Quantum well polaritons : strong and weak coupling regimes

The work described in this thesis is a theoretical investigation of the properties of exciton-polaritons in quantum wells (QWs). The polariton effect is first studied in the case of a completely coherent interaction between QW excitons and bulk photons, i.e. in the so called strong coupling limit. Then, an incoherent damping rate for the exciton states is included and the resulting modifications in the polariton dispersion are analyzed. A microscopic model which accounts for the scattering of QW excitons by random impurities is also proposed. In the strong coupling limit, a definitive and correct description of the QW polariton dispersion, for both confined and radiative modes, is obtained when the exciton- photon coupling is treated non perturbatively. A self-consistent perturbation theory which qualitatively agrees with the obtained results is also formulated. With increasing the incoherent damping, the orthogonality between radiative and confined polariton states is not affected, but a phase transition from the strong coupling regime to a weak coupling one occurs for both modes. The crossover between the two regimes is attributed to a topological change of the polariton dispersion curves when the damping rate reaches a critical value. A microscopic approach dealing with scattering of excitons by random impurities is formulated in terms of a quadratic Hamiltonian for QW excitons, bulk photons and localised impurities. By analyzing the preliminary results based on the calculation of the relevant eigenstates, the mixing between radiative and confined modes is observed.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Quantum well polaritons: Strong and weak coupling regimes

The work described in this thesis is a theoretical investigation of the properties of exciton-polaritons in quantum wells (QWs). The polariton effect is first studied in the case of a completely coherent interaction between QW excitons and bulk photons, i.e. in the so called strong coupling limit. Then, an incoherent damping rate for the exciton states is included and the resulting modifications in the polariton dispersion are analyzed. A microscopic model which accounts for the scattering of QW excitons by random impurities is also proposed. In the strong coupling limit, a definitive and correct description of the QW polariton dispersion, for both confined and radiative modes, is obtained when the exciton- photon coupling is treated non perturbatively. A self-consistent perturbation theory which qualitatively agrees with the obtained results is also formulated. With increasing the incoherent damping, the orthogonality between radiative and confined polariton states is not affected, but a phase transition from the strong coupling regime to a weak coupling one occurs for both modes. The crossover between the two regimes is attributed to a topological change of the polariton dispersion curves when the damping rate reaches a critical value. A microscopic approach dealing with scattering of excitons by random impurities is formulated in terms of a quadratic Hamiltonian for QW excitons, bulk photons and localised impurities. By analyzing the preliminary results based on the calculation of the relevant eigenstates, the mixing between radiative and confined modes is observed

Microcavity polariton-like dispersion doublet in resonant Bragg gratings

Periodic structures resonantly coupled to excitonic media allow the existence of extra intragap modes ('Braggoritons'), due to the coupling between Bragg photon modes and 3D bulk excitons. This induces unique and unexplored dispersive features, which can be tailored by properly designing the photonic bandgap around the exciton resonance. We report that one-dimensional Braggoritons realized with semiconductor gratings have the ability to mimic the dispersion of quantum-well microcavity polaritons. This will allow the observation of new nonlinear phenomena, such as slow-light-enhanced nonlinear propagation and an efficient parametric scattering at two 'magic frequencies'

Studying Light-Harvesting Models with Superconducting Circuits

The process of photosynthesis, the main source of energy in the animate world, converts sunlight into chemical energy. The surprisingly high efficiency of this process is believed to be enabled by an intricate interplay between the quantum nature of molecular structures in photosynthetic complexes and their interaction with the environment. Investigating these effects in biological samples is challenging due to their complex and disordered structure. Here we experimentally demonstrate a new approach for studying photosynthetic models based on superconducting quantum circuits. In particular, we demonstrate the unprecedented versatility and control of our method in an engineered three-site model of a pigment protein complex with realistic parameters scaled down in energy by a factor of $10^5$. With this system we show that the excitation transport between quantum coherent sites disordered in energy can be enabled through the interaction with environmental noise. We also show that the efficiency of the process is maximized for structured noise resembling intramolecular phononic environments found in photosynthetic complexes.Comment: 8+12 pages, 4+12 figure

Programming Light-Harvesting Efficiency Using DNA Origami.

The remarkable performance and quantum efficiency of biological light-harvesting complexes has prompted a multidisciplinary interest in engineering biologically inspired antenna systems as a possible route to novel solar cell technologies. Key to the effectiveness of biological "nanomachines" in light capture and energy transport is their highly ordered nanoscale architecture of photoactive molecules. Recently, DNA origami has emerged as a powerful tool for organizing multiple chromophores with base-pair accuracy and full geometric freedom. Here, we present a programmable antenna array on a DNA origami platform that enables the implementation of rationally designed antenna structures. We systematically analyze the light-harvesting efficiency with respect to number of donors and interdye distances of a ring-like antenna using ensemble and single-molecule fluorescence spectroscopy and detailed Förster modeling. This comprehensive study demonstrates exquisite and reliable structural control over multichromophoric geometries and points to DNA origami as highly versatile platform for testing design concepts in artificial light-harvesting networks.A. W. C. acknowledges support from the Winton Programme for the Physics of Sustainability. U. F. K. was partly supported by an ERC starting grant (PassMembrane, EY 261101). E. A.H. acknowledges support from Janggen-Pöhn Stiftung and the Schweizerischer Nationalfonds (SNF). P. T. acknowledges support by a starting grant (SiMBA, EU 261162) of the European Research Council (ERC). B. W. gratefully acknowledges support by the Braunschweig International Graduate School of Metrology B-IGSM and the DFG Research Training Group GrK1952/1 ‘Metrology for Complex Nanosystems’. P. M. thankfully acknowledges the support of the EPSRC Centre for Doctoral Training in Sensor Technologies and Applications EP/L015889/1.This is the final version of the article. It first appeared from ACS via https://doi.org/10.1021/acs.nanolett.5b0513

Quantum theory of spontaneous emission in multilayer dielectric structures

We present a fully quantum-electrodynamical formalism suitable to evaluate the spontaneous emission rate and pattern from a dipole embedded in a non-absorbing and lossless multilayer dielectric structure. In the model here developed the electromagnetic field is quantized by a proper choice of a complete and orthonormal set of classical spatial modes, which consists of guided and radiative (partially and fully) states. In particular, by choosing a set of radiative states characterized by a single outgoing component, we get rid of the problem related to the quantum interference between different outgoing modes, which arises when the standard radiative basis is used to calculate spontaneous emission patterns. After the derivation of the local density of states, the analytical expressions for the emission rates are obtained within the framework of perturbation theory. First we apply our model to realistic Silicon-based structures such as a single Silicon/air interface and a Silicon waveguide in both the symmetric and asymmetric configurations. Then, we focus on the analysis of the spontaneous emission process in a silicon-on-insulator (SOI) Slot waveguide (a 6 layers model structure) doped with erbium ions (emitting at the telecom wavelength). In this latter case we find a very good agreement with the experimental evidence [M. Galli et al., Appl. Phys. Lett. 89, 241114 (2006)] of an enhanced TM/TE photoluminescence signal. Hence, this model is relevant to study the spontaneous emission in Silicon-based multilayer structures which nowadays play a fundamental role for the development of highly integrated multifunctional devices.Comment: 36 pages, 13 figure

Learning exact enumeration and approximate estimation in deep neural network models

A system for approximate number discrimination has been shown to arise in at least two types of hierarchical neural network models—a generative Deep Belief Network (DBN) and a Hierarchical Convolutional Neural Network (HCNN) trained to classify natural objects. Here, we investigate whether the same two network architectures can learn to recognise exact numerosity. A clear difference in performance could be traced to the specificity of the unit responses that emerged in the last hidden layer of each network. In the DBN, the emergence of a layer of monotonic ‘summation units’ was sufficient to produce classification behaviour consistent with the behavioural signature of the approximate number system. In the HCNN, a layer of units uniquely tuned to the transition between particular numerosities effectively encoded a thermometer-like ‘numerosity code’ that ensured near-perfect classification accuracy. The results support the notion that parallel pattern-recognition mechanisms may give rise to exact and approximate number concepts, both of which may contribute to the learning of symbolic numbers and arithmetic

Modification of erbium radiative lifetime in planar silicon slot waveguides

Nel lavoro vengono determinati i tempi di vita radiativi e non radiativi dello ione Erbio in guide d'onda slot con strato sottile di SiO2 in un core di silicio. I risultati sono ottenuti combinando esperimenti con una trattazione teorica quantistica. The authors report a systematic study of the lifetime of the 1.54 micron transition of Er3+-doped SiO2 thin film as active material in planar slot waveguides in polycrystalline silicon. The lifetime shows a strong reduction when compared with values measured in three other configurations. The experimental results, combined with a rigorous quantum-electrodynamical formalism, are consistent with a sizable increase in both the radiative and nonradiative decay rates of Er3+ transition in slot waveguide. The radiative efficiency is only slightly reduced with respect to Er3+ in the bulk oxide, this result being important for future realization of Si-compatible active optical devices