Systematic study of the influence of coherent phonon wave packets on the lasing properties of a quantum dot ensemble

Kohärente Phononen können die Licht-Materie-Wechselwirkung in Halbleiter Nanostrukturen stark ändern. Bei einem Ensemble von Quantenpunkten (QP) als aktivem Lasermedium sind Phononen im Stande, die Laserintensität deutlich zu verstärken oder abzuschwächen. Die Physik des gekoppelten Phonon-Exziton-Licht-Systems wird von verschiedenen Mechanismen dominiert, die im Experiment nicht eindeutig unterschieden werden können, da die komplizierte Probenstruktur zu einem komplexen Verspannungspuls führt, der auf das QP-Ensemble trifft. Hier zeigen wir durch eine umfassende theoretische Studie, wie die Laseremission durch Phononpulse verschiedener Form und QP-Ensembles verschiedener spektraler Verteilung beeinflusst wird. Dies erlaubt einen Einblick in die grundlegenden Wechselspiele des gekoppelten Gesamtsystems. Dadurch können wir zwischen zwei Mechanismen unterschieden: der adiabatischen Verschiebung des Ensembles und dem Schüttel-Effekt. Dies ebnet den Weg zu einer gezielten Kontrolle der Laser Emission durch kohärente Phononen.Coherent phonons can greatly vary light–matter interaction in semiconductor nanostructures placed inside an optical resonator on a picosecond time scale. For an ensemble of quantum dots (QDs) as active laser medium, phonons are able to induce a large enhancement or attenuation of the emission intensity, as has been recently demonstrated. The physics of this coupled phonon–exciton–light system consists of various effects, which in the experiment typically cannot be clearly separated, in particular, due to the complicated sample structure a rather complex strain pulse impinges on the QD ensemble. Here we present a comprehensive theoretical study how the laser emission is affected by phonon pulses of various shapes as well as by ensembles with different spectral distributions of the QDs. This gives insight into the fundamental interaction dynamics of the coupled phonon–exciton–light system, while it allows us to clearly discriminate between two prominent effects: the adiabatic shifting of the ensemble and the shaking effect. This paves the way to a tailored laser emission controlled by phonons.</p

Exploring coherence of individual excitons in InAs quantum dots embedded in natural photonic defects : influence of the excitation intensity

We acknowledge the financial support by the European Research Council (ERC) Starting Grant PICSEN (grant no. 306387)The exact optical response of quantum few-level systems depends crucially on the exact choice of the incoming pulse areas. We use four-wave mixing (FWM) spectroscopy to infer the coherent response and dynamics of single InAs quantum dots (QDs) and study their pulse area dependence. By combining atomic force microscopy with FWM hyperspectral imaging, we show that the retrieved FWM signals originate from individual QDs enclosed in natural photonic defects. The optimized light-matter coupling in these defects allows us to perform our studies in a wide range of driving field amplitudes. When varying the pulse areas of the exciting laser pulses Rabi rotations of microscopic interband coherences can be resolved by the two-pulse FWM technique. We investigate these Rabi coherence rotations within two- and three-level systems, both theoretically and experimentally, and explain their damping by the coupling to acoustic phonons. To highlight the importance of the pulse area in uence, we show that the phonon-induced dephasing of QD excitons depends on the pulse intensity.PostprintPeer reviewe

Entropy Dynamics of Phonon Quantum States Generated by Optical Excitation of a Two-Level System

In quantum physics, two prototypical model systems stand out due to their wide range of applications. These are the two-level system (TLS) and the harmonic oscillator. The former is often an ideal model for confined charge or spin systems and the latter for lattice vibrations, i.e., phonons. Here, we couple these two systems, which leads to numerous fascinating physical phenomena. Practically, we consider different optical excitations and decay scenarios of a TLS, focusing on the generated dynamics of a single phonon mode that couples to the TLS. Special emphasis is placed on the entropy of the different parts of the system, predominantly the phonons. While, without any decay, the entire system is always in a pure state, resulting in a vanishing entropy, the complex interplay between the single parts results in non-vanishing respective entanglement entropies and non-trivial dynamics of them. Taking a decay of the TLS into account leads to a non-vanishing entropy of the full system and additional aspects in its dynamics. We demonstrate that all aspects of the entropy's behavior can be traced back to the purity of the states and are illustrated by phonon Wigner functions in phase space

Theory of phonon sidebands in the absorption spectra of moir\'e exciton-polaritons

Excitons in twisted bilayers of transition metal dichalcogenides have strongly modified dispersion relations due to the formation of periodic moir\'e potentials. The strong coupling to a light field in an optical cavity leads to the appearance of moir\'e polaritons. In this paper, we derive a theoretical model for the linear absorption spectrum of the coupled moir\'e polariton-phonon system based on the time-convolutionless (TCL) approach. Results obtained by numerically solving the TCL equation are compared to those obtained in the Markovian limit and from a perturbative treatment of non-Markovian corrections. A key quantity for the interpretation of the findings is the generalized phonon spectral density. We discuss the phonon impact on the spectrum for realistic moir\'e exciton dispersions by varying twist angle and temperature. Key features introduced by the coupling to phonons are broadenings and energy shifts of the upper and lower polariton peak and the appearance of phonon sidebands between them. We analyze these features with respect to the role of Markovian and non-Markovian effects and find that they strongly depend on the twist angle. We can distinguish between the regimes of large, small, and intermediate twist angles. In the latter phonon effects are particularly pronounced due to dominating phonon transitions into regions which are characterized by van Hove singularities in the density of states

In search of the authentic nation: landscape and national identity in Canada and Switzerland

While the study of nationalism and national identity has flourished in the last decade, little attention has been devoted to the conditions under which natural environments acquire significance in definitions of nationhood. This article examines the identity-forming role of landscape depictions in two polyethnic nation-states: Canada and Switzerland. Two types of geographical national identity are identified. The first – what we call the ‘nationalisation of nature’– portrays zarticular landscapes as expressions of national authenticity. The second pattern – what we refer to as the ‘naturalisation of the nation’– rests upon a notion of geographical determinism that depicts specific landscapes as forces capable of determining national identity. The authors offer two reasons why the second pattern came to prevail in the cases under consideration: (1) the affinity between wild landscape and the Romantic ideal of pure, rugged nature, and (2) a divergence between the nationalist ideal of ethnic homogeneity and the polyethnic composition of the two societies under consideration

Beitrag zur Analyse des elektrischen Verhaltens von hoch-sperrenden rückwärts leitfähigen Insulated Gate Bipolar Transistoren

Rückwärts leitfähige IGBTs bilden einen neuen Typ von Leistungshalbleitern, welche die Funktionalität von IGBT und antiparallelen Dioden in einem Chip integrieren. Zur Prävention eines Snapbacks der Durchlassspannung besteht der Bi-mode Insulated Gate Bipolar Transistor aus einer Parallelschaltung eines konventionellen IGBTs und eines RC-IGBTs. Der Chipaufbau von rückwärts leitfähigen IGBT führt zu deutlichen Änderungen des elektrischen Verhaltens im Vergleich zu konventinellen IGBTs

Quantum dots as optimized chiral emitters for photonic integrated circuits

Chiral coupling, which allows directional interactions between quantum dots (QDs) and photonic crystal waveguide modes, holds promise for enhancing the functionality of quantum photonic integrated circuits. Elliptical polarizations of QD transitions offer a considerable enhancement in directionality. However, in epitaxial QD fabrication, the lack of precise control over lateral QD positions still poses a challenge in achieving efficient chiral interfaces. Here, we present a theoretical analysis in which we propose to optimize the polarization of a QD emitter against the spatially averaged directionality and demonstrate that the resulting emitter offers a considerable technological advantage in terms of the size and location of high-directionality areas of the waveguide as well as their overlap with the regions of large Purcell enhancement, thereby improving the scalability of the device. Moreover, using $\mathbf{\mathit{k}}\cdot\mathbf{\mathit{p}}$ modeling, we demonstrate that the optimal elliptical polarization can be achieved for neutral exciton transitions in a realistic QD structure. Our results present a viable path for efficient chiral coupling in QD-based photonic integrated circuits, to a large extent overcoming the challenges and limitations of the present manufacturing technology.Comment: Some text modifications in the Introduction, references added, typos corrected, Fig. 7 updated, and the title change