystal 4-cyano-4’-pentylbiphenyl (5CB). The quantum dots serve as emitters feeding the optical modes of the photonic cavity. By changing temperature, the liquid crystal undergoes a phase transition from the isotropic to the nematic state, which can be used as an effective tuning mechanism of the photonic modes of the cavity. In the nematic state, the uniaxial electrical anisotropy of the liquid crystal molecules can be exploited for orienting the material in an electric field, thus externally controlling the birefringence of the material. Using this effect, an electric field induced tuning of the modes is achieved. Numerical simulations using the finite-differences time-domain (FDTD) technique employing an anisotropic dielectric medium allow to understand the alignment of the liquid crystal molecules on the surface of the microdisk resonator.}}, author = {{Piegdon, Karoline A. and Declair, Stefan and Förstner, Jens and Meier, Torsten and Matthias, Heiner and Urbanski, Martin and Kitzerow, Heinz-Siegfried and Reuter, Dirk and Wieck, Andreas D. and Lorke, Axel and Meier, Cedrik}}, issn = {{1094-4087}}, journal = {{Optics Express}}, keywords = {{tet_topic_qd, tet_topic_microdisk}}, number = {{8}}, publisher = {{The Optical Society}}, title = {{{Tuning quantum-dot based photonic devices with liquid crystals}}}, doi = {{10.1364/oe.18.007946}}, volume = {{18}}, year = {{2010}}, } @inproceedings{4180, abstract = {{We show that an optically driven carrier spin undergoes indirect dephasing even in the absence of spin-reservoir coupling and illustrate it for phonon-induced decoherence during optical spin rotation in a single quantum dot.}}, author = {{Grodecka, Anna and Machnikowski, Pawel and Förstner, Jens}}, booktitle = {{Advances in Optical Sciences Congress}}, isbn = {{9781557528735}}, keywords = {{tet_topic_qd}}, publisher = {{OSA Technical Digest (CD) (Optical Society of America, 2009)}}, title = {{{Indirect Dephasing Channel for Optically Controlled Spin in a Single Quantum Dot}}}, doi = {{10.1364/nlo.2009.nma1}}, year = {{2009}}, } @article{4182, abstract = {{We demonstrate that an optically driven spin of a carrier in a quantum dot undergoes indirect dephasing via conditional optically induced charge evolution even in the absence of any direct interaction between the spin and its environment. A generic model for the indirect dephasing with a three-component system with spin, charge, and reservoir is proposed. This indirect decoherence channel is studied for the optical spin manipulation in a quantum dot with a microscopic description of the charge-phonon interaction taking into account its non-Markovian nature.}}, author = {{Grodecka, A. and Machnikowski, P. and Förstner, Jens}}, issn = {{1050-2947}}, journal = {{Physical Review A}}, keywords = {{tet_topic_qd}}, number = {{4}}, publisher = {{American Physical Society (APS)}}, title = {{{Indirect spin dephasing via charge-state decoherence in optical control schemes in quantum dots}}}, doi = {{10.1103/physreva.79.042331}}, volume = {{79}}, year = {{2009}}, } @inproceedings{4179, abstract = {{We numerically investigate Whispering Gallery Modes (WGM) in a subwavelength microdisk resonator [1] embedded in an uniaxial anisotropic liquid crystal environment. It is shown that the WGMs have anticrossing behavior when modes of different radial mode order M or azimuthal order N approach each other spectrally. }}, author = {{Förstner, Jens and Declair, S. and Meier, Cedrik and Meier, Torsten}}, booktitle = {{Theoretical and Computational Nanophotonics Tacona-Photonics}}, keywords = {{tet_topic_microdisk}}, location = {{Tacona}}, number = {{1}}, pages = {{60--62}}, title = {{{Anticrossing of Whispering Gallery Modes in Microdisk Resonators Embedded in a Liquid Crystal}}}, doi = {{10.1063/1.3253921}}, volume = {{1176}}, year = {{2009}}, } @inproceedings{4184, abstract = {{We have experimentally investigated injection currents generated by all-optical excitation of GaAs/AlGaAs quantum wells excited with 130 fs optical pulses. The currents have been detected via free-space THz experiments at room temperature. Our experiments prove that Coulomb effects strongly influence injection currents. This becomes most prominently visible when exciting light-hole exciton transitions. At this photon energy we observe a pronounced phase shift of the current transients which is due to oppositely oriented heavy-hole and light-hole type contributions. We are currently developing a microscopic theory based on a 14×14 k.p model in combination with the semiconductor Bloch equations to describe the observed features quantitatively. The combined theoretical and experimental approach will allow us to analyze the influence of the bandstructure and interaction effects on the injection current amplitude and current dynamics.}}, author = {{Bieler, M. and Pierz, K. and Siegner, U. and Dawson, P. and Duc, H. T. and Förstner, Jens and Meier, Torsten}}, booktitle = {{Ultrafast Phenomena in Semiconductors and Nanostructure Materials XIII}}, editor = {{Tsen, Kong-Thon and Song, Jin-Joo and Betz, Markus and Elezzabi, Abdulhakem Y.}}, keywords = {{tet_topic_qw}}, location = {{San Jose (California / USA)}}, pages = {{721404--721404--13}}, publisher = {{SPIE}}, title = {{{Generation of injection currents in (110)-oriented GaAs quantum wells: experimental observation and development of a microscopic theory}}}, doi = {{10.1117/12.811841}}, volume = {{7214}}, year = {{2009}}, } @inproceedings{4181, abstract = {{We experimentally and theoretically investigate microdisk resonators with embedded quantum dots immersed in a liquid crystal in its nematic phase, showing the tunabililty of the photonic modes via external parameters like temperature or electric field.}}, author = {{Förstner, Jens and Meier, Cedrik and Piegdon, Karoline and Declair, Stefan and Hoischen, Andreas and Urbanski, Mark and Meier, Torsten and Kitzerow, Heinz-Siegfried}}, booktitle = {{Advances in Optical Sciences Congress}}, isbn = {{9781557528735}}, keywords = {{tet_topic_microdisk}}, location = {{Honolulu, Hawaii United States}}, publisher = {{OSA Technical Digest (CD) (Optical Society of America, 2009), paper NTuC2}}, title = {{{Coupling Dynamics of Quantum Dots in a Liquid-Crystal-Tunable Microdisk Resonator}}}, doi = {{10.1364/nlo.2009.ntuc2}}, year = {{2009}}, } @article{4187, abstract = {{We study phonon-assisted electron tunneling in semiconductor quantum dot molecules. In particular, singletsinglet relaxation in a two-electron-doped structure is considered. The influence of Coulomb interaction is discussed via comparison with a single-electron system. We find that the relaxation rate reaches similar values in the two cases but the Coulomb interaction shifts the maximum rates toward larger separations between the dots. The difference in electron-phonon interaction between deformation potential and piezoelectric coupling is investigated. We show that the phonon-induced tunneling between two-electron singlet states is a fast process, taking place on the time scales of the order of a few tens of picoseconds.}}, author = {{Grodecka, A. and Machnikowski, P. and Förstner, Jens}}, issn = {{1098-0121}}, journal = {{Physical Review B}}, keywords = {{tet_topic_qd}}, number = {{8}}, publisher = {{American Physical Society (APS)}}, title = {{{Phonon-assisted tunneling between singlet states in two-electron quantum dot molecules}}}, doi = {{10.1103/physrevb.78.085302}}, volume = {{78}}, year = {{2008}}, } @article{4256, abstract = {{We present phase-resolved pulse propagation measurements that allow us to fully describe the transition between several light–matter interaction regimes. The complete range from linear excitation to the breakdown of the photonic bandgap on to self-induced transmission and self-phase modulation is studied on a high-quality multiple-quantum-well Bragg structure. An improved fast-scanning cross-correlation frequency-resolved optical gating setup is applied to retrieve the pulse phase with an excellent signal-tonoise ratio. Calculations using the semiconductor Maxwell–Bloch equations show qualitative agreement with the experimental findings. }}, author = {{zu Siederdissen, Tilman Höner and Nielsen, Nils C. and Kuhl, Jürgen and Schaarschmidt, Martin and Förstner, Jens and Knorr, Andreas and Khitrova, Galina and Gibbs, Hyatt M. and Koch, Stephan W. and Giessen, Harald}}, issn = {{0146-9592}}, journal = {{Optics Letters}}, keywords = {{tet_topic_polariton, tet_topic_qw}}, number = {{11}}, publisher = {{The Optical Society}}, title = {{{Transition between different coherent light–matter interaction regimes analyzed by phase-resolved pulse propagation}}}, doi = {{10.1364/ol.30.001384}}, volume = {{30}}, year = {{2008}}, } @article{4317, abstract = {{Phonon-assisted singlet-singlet relaxation in semiconductor quantum dot molecules is studied theoretically. Laterally coupled quantum dot structures doped with two electrons are considered. We take into account interaction with acoustic phonon modes via deformation potential and piezoelectric coupling. We show that piezoelectric mechanism for the considered system is of great importance and for some ranges of quantum dot molecule parameters is the dominant contribution to relaxation. It is shown that the phonon-assisted tunneling is much faster (down to ∼ 6 ps even at zero temperature) in comparison with other decoherence processes. The influence of Coulomb interaction is discussed and its consequences are indicated. We calculate the relaxation rates for GaAs quantum dot molecules and study the dependence on quantum dot size, distance and offset between the constituent quantum dots. In addition the temperature dependence of the tunneling rates is analyzed.}}, author = {{Grodecka, Anna and Machnikowski, Paweł and Förstner, Jens}}, issn = {{1610-1634}}, journal = {{physica status solidi (c)}}, keywords = {{tet_topic_qd}}, number = {{2}}, pages = {{474--478}}, publisher = {{Wiley}}, title = {{{Theoretical study of phononassisted singlet-singlet relaxation in two-electron semiconductor quantum dot molecules}}}, doi = {{10.1002/pssc.200880319}}, volume = {{6}}, year = {{2008}}, } @article{4303, abstract = {{The interaction of strong low-area pulses with two-level systems shows absorption line narrowing and hole burning within the homogeneous linewidth as a result of nonlinear wave mixing. The wave mixing results from the two-level electronic saturation nonlinearity and occurs, depending on the sign of the pulse area, as a strong absorption enhancement or gain at the transition frequency of the two-level system for resonant excitation.}}, author = {{Förstner, Jens and Knorr, A. and Lindberg, M. and Koch, S. W.}}, issn = {{0146-9592}}, journal = {{Optics Letters}}, keywords = {{tet_topic_polariton}}, number = {{20}}, publisher = {{The Optical Society}}, title = {{{Line narrowing and hole burning within the homogeneous linewidth: a new wave-mixing effect in two-level systems}}}, doi = {{10.1364/ol.27.001830}}, volume = {{27}}, year = {{2007}}, } @article{4189, abstract = {{We theoretically study the biexciton-phonon interaction in strongly confined semiconductor quantum dots. For spectrally narrow single-pulse excitation generation of biexcitonic occupations is only possible via a two-photon cascade, which exhibits renormalized Rabi oscillations and spectrally compressed phononsidebands.}}, author = {{Förstner, Jens and Knorr, A. and Moloney, J. V.}}, issn = {{1610-1634}}, journal = {{physica status solidi (c)}}, keywords = {{tet_topic_qd}}, number = {{7}}, pages = {{2389--2392}}, publisher = {{Wiley}}, title = {{{Interplay of electron-phonon and Coulomb interaction in semiconductor quantum dots}}}, doi = {{10.1002/pssc.200668111}}, volume = {{3}}, year = {{2006}}, } @inproceedings{4241, abstract = {{We discuss second-harmonic generation experiments on planar arrays of magnetic split-ring resonators, using 150 fs pulses at 1.5 mum wavelength. Lithographic tuning reveals by far the largest signals when exciting the magnetic-dipole resonance.}}, author = {{Klein, M. W. and Enkrich, C. and Wegener, M. and Förstner, Jens and Moloney, J. V. and Hoyer, W. and Stroucken, T. and Meier, Torsten and Koch, S. W. and Linden, S.}}, booktitle = {{2006 Conference on Lasers and Electro-Optics and 2006 Quantum Electronics and Laser Science Conference}}, isbn = {{9781557528131}}, issn = {{2160-9004 }}, keywords = {{tet_topic_meta}}, location = {{Long Beach, CA (USA)}}, publisher = {{IEEE}}, title = {{{Optical experiments on second-harmonic generation from metamaterials consisting of split-ring resonators}}}, doi = {{10.1109/cleo.2006.4629006}}, year = {{2006}}, } @article{4243, abstract = {{We investigate the optical properties of a Coulomb-coupled double-quantum dot system excited by coherent light pulses. Basic effects of Coulomb coupling regarding linear and nonlinear optical processes are discussed. By numerically solving the Heisenberg equation of motion we are able to present the temporal evolution of the system’s density matrix for a wide range of coupling parameters. The two main coupling effects in dipole approximation, biexcitonic shift and Förster energy transfer, are investigated and their qualitative and quantitative influence on absorption spectra, Rabi oscillations, and single- and two-pulse excitation is discussed. We present simulated differential transmission spectra to allow for comparison with recent experimental studies.}}, author = {{Danckwerts, J. and Ahn, K. J. and Förstner, Jens and Knorr, A.}}, issn = {{1098-0121}}, journal = {{Physical Review B}}, keywords = {{tet_topic_qd}}, number = {{16}}, pages = {{165318--165318--18}}, publisher = {{American Physical Society (APS)}}, title = {{{Theory of ultrafast nonlinear optics of Coulomb-coupled semiconductor quantum dots: Rabi oscillations and pump-probe spectra}}}, doi = {{10.1103/physrevb.73.165318}}, volume = {{73}}, year = {{2006}}, } @inproceedings{4191, abstract = {{We study optical second-harmonic generation from planar arrays of magnetic split-ring resonators at 1.5 microns resonance wavelength. We obtain by far the largest signals when exciting the magnetic-dipole resonance. }}, author = {{Klein, Matthias W. and Enkrich, Christian and Wegener, Martin and Förstner, Jens and Moloney, Jerome V. and Hoyer, Walter and Stroucken, Tineke and Meier, Torsten and Koch, Stephan W. and Linden, Stefan}}, booktitle = {{Photonic Metamaterials: From Random to Periodic}}, isbn = {{155752808X}}, keywords = {{tet_topic_meta}}, publisher = {{OSA}}, title = {{{Optical Experiments on Second-Harmonic Generation with Metamaterials Composed of Split-Ring Resonators}}}, doi = {{10.1364/meta.2006.tuc5}}, year = {{2006}}, }