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Semiconductor Theory

We describe the nonlinear optical dynamics in tailored quantum dots, quantum wells and bulk semiconductors.

Related publications by the TET group


Open list in Research Information System

Electrically controlled rapid adiabatic passage in a single quantum dot

A. Mukherjee, A. Widhalm, D. Siebert, S. Krehs, N. Sharma, A. Thiede, D. Reuter, J. Förstner, A. Zrenner, Applied Physics Letters (2020)


Ultrafast electric phase control of a single exciton qubit

A. Widhalm, A. Mukherjee, S. Krehs, N. Sharma, P. Kölling, A. Thiede, D. Reuter, J. Förstner, A. Zrenner, Applied Physics Letters (2018), 112(11), pp. 111105

We report on the coherent phase manipulation of quantum dot excitons by electric means. For our experiments, we use a low capacitance single quantum dot photodiode which is electrically controlled by a custom designed SiGe:C BiCMOS chip. The phase manipulation is performed and quantified in a Ramsey experiment, where ultrafast transient detuning of the exciton energy is performed synchronous to double pulse p/2 ps laser excitation. We are able to demonstrate electrically controlled phase manipulations with magnitudes up to 3p within 100 ps which is below the dephasing time of the quantum dot exciton.


    Fabrication and characterization of two-dimensional cubic AlN photonic crystal membranes containing zincblende GaN quantum dots

    S. Blumenthal, M. Bürger, A. Hildebrandt, J. Förstner, N. Weber, C. Meier, D. Reuter, D.J. As, physica status solidi (c) (2016), 13(5-6), pp. 292-296

    We successfully developed a process to fabricate freestanding cubic aluminium nitride (c-AlN) membranes containing cubic gallium nitride (c-GaN) quantum dots (QDs). The samples were grown by plasma assisted molecular beam epitaxy (MBE). To realize the photonic crystal (PhC) membrane we have chosen a triangular array of holes. The array was fabricated by electron beam lithography and several steps of reactive ion etching (RIE) with the help of a hard mask and an undercut of the active layer. The r/a- ratio of 0.35 was deter- mined by numerical simulations to obtain a preferably wide photonic band gap. Micro-photoluminescence (μ-PL) measurements of the photonic crystals, in particular of a H1 and a L3 cavity, and the emission of the QD ensemble were performed to characterize the samples. The PhCs show high quality factors of 4400 for the H1 cavity and about 5000/3000 for two different modes of the L3 cavity, respectively. The energy of the fundamental modes is in good agreement to the numerical simulations.


      A process for the preparation of a population inversion in a quantum system using multi-pulse excitation

      A. Zrenner, J. Förstner, D. Mantei. A process for the preparation of a population inversion in a quantum system using multi-pulse excitation, Patent DE102013012682A1. 2015.

      The invention relates to a process for the preparation of a population inversion in a quantum system (Q) by means of multi-pulse excitation, wherein a quantum system (Q) comprising at least one quantum dot with two orthogonal states (/ X> / Y>), particularly the (mutually orthogonal polarizations P1 , P2) are optically excitable, is illuminated with a first laser pulse (L1) which is (for resonant excitation of the first (/ Y>) of the two states of / X, / Y>) is set> and temporally below (with a second laser pulse of (for resonant excitation of the second (/ X>) of the two states of / X, / Y>) is set> L2) is illuminated.


        Robust Population Inversion by Polarization Selective Pulsed Excitation

        D. Mantei, J. Förstner, S. Gordon, Y.A. Leier, A.K. Rai, D. Reuter, A.D. Wieck, A. Zrenner, Scientific Reports (2015), 5(1), pp. 10313

        The coherent state preparation and control of single quantum systems is an important prerequisite for the implementation of functional quantum devices. Prominent examples for such systems are semiconductor quantum dots, which exhibit a fine structure split single exciton state and a V-type three level structure, given by a common ground state and two distinguishable and separately excitable transitions. In this work we introduce a novel concept for the preparation of a robust inversion by the sequential excitation in a V-type system via distinguishable paths.


        Cubic GaN quantum dots embedded in zinc-blende AlN microdisks

        M. Bürger, R. Kemper, C. Bader, M. Ruth, S. Declair, C. Meier, J. Förstner, D. As, Journal of Crystal Growth (2013), 378, pp. 287-290

        Microresonators containing quantum dots find application in devices like single photon emitters for quantum information technology as well as low threshold laser devices. We demonstrate the fabrication of 60 nm thin zinc-blende AlN microdisks including cubic GaN quantum dots using dry chemical etching techniques. Scanning electron microscopy analysis reveals the morphology with smooth surfaces of the microdisks. Micro-photoluminescence measurements exhibit optically active quantum dots. Furthermore this is the first report of resonator modes in the emission spectrum of a cubic AlN microdisk.


          Whispering gallery modes in zinc-blende AlN microdisks containing non-polar GaN quantum dots

          M. Bürger, M. Ruth, S. Declair, J. Förstner, C. Meier, D.J. As, Applied Physics Letters (2013), 102(8), pp. 081105

          Whispering gallery modes (WGMs) were observed in 60 nm thin cubic AlN microdisk resonators containing a single layer of non-polar cubic GaN quantum dots. Freestanding microdisks were patterned by means of electron beam lithography and a two step reactive ion etching process. Micro-photoluminescence spectroscopy investigations were performed for optical characterization. We analyzed the mode spacing for disk diameters ranging from 2-4 lm. Numerical investigations using three dimensional finite difference time domain calculations were in good agreement with the experimental data. Whispering gallery modes of the radial orders 1 and 2 were identified by means of simulated mode field distributions.


          Photonic crystal waveguides intersection for resonant quantum dot optical spectroscopy detection

          X. Song, S. Declair, T. Meier, A. Zrenner, J. Förstner, Optics Express (2012), 20(13)

          Using a finite-difference time-domain method, we theoretically investigate the optical spectra of crossing perpendicular photonic crystal waveguides with quantum dots embedded in the central rod. The waveguides are designed so that the light mainly propagates along one direction and the cross talk is greatly reduced in the transverse direction. It is shown that when a quantum dot (QD) is resonant with the cavity, strong coupling can be observed via both the transmission and crosstalk spectrum. If the cavity is far off-resonant from the QD, both the cavity mode and the QD signal can be detected in the transverse direction since the laser field is greatly suppressed in this direction. This structure could have strong implications for resonant excitation and in-plane detection of QD optical spectroscopy.


            Cavity-assisted emission of polarization-entangled photons from biexcitons in quantum dots with fine-structure splitting

            S. Schumacher, J. Förstner, A. Zrenner, M. Florian, C. Gies, P. Gartner, F. Jahnke, Optics Express (2012), 20(5)

            We study the quantum properties and statistics of photons emitted by a quantum-dot biexciton inside a cavity. In the biexciton-exciton cascade, fine-structure splitting between exciton levels degrades polarization-entanglement for the emitted pair of photons. However, here we show that the polarization-entanglement can be preserved in such a system through simultaneous emission of two degenerate photons into cavity modes tuned to half the biexciton energy. Based on detailed theoretical calculations for realistic quantum-dot and cavity parameters, we quantify the degree of achievable entanglement.


            Intensity dependence of optically-induced injection currents in semiconductor quantum wells

            M. Pochwala, H.T. Duc, J. Förstner, T. Meier, in: CLEO:2011 - Laser Applications to Photonic Applications, OSA, 2011

            The intensity dependence of optically-induced injection currents in semiconductor quantum wells is investigated numerically. Oscillatory behavior of the electron charge current transients as function of intensity and time is predicted and explained.


              Injection currents in (110)-oriented GaAs/AlGaAs quantum wells: recent progress in theory and experiment

              H.T. Duc, M. Pochwala, J. Förstner, T. Meier, S. Priyadarshi, A.M. Racu, K. Pierz, U. Siegner, M. Bieler, in: Ultrafast Phenomena in Semiconductors and Nanostructure Materials XV, SPIE, 2011

              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.


                Intensity-dependent ultrafast dynamics of injection currents in unbiased GaAs quantum wells

                M. Pochwała, H.T. Duc, J. Förstner, T. Meier, physica status solidi (RRL) - Rapid Research Letters (2011), 5(3), pp. 119-121

                The intensity dependence of optically-induced injection currents in unbiased GaAs semiconductor quantum wells grown in [110] direction is investigated theoretically for a number of well widths. Our microscopic analysis is based on a 14 x 14 band k . p method in combination with the multisubband semiconductor Bloch equations. An oscillatory dependence of the injection current transients as function of intensity and time is predicted and explained. It is demonstrated that optical excitations involving different subbands and Rabi flopping are responsible for this complex dynamics.


                  Electrong-factor anisotropy in symmetric (110)-oriented GaAs quantum wells

                  J. Hübner, S. Kunz, S. Oertel, D. Schuh, M. Pochwała, H.T. Duc, J. Förstner, T. Meier, M. Oestreich, Physical Review B (2011), 84(4), pp. 041301 (R)

                  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.


                  Oscillatory excitation energy dependence of injection currents in GaAs/AlGaAs quantum wells

                  H. Thanh Duc, J. Förstner, T. Meier, S. Priyadarshi, A.M. Racu, K. Pierz, U. Siegner, M. Bieler, physica status solidi (c) (2011), 8(4), pp. 1137-1140

                  The injection of photocurrents by femtosecond laser pulses in (110)-orientedGaAs/AlGaAs quantum wells is investigated theoretically and experimentally. The roomtemperature measurements show an oscillatory dependence of the injection current amplitude and direction on the excitation photon energy. Microscopic calculations using the semiconductor Bloch equations that are set up on the basis of k.p band structure calculations provide a detailed understanding of the experimental findings.


                    Method for transmission of information about polarization state of photons to stationary system

                    J. Förstner, D. Mantei, S.M.. de Vasconcellos, A. Zrenner. Method for transmission of information about polarization state of photons to stationary system, Patent DE102010020817A1. 2011.

                    Die Erfindung betrifft ein Verfahren zur Übertragung des Polarisationszustandes von Photonen in ein stationäres System, bei dem mit Photonen eines Polarisationszustandes ein Quanten-System angeregt wird, das zwei Zustände aufweist, die mit zueinander orthogonalen Polarisationen anregbar sind und deren energetischer Abstand kleiner ist als die energetische Bandbreite der Photonen, wobei beide Zustände in Abhängigkeit von der Polarisation besetzt werden und das Quantensystem einen Superpositionszustand beider Zustände einnimmt.


                      Phonon-assisted decoherence and tunneling in quantum dot molecules

                      A. Grodecka-Grad, J. Förstner, physica status solidi (c) (2011), 8(4), pp. 1125-1128

                      We study the influence of the phonon environment on the electron dynamics in a doped quantum dot molecule. A non-perturbative quantumkinetic theory based on correlation expansion is used in order to describe both diagonal and off-diagonal electron-phonon couplings representing real and virtual processes with relevant acoustic phonons. We show that the relaxation is dominated by phononassisted electron tunneling between constituent quantum dots and occurs on a picosecond time scale. The dependence of the time evolution of the quantum dot occupation probabilities on the energy mismatch between the quantum dots is studied in detail.


                        Tuning quantum-dot based photonic devices with liquid crystals

                        K.A. Piegdon, S. Declair, J. Förstner, T. Meier, H. Matthias, M. Urbanski, H. Kitzerow, D. Reuter, A.D. Wieck, A. Lorke, C. Meier, Optics Express (2010), 18(8)

                        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.


                        Modeling excitonic line shapes in weakly disordered semiconductor nanostructures

                        I. Kuznetsova, N. Gőgh, J. Förstner, T. Meier, S.T. Cundiff, I. Varga, P. Thomas, Physical Review B (2010), 81(7)

                        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.


                          Reversal of Coherently Controlled Ultrafast Photocurrents by Band Mixing in Undoped GaAs Quantum Wells

                          S. Priyadarshi, A.M. Racu, K. Pierz, U. Siegner, M. Bieler, H.T. Duc, J. Förstner, T. Meier, Physical Review Letters (2010), 104(21)

                          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.


                            Microscopic theoretical analysis of optically generated injection currents in semiconductor quantum wells

                            H.T. Duc, J. Förstner, T. Meier, in: Ultrafast Phenomena in Semiconductors and Nanostructure Materials XIV, SPIE, 2010, pp. 76000S-76000S-9

                            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.


                              Microscopic analysis of charge and spin photocurrents injected by circularly polarized one-color laser pulses in GaAs quantum wells

                              H.T. Duc, J. Förstner, T. Meier, Physical Review B (2010), 82(11), pp. 115316-1

                              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.


                                Self-assembled quantum dots in a liquid-crystal-tunable microdisk resonator

                                K.A. Piegdon, M. Offer, A. Lorke, M. Urbanski, A. Hoischen, H. Kitzerow, S. Declair, J. Förstner, T. Meier, D. Reuter, A.D. Wieck, C. Meier, Physica E: Low-dimensional Systems and Nanostructures (2010), 42(10), pp. 2552-2555

                                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.


                                  Theory of phonon-mediated relaxation in doped quantum dot molecules

                                  A. Grodecka-Grad, J. Förstner, Physical Review B (2010), 81(11)

                                  A quantum dot molecule doped with a single electron in the presence of diagonal and off-diagonal carrierphonon couplings is studied by means of a nonperturbative quantum kinetic theory. The interaction with acoustic phonons by deformation potential and piezoelectric coupling is taken into account. We show that the phonon-mediated relaxation is fast on a picosecond time scale and is dominated by the usually neglected off-diagonal coupling to the lattice degrees of freedom leading to phonon-assisted electron tunneling. We show that in the parameter regime of current electrical and optical experiments, the microscopic non-Markovian theory has to be employed.


                                  Phonon-mediated relaxation in doped quantum dot molecules

                                  A. Grodecka-Grad, J. Förstner, Journal of Physics: Conference Series (2010), 245

                                  We study a single quantum dot molecule doped with one electron in the presence of electron-phonon coupling. Both diagonal and off-diagonal interactions representing real and virtual processes with acoustic phonons via deformation potential and piezoelectric coupling are taken into account. We employ a non-perturbative quantum kinetic theory and show that the phonon-mediated relaxation is dominated by an electron tunneling on a picosecond time scale.A dependence of the relaxation on the temperature and the strength of the tunneling coupling is analyzed.


                                    Generation of injection currents in (110)-oriented GaAs quantum wells: experimental observation and development of a microscopic theory

                                    M. Bieler, K. Pierz, U. Siegner, P. Dawson, H.T. Duc, J. Förstner, T. Meier, in: Ultrafast Phenomena in Semiconductors and Nanostructure Materials XIII, SPIE, 2009, pp. 721404-721404-13

                                    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.


                                      Indirect Dephasing Channel for Optically Controlled Spin in a Single Quantum Dot

                                      A. Grodecka, P. Machnikowski, J. Förstner, in: Advances in Optical Sciences Congress, OSA Technical Digest (CD) (Optical Society of America, 2009), 2009

                                      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.


                                        Indirect spin dephasing via charge-state decoherence in optical control schemes in quantum dots

                                        A. Grodecka, P. Machnikowski, J. Förstner, Physical Review A (2009), 79(4)

                                        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.


                                        Theoretical study of phononassisted singlet-singlet relaxation in two-electron semiconductor quantum dot molecules

                                        A. Grodecka, P. Machnikowski, J. Förstner, physica status solidi (c) (2008), 6(2), pp. 474-478

                                        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.


                                          Transition between different coherent light–matter interaction regimes analyzed by phase-resolved pulse propagation

                                          T.H. zu Siederdissen, N.C. Nielsen, J. Kuhl, M. Schaarschmidt, J. Förstner, A. Knorr, G. Khitrova, H.M. Gibbs, S.W. Koch, H. Giessen, Optics Letters (2008), 30(11)

                                          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.


                                          Phonon-assisted tunneling between singlet states in two-electron quantum dot molecules

                                          A. Grodecka, P. Machnikowski, J. Förstner, Physical Review B (2008), 78(8)

                                          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.


                                            Theory of ultrafast nonlinear optics of Coulomb-coupled semiconductor quantum dots: Rabi oscillations and pump-probe spectra

                                            J. Danckwerts, K.J. Ahn, J. Förstner, A. Knorr, Physical Review B (2006), 73(16), pp. 165318-165318-18

                                            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.


                                            Interplay of electron-phonon and Coulomb interaction in semiconductor quantum dots

                                            J. Förstner, A. Knorr, J.V. Moloney, physica status solidi (c) (2006), 3(7), pp. 2389-2392

                                            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.


                                              Quantum information processing using Coulomb-coupled quantum dots

                                              J. Danckwerts, J. Förstner, A. Knorr, in: AIP Conference Proceedings, AIP, 2005

                                              A system of two quantum dots coupled by dipole‐dipole interaction is investigated within a density matrix approach. We compute the temporal evolution of the system in the linear and nonlinear optical regime and discuss the possibility of performing basic quantum information gates. The influence of the Förster energy transfer on Rabi oscillations is discussed.


                                                Phase Evolution of Solitonlike Optical Pulses during Excitonic Rabi Flopping in a Semiconductor

                                                N.C. Nielsen, T.H. zu Siederdissen, J. Kuhl, M. Schaarschmidt, J. Förstner, A. Knorr, H. Giessen, Physical Review Letters (2005), 94(5)

                                                We demonstrate that the temporal pulse phase remains essentially unaltered before separate phase characteristics are developed when propagating high-intensity pulses coherently on the exciton resonance of an optically thick semiconductor. This behavior is a clear manifestation of self-induced transmission and pulse breakup into solitonlike pulses due to Rabi flopping of the carrier density. Experiments using a novel fast-scan cross-correlation frequency-resolved optical gating (XFROG) method are in good agreement with numerical calculations based on the semiconductor Bloch equations.


                                                  Resonance fluorescence of semiconductor quantum dots: Signatures of the electron-phonon interaction

                                                  K.J. Ahn, J. Förstner, A. Knorr, Physical Review B (2005), 71(15)

                                                  Using a fully quantized description of strongly confined electrons interacting with acoustic phonons and the photon field, the nonstationary resonance-fluorescence spectra of a semiconductor quantum dot are investigated. For excitation pulses with durations approaching typical electron-phonon scattering times, the virtual quantum processes yield an observable electron-phonon sideband broadening.


                                                    Ultrafast electron-phonon interaction of intersubband transitions: Quantum kinetics from adiabatic following to Rabi-oscillations

                                                    S. Butscher, J. Förstner, I. Waldmüller, A. Knorr, Physical Review B (2005), 72(4), pp. 045314-045314-4

                                                    The interaction of electrons with LO phonons provides an important mechanism of optical dephasing and carrier scattering for the two-dimensional electron gas in semiconductor quantum wells. In this paper, the corresponding ultrafast nonlinearities for off-resonant and resonant intersubband excitations are investigated. Quantum kinetic effects of the electron-phonon interaction and the corresponding violation of the microscopic energy conservation yield a qualitative different picture compared to the standard Markovian theory, if the phonon energy is larger than the intersubband-gap energy.


                                                    Light Propagation and Many-Particle Effects in Semiconductor Nanostructures

                                                    J. Förstner, 2004

                                                    In dieser Arbeit wird eine Theorie vorgestellt, welche die quantenmechanische Vielteilchenphysik der Licht-Materie Wechselwirkung in Halbleiternanostrukturen beschreibt. Diese mikroskopische Beschreibung wird durch Kombination eines allgemeinen Dichtematrixansatzes mit speziellen Methoden zur Auswertung der Maxwellgleichungen wie der zeitaufgelösten Finite-Differenzen-Methode (FDTD) erreicht. Die Theorie wird auf verschiedene physikalische Situationen angewendet, wie z.B. Lichtausbreitung in Volumenhalbleitern, Interband- und Intersubbandübergänge in Quantenfilmstrukturen und optische Anregung von Quantenpunkten. Der Fokus liegt dabei auf der Beschreibung der linearen und nichtlinearen Antwort des Vielteilchensystems und seiner Ankopplung an das elektromagnetische Feld. In diesem Zusammenhang wird sowohl die Erzeugung als auch der Zerfall von optischen Anregungen untersucht, indem verschiedene Kopplungsmechanismen wie Elektron-Phonon-, Elektron-Photon- und Elektron-Elektron-Wechselwirkung berücksichtigt werden. Im Bereich der linearen Optik, also für Anregung mit geringer Intensität, ermöglicht die Theorie die Berechnung von Absorptionsspektren. Verschiedene Effekte in linearer Optik werden in dieser Arbeit untersucht und beschrieben: Linienaufspaltung durch Polaritonen im Volumenmaterial, Zunahme der Linienbreite bei Intersubbandübergängen verursacht durch Elektron-Elektron- und Elektron- Phonon-Streuung in einzelnen Quantenfilmen, Bildung einer optischen Bandlücke durch starke radiative Kopplung in Vielfilmstrukturen in Bragg-Geometrie, Phononenseitenbänder verursacht durch quantenkinetische Effekte in einzelnen Quantenpunkten und schliesslich Superradianz und Interferenzeffekte in Quantenpunktgittern. Bei nichtlinearer Anregung treten Dichte-Rabiflops als fundamentale Prozesse in allen betrachteten Systemen auf und können als kohärente Be- und Entvölkerung von quantenmechanischen Zuständen beobachtet werden. Der Einfluss von starker Lichtkopplung und verschiedenen Wechselwirkungen auf dynamische Größen wie die Besetzung wird untersucht. Bei nichtlinearer Propagation, bei der sich ein starker Lichtpuls über längere Strecken in einem System bewegt, wird selbstinduzierte Verstärkung der Transmission näher betrachtet. Des weiteren werden von der Coulombwechselwirkung verursachte nichtlineare Effekte wie exzitoninduziertes Dephasieren in Volumenmaterial und verschränkte Zustände in Quantenpunkten untersucht, die einen Zusammenbruch der Hartree-Fock- Näherung darstellen. Zusammenfassend werden in dieser Arbeit verschiedene lineare und nichtlineare optische Effekte in Halbleiternanostrukturen verschiedener Dimensionalität mit Hilfe einer allgemeinen Theorie, die einen Dichtematrixansatz mit den Maxwellschen Gleichungen kombiniert, untersucht.


                                                    Self-consistent Projection Operator Theory of Intersubband Absorbance in Semiconductor Quantum Wells

                                                    I. Waldmüller, J. Förstner, A.. Knorr, in: Nonequilibrium Physics at Short Time Scales, Springer Berlin Heidelberg, 2004

                                                    Due to their many-particle character and their application in quantum cascade lasers, optical intersubband excitations in semiconductor quantum wells have become the focus of many recent publications [1,2]. In samples of high quality, intrinsic processes like electron-electron and electron-phonon many particle correlations determine the basic optical and transport properties such as lineshape and ultrafast dynamics. At the same time, intersubband excitations allow the direct investigation of dynamical properties of an important model system of many particle physics - the two-dimensional electron gas. We here present a microscopic theory for the intersubband dynamics and absorption. The calculation of absorption spectra of MQW systems is in principle composed of two parts: the determination of the polarization in a single quantum well within a density matrix approach as the source of electromagnetic radiation (Fig. 1a) and the calculation of the generated fields in the geometry of interest (Fig. 1b) within a Green's function approach [3,4]. We will here focus on the so-called single-pass geometry (cf. Fig. 1b, [5]).


                                                      Nonlinear light pulse propagation in Bragg-periodic multiple semiconductor quantum well samples: ultrafast switching of a resonant photonic band gap

                                                      M. Schaarschmidt, J. Förstner, A. Knorr, J.P. Prineas, N.C. Nielsen, J. Kuhl, G. Kithrova, H.M. Gibbs, H. Giessen, S.W. Koch, in: Conference on Lasers and Electro-Optics/International Quantum Electronics Conference and Photonic Applications Systems Technologies, OSA, 2004

                                                      We investigate theoretically the ultrafast nonlinear suppression of the resonant photonic band gap by strong laser pulses in semiconductor multiple qnantum wells. We achieve good agreement with our measurements on reflection samples.


                                                        Temporal phase evolution during excitonic Rabi flopping in semiconductors

                                                        T. Höner zu Siederdissen, N.C. Nielsen, J. Kuhl, J. Förstner, A. Knorr, H. Giessen, in: International Quantum Electronics Conference and Photonic Applications Systems Technologies, OSA, 2004

                                                        Theoretically and experimentally, we investigate temporal phase evolution during Rabi-flopping on the A-exciton resonance in CdSe using a novel fast-scanning XFROG method and observe phase changes smaller than π/2 compared to the slightly-chirped input pulse.


                                                          Polaron signatures in the line shape of semiconductor ;intersubband transitions: quantum kinetics of the electron–phonon interaction

                                                          S. Butscher, J. Förstner, I. Waldmüller, A. Knorr, physica status solidi (b) (2004), 241(11), pp. R49-R51

                                                          We present a theory of the optical line shape of coherent intersubband transitions in a semiconductor quantum well, considering non-Markovian LO-phonon scattering as major broadening mechanism. We show that a quantum kinetic approach leads to additional polaron resonances and a resonance enhancement for gap energies close to the phonon energy.


                                                            Adiabatically driven electron dynamics in a resonant photonic band gap: Optical switching of a Bragg periodic semiconductor

                                                            M. Schaarschmidt, J. Förstner, A. Knorr, J.P. Prineas, N.C. Nielsen, J. Kuhl, G. Khitrova, H.M. Gibbs, H. Giessen, S.W. Koch, Physical Review B (2004), 70(23)

                                                            The adiabatic driving of the resonant electron dynamics in a one-dimensional resonant photonic band gap is proposed as an optical mechanism for nonlinear ultrafast switching. Pulsed excitation inside the photonic gap results in an ultrafast suppression and recovery of the gap. This behavior results from the adiabatic carrier dynamics due to rapid radiative damping inside the band gap.


                                                            Linear and nonlinear pulse propagation in a multiple-quantum-well photonic crystal

                                                            N.C. Nielsen, J. Kuhl, M. Schaarschmidt, J. Förstner, A. Knorr, S.W. Koch, G. Khitrova, H.M. Gibbs, H. Giessen, Physical Review B (2004), 70(7)

                                                            We investigate the temporal and spectral properties of subpicosecond pulses transmitted on the heavy-hole exciton transition through a multiple-quantum-well Bragg structure, exhibiting a one-dimensional photonic band gap. At low light intensities, a temporal propagation beating is observed. This beating is strongly dependent on the optical dephasing time T2 which is dominated by the radiative interwell coupling. In an intermediate intensity regime, the Pauli-blocking nonlinearity leads to gradual suppression of the photonic band gap and vanishing of the linear propagation beating. For highly nonlinear excitation, we find signatures of selfinduced transmission due to Rabi flopping and adiabatic following of the carrier density. Numerical simulations using the semiconductor Maxwell-Bloch equations are in excellent agreement with the experimental data up to intensities for which higher many-particle correlations become more important and self-phase modulation occurs in the sample substrate.


                                                              Phonon-Assisted Damping of Rabi Oscillations in Semiconductor Quantum Dots

                                                              J. Förstner, C. Weber, J. Danckwerts, A. Knorr, Physical Review Letters (2003), 91(12), pp. 127401


                                                              Self-induced transparency in InGaAs quantum dot waveguides

                                                              S. Schneider, P. Borri, W. Langbein, U. Woggon, J. Förstner, A. Knorr, R.L. Sellin, D. Ouyang, D. Bimberg, physica status solidi (c) (2003)(5), pp. 1548-1551

                                                              We present the experimental observation and the theoretical modelling of self-induced transparency signatures such as nonlinear transmission, pulse retardation and reshaping for subpicosecond pulse propagation in a 2 mm-long InGaAs quantum-dot ridge waveguide at 10 K. The measurements were obtained using a cross-correlation frequency resolved optical gating technique which allows us to retrieve the field amplitude of the propagating pulses.


                                                                Theory of the lineshape of quantum well intersubband transitions: optical dephasing and light propagation effects

                                                                I. Waldmüller, M. Woerner, J. Förstner, A. Knorr, physica status solidi (b) (2003), 238(3), pp. 474-477

                                                                We outline a theoretical description of the absorption linewidth of quantum well intersubband transitions by solving Maxwell’s equations for a non-local susceptibility including many particle effects. We show that the intersubband absorption results from a complex interplay between mean-field effects, dephasing contributions and light propagation effects, all being very sensitive to subband dispersion.


                                                                  Correlated influence of carrier-carrier/carrier-phonon interaction and radiative damping on semiconductor intersubband transitions

                                                                  I. Waldmuller, J. Förstner, A. Knorr, M. Woerner, K. Reimann, R. Kaindl, R. Hey, K. Ploog, in: Postconference Digest Quantum Electronics and Laser Science, 2003. QELS., IEEE, 2003

                                                                  The linewidth of intersubband transitions resulting from simultaneous action of many-body contributions and radiative damping is analyzed. The processes are non-additive and a self-consistent treatment is necessary to explain recent experiments.


                                                                    Phonon-induced damping of Rabi oscillations in semiconductor quantum dots

                                                                    J. Förstner, C. Weber, J. Danckwerts, A. Knorr, physica status solidi (b) (2003), 238(3), pp. 419-422

                                                                    The phonon-induced dephasing dynamics of semiconductor quantum dots during nonlinear optical excitation is studied using quantum kinetic equations. We find that despite the decoherence process Rabi oscillations occur even for relatively long pulse durations and that their signatures in pump-probe experiments only get suppressed for high input pulse areas.


                                                                      Self-induced transparency in InGaAs quantum-dot waveguides

                                                                      S. Schneider, P. Borri, W. Langbein, U. Woggon, J. Förstner, A. Knorr, R.L. Sellin, D. Ouyang, D. Bimberg, Applied Physics Letters (2003), 83(18), pp. 3668-3670

                                                                      We report the experimental observation and the theoretical modeling of self-induced-transparency signatures such as nonlinear transmission, pulse retardation and reshaping, for subpicosecond pulse propagation in a 2-mm-long InGaAs quantum-dot ridge waveguide in resonance with the excitonic ground-state transition at 10 K. The measurements were obtained by using a cross-correlation frequency-resolved optical gating technique which allows us to retrieve the field amplitude of the propagating pulses.


                                                                      Pulse propagation in Bragg-resonant multiple quantum wells: from pulse breakup to compression

                                                                      N.C. Nielsen, J. Kuhl, M. Schaarschmidt, J. Förstner, A. Knorr, S.W. Koch, H.M. Gibbs, G. Khitrova, H. Giessen, physica status solidi (c) (2003)(5), pp. 1484-1487

                                                                      The nonlinear propagation of subpicosecond pulses resonant to the hh 1s exciton in Bragg-periodic multiple quantum wells is investigated experimentally and theoretically. We show coherent pulse breakup and its suppression for increasing pulse intensity in good agreement with calculations based on the semiconductor Maxwell-Bloch equations. For highly nonlinear excitation, pulse compression is observed which is strongly enhanced by the additional contribution of self-phase modulation in the barrier and substrate material.


                                                                        Damping of electron density Rabi-oscillations and self-induced-transparency in semiconductor quantum dots

                                                                        J. Förstner, C. Weber, J. Danckwerts, A. Knorr, in: Postconference Digest Quantum Electronics and Laser Science, 2003. QELS., IEEE, 2003

                                                                        A non-Markovian quantum kinetic theory of the coupled electron-phonon system in semiconductor quantum dots is used to analyze the nonlinear dipole decoherence and light propagation dynamics for arbitrary pulse strengths and lengths.


                                                                          Theory of ultrafast dynamics and lineshape of semiconductor quantum well intersubband emitters

                                                                          I. Waldmüller, J. Förstner, A. Knorr, in: Nonlinear Optics: Materials, Fundamentals and Applications, OSA, 2002

                                                                          On the basis of a density matrix approach including electron-electron scattering, a detailed analysis of the temporal dynamics and the dephasing process after optical excitation in intersubband emitters is presented for a wide range of parameters.


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                                                                            Prof. Dr. Jens Förstner

                                                                            Theoretical Electrical Engineering

                                                                            Jens Förstner
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