Jahnke, Frank

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Jahnke, Frank

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Jahnke, Frank

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Licht-Materie-Wechselwirkung in Halbleiter-Nanostrukturen zur Erzeugung nichtklassischen Lichts und stimulierter Emission
(2014-11-17)
Florian, Matthias
;
Wehling, Tim
;
Jahnke, Frank
;
Jahnke, Frank
The framework of quantum optics was developed side by side with groundbreaking experiments involving lasers and atoms as active medium. The amount of control one nowadays has on the design of semiconductor nanostructures is constantly leading to new progress in this field, and we can use quantum dots (QDs) that possess atom-like discrete states as emitters instead. With a single QD in a high-quality cavity with three-dimensional mode confinement, the ultimate limit of miniaturization is reached, where one electronic transition interacts with a single mode of the electromagnetic field. The application potential of this system lies in efficient light sources, new devices for quantum information technologies, as well as in highly tunable platforms to perform fundamental studies in the field of semiconductor quantum optics. We investigate the characteristics of microcavity lasers with single QD gain and discuss the possibility to realize stimulated emission in the strong-coupling regime. The impact of non-resonant background emitters present in experimental realizations is addressed, where off-resonant coupling between emitter resonances and the cavity mode is enabled via phonons or additional carriers in delocalized states. Furthermore, new schemes for electrically driven single-photon sources and the generation of polarization-entangled photons are proposed. Our theoretical analysis is based on microscopic theories going beyond simple atomic models. This allows us to investigate many-body effects and incorporates carrier-photon correlations, providing direct access to the statistical properties of the emission. The dynamical evolution of the system is described by means of density matrix approaches, or relies on cumulant expansion techniques where the first is numerically not possible. Multi-exciton effects of the quantum dot carriers play an important role, as well as the coupling to continuum states of the embedding material.
doctoral thesis
315 238
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Optical and Electronic Properties of InGaAs and Nitride Quantum Dots
(2006-07-21)
Baer, Norman
;
Czycholl, Gerd
;
Jahnke, Frank
;
Jahnke, Frank
Semiconductor quantum dots (QDs) attract considerable attention due to their potential for fundamental studies and device applications. Here, the electronic and optical properties of such structures are investigated for the well-established III-V and the new group-III nitride system.The influence of Coulomb correlations on the optical spectra of InGaAs QDs are studied for up to twelve excitons using a full configuration interaction approach. Characteristic features are explained using simplified Hamiltonians. Recent experiments involving charged exciton complexes are analyzed.Furthermore, properties of InN/GaN quantum dots are studied. The one-particle states are provided by a tight-binding model and the existence of an exactly degenerate p-shell is discussed. We present multi-exciton spectra and investigate the influence of built-in fields. Dark (dipole forbidden) exciton and biexciton ground states are predicted for small lens-shaped dots. For larger QDs, a level reordering can lead to a bright exciton ground state. In both cases a semi-analytic discussion provides deeper insight.The photoluminescence (PL) dynamics of an initially excited QD system is studied using a microscopic theory. The influence of correlations on the PL dynamics is investigated for different photonic environments. Our theory reveals that the lack of full correlation can lead to a non-exponential and excitation-intensity dependent decay, which is also found in recent experiments.
doctoral thesis
360 208
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Optical gain and laser properties of semiconductor quantum-dot systems
(2008-02-11)
Lorke, Michael
;
Knorr, Andreas
;
Jahnke, Frank
;
Jahnke, Frank
For practical applications of quantum dots in light emitters as well as for fundamental studies of their emission properties, the understanding of many-body processes plays a central role. We employ a microscopic theory to study the optical properties of semiconductor quantum dots. The excitation-induced polarization dephasing due to carrier-phonon and carrier-carrier Coulomb interaction as well as the corresponding lineshifts of the optical interband transitions are determined onthe basis of a quantum-kinetic treatment of correlation processes.Our theoretical model includes non-Markovian effects as well as renormalized single-particle states.Thus we achieve an accurate description of the partial compensation between different dephasing contributions and are able to systematically study their temperature and density dependencies.Applications of this theoretical model include optical gain spectra for quantum-dot systems that reveal a novel effect, not present in other gain materials. For large carrier densities, the maximum gain can decrease with increasing carrier density. This behavior arises from a delicate balancing of state filling and dephasing, and implies the necessity of an accurate treatment of the carrier-density dependence of correlations. Measurements of the coherence properties of the light emitted from semiconductor quantum-dot lasers have raised considerable attention in recent years. We study the correlations between individual emission events on the basis of a microscopic semiconductor laser theory. This allows for a study of effects like Pauli blocking, modifications to the source term of spontaneous emission,and the absence of complete inversion, that strongly influence the emission characteristics of quantum dot based devices. A new and challenging material system for applications in the visible spectral range are nitride semiconductors. As crystal symmetry and bandmixing effects strongly influence the opticalselection rules, the single particle properties of quantum dot and wetting layer states are determinedon an atomistic level from tight-binding calculations. The resulting tight-binding wave functionsare used to calculate dipole transition matrix elements and Coulomb interaction matrix elements.As an example for the combination of microscopic single-particle calculations and many-body theory, optical spectra of quantum-dot wetting-layer systems including multiple subbands and the influence of theatomic structure and strong bandmixing effects are presented.
doctoral thesis
258 111
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Carrier-Phonon Interaction in Semiconductor Quantum Dots
(2009-03-10)
Seebeck, Jan
;
Schneider, Hans Christian
;
Jahnke, Frank
;
Jahnke, Frank
The goals of this PhD thesis are theoretical investigations of carrier scattering processes in semiconductor quantum dots on a quantum-kinetic basis. A consistent treatment of quasi-particle renormalizations and carrier kinetics for non-equilibrium conditions is presented, using the framework of non-equilibrium Green's functions. The focus of our investigations is the interaction of carriers with LO phonons. Starting from a detailed study of quantum-dot polarons, scattering and dephasing processes are studied. For the optical response of semiconductor quantum dots a consistent treatment of the carrier-LO-phonon and the carrier-carrier interaction is presented. Finally, the combination of an tight-binding description of the single-particle properties with a many-body theory is demonstrated for nitride quantum dots where intrinsic material properties are of central importance.
doctoral thesis
242 162
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Theoretische Beschreibung kohärenter optischer Nichtlinearitäten in Halbleiter-Quantenfilmen
(2005-01-24)
Buck, Michael
;
Schneider, Hans Christian
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Jahnke, Frank
;
Jahnke, Frank
In order to optimize opto-electronic applications of semiconductor nanostructures, a deep understanding of optical nonlinearities is essential.One fundamental method currently used within this field of research is ultrafast optical spectroscopy.This work presents a theoretical study of optical nonlinearities in semiconductor quantum wells in the coherent time regime.The theory is based on the equations of motion for the coherent exciton and biexciton transition amplitudes.These equations are derived using two different ways: Heisenberg equations of motion in combination with the dynamics-controlled truncation theory and alternatively the non-equilibrium Green´s function technique. For the coherent regime both approaches are equivalent and can be used to formulate a coupled set of equations for the coherent excitation dynamics that is correct up to the third order in the optical field.In contrast to the dynamics-controlled truncation theory the non-equilibrium Green´s function technique allows the inclusion of incoherent scattering processes into the description.The theory is evaluated to describe pump and probe as well as four-wave-mixing experiments with ZnSe quantum wells.Numerical calculations for the coherent control of the macroscopic polarization andthe dependence of transient four-wave-mixing on the light-polarization and intensity are both in good agreement with experiments.In order to correctly describe elevated excitation conditions for the latter investigation, the theory is self-consistently extended such that the included excitonic and biexcitonic nonlinearities contribute up to arbitrary order in the optical field.
doctoral thesis
260 779
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Vielteilcheneffekte in Halbleiterquantenpunkten - Ladungsträgerdynamik und optische Eigenschaften
(2012-09-03)
Schuh, Kolja
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Czycholl, Gerd
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Jahnke, Frank
;
Jahnke, Frank
In this thesis theoretical models for the description of optical properties of semiconductor quantum dots are developed. Particular attention is given to the inclusion of many-body effects. The carrier dynamics and spectral properties due to the many-body interactions are calculated using a non-equilibrium Green's function formalism. In particular, the simultaneous influence of the carrier-phonon and Coulomb interaction on scattering efficiency in a quantum-dot system is investigated. Furthermore, the interplay of structural properties and many-body interactions has a large influence on optical properties. By implementing a configuration interaction scheme, we find that the Coulomb interaction strongly enhances the ground-state transitions by partly neutralizing the quantum-confined Stark effect in nonpolar nitride quantum dots.
doctoral thesis
284 172
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Many-body effects and quantum-optical mode coupling in semiconductor lasers
(2025-09-29)
Drechsler, Monty L.
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Jahnke, Frank
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Wiersig, Jan
;
Jahnke, Frank
Semiconductor lasers are invaluable tools for technological progress. They are indispensable for applications in optical communications, integrated photonic circuits, entanglement generation, quantum computing and sensing. This work develops a comprehensive quantum-optical semiconductor laser theory based on an equation of motion approach, capturing a broad spectrum of laser phenomena. A key focus is the linewidth behavior of singlemode quantum-well lasers. Deviations from the Schawlow-Townes behavior, particularly in micro- and nanolasers, are investigated. Many-body effects, including Coulomb interactions and electron-phonon coupling, significantly influence laser characteristics and alter the gain spectrum. Furthermore, the work extends the laser theory to multimode operation, identifying critical mode coupling mechanisms and introducing the concept of a “coherent phase” as a criterion for distinguishing between quantum and classical regimes. It is discussed that spontaneous emission disrupts phase coherence, while conventional lasers maintain it above the threshold. The research also explores the role of electron scattering in multimode lasing, demonstrating its major impact on laser characteristics. Additionally, the developed multimode laser theory is used to investigate mode locking in single-section quantum-dot lasers. Established theories for mode locking are based on a classical description of the light field, whereas here a quantum-optical theory is developed. Quantum fluctuations and relaxation dynamics are shown to play a crucial role for pulse generation. An analytical lower limit for the width of the beat-note spectrum is derived, with future work suggested to explore intensity-dependent effects on mode locking stability. These findings contribute to the theoretical foundation necessary for advancements in on-chip quantum computing, quantum communication, and integrated photonics.
doctoral thesis
34 73
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Korrelationseffekte angeregter Ladungsträger in Halbleiter-Nanostrukturen am Beispiel von InGaAs-Quantenpunkten und atomaren MoS2-Monolagen
(2014-11-10)
Steinhoff, Alexander
;
Wehling, Tim
;
Jahnke, Frank
;
Jahnke, Frank
Semiconductor nanostructures are applied in various electronic and optoelectronic devices. As miniaturization of these devices progresses, a microscopic treatment of correlations between excited carriers is essential for understanding and describing the governing physics. We investigate two different types of semiconductor nanostructures, which have each received considerable attention over the last years. These are self-assembled InGaAs quantum dots (QDs) on the one hand and atomic monolayers of MoS2 on the other hand. Self-assembled semiconductor QDs are used as active material in conventional lasers and as efficient non-classical light sources with applications in quantum information. As they can confine a small number of carriers in localized stats with discrete energies, it is questionable to neglect correlations between the carriers when describing their dynamics. We analyze the influence of carrier correlations in a single QD on Coulomb scattering processes, which are due to the contact with a quasi-continuum of wetting-layer (WL) states. Results obtained from a Boltzmann equation are compared with the fully correlated dynamics governed by a von-Neumann-Lindblad equation. In a first step, we take into account correlations generated by the exact treatment of Pauli blocking due to the contributing QD carrier configurations. Subsequently, we include correlations generated by energy renormalizations due to Coulomb interaction between the QD carriers. It is shown that at low WL carrier densities, neither Pauli correlations nor Coulomb correlations can be safely neglected, if the dynamics of single-particle states in the QD are to be predicted qualitatively and quantitatively. In the high-density regime, both types of correlations play a lesser role and thus a description of carrier dynamics by a Boltzmann equation becomes reliable. Furthermore, the efficiency of WL-assisted scattering processes as well as scattering-induced dephasing rates depending on the WL carrier density are discussed. Subsequently, experimental results for the carrier capture and relaxation dynamics in QDs are analyzed using a microscopic theory including also carrier-LO-phonon interaction. Time-resolved differential transmission changes of the QD transitions after ultrafast optical excitation of the barrier states are studied in a wide range of carrier temperatures and excitation densities. The measurements can be explained by QD polaron scattering and their excitation-dependent renormalization due to additional Coulomb scattering processes. Results of a configuration-picture and single-particle picture description, both with non-perturbative transition rates, show good agreement with the experiments while Boltzmann scattering rates lead to a different excitation density and temperature dependence. Monolayer MoS2 is the most prominent member of the class of two-dimensional transition-metal dichalcogenides and exhibits a direct band gap, making it a promising candidate for optical applications. We study the ground-state and finite-density optical response of MoS2 by solving the semiconductor Bloch equations, using ab-initio band structures and Coulomb interaction matrix elements. Spectra for excited carrier densities up to 10^13/cm^2 reveal a redshift of the excitonic ground-state absorption, whereas higher excitonic lines are found to disappear successively due to Coulomb-induced band-gap shrinkage of more than 500 meV and binding-energy reduction. Strain-induced band variations lead to a redshift of the lowest exciton line by approx. 110 meV/% and change the direct transition to indirect while maintaining the magnitude of the optical response.
doctoral thesis
308 139
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Carrier-Carrier and Carrier-Phonon Scattering in Self-Assembled Quantum Dots
(2005-10-05)
Nielsen, Torben Roland
;
Schlitzer, Reiner
;
Lemke, Peter
;
Jahnke, Frank
In this work theoretical investigations of the carrier-carrier interaction and the carrier-phonon interaction in self-assembled quantum dots are presented. Based on the nonequilibrium Green's function technique, equations of motion are derived for the one-particle Green's function. To demonstrate our method we focus on lens-shaped quantum dots grown on top of a wetting layer, using typical InGaAs and InGaN material parameters. For the carrier-carrier interaction, we study the carrier-carrier scattering processes leading to carrier capture into and relaxation inside the quantum dots. The corresponding scattering rates are evaluated within the Boltzmann approximation under quasi-equilibrium conditions. For the InGaAs material system we find at elevated carrier densities in the wetting layer that Coulomb scattering provides processes with capture (relaxation) times typically faster than 10ps (1ps). For the InGaN material system the combined influence of the quantum-confined Stark effect and many-body renormalizations is furthermore taken into account. The charge separation induced by the built-in electrostatic field has important consequences on the capture and relaxation rates. It is shown that its main effect comes through the renormalization of the energies of the states involved in the collisions, and leads to an increase in the scattering efficency. The carrier-phonon scattering is studied for the InGaAs material system. The interaction of carriers with longitudinal-optical (LO) phonons at the Boltzmann level predicts inefficient scattering (phonon bottleneck) when the transition energies of the quantum dot states do not match the LO-phonon energy. In contrast, we demonstrate that a quantum kinetic description of the carrier-phonon interaction supports experimental observed fast scattering processes.
doctoral thesis
338 198
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Theory for Light-Matter Interaction in Semiconductor Quantum Dots
(2008-07-17)
Gies, Christopher
;
Knorr, Andreas
;
Jahnke, Frank
;
Jahnke, Frank
A microscopic theory is developed and applied to describe luminescence from semiconductor quantum dots (QDs). The radiative emission dynamics is studied by the investigation of time-resolved photoluminescence. Special emphasis is placed on the role of carrier correlations and the differences between QDs and atoms. From the most general form of the theory a laser model for QDs in microresonators is developed, which is the central achievement of this thesis. In this model semiconductor effects can be included in a consistent manner. Going beyond the rate equation limit, we calculate the first- and second-order correlation functions to characterize the laser threshold properties, which are, in the classical sense, no longer well defined in current state-of-the-art microcavity lasers with high spontaneous emission coupling into the laser mode. To underline the close connection to applications, all results are presented together with results from recent experiments. To gain a deeper understanding of the derived laser theory and of the difference between QDs and atoms, a detailed comparison with quantum-optical models is performed, namely with the rate equations, a master equation approach and the Liouville/von-Neumann equation for the full density matrix.
doctoral thesis
709 504

Jahnke, Frank

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