Exciton lifetime and emission polarization dispersion in strongly in-plane asymmetric nanostructures

We present experimental and theoretical investigation of exciton recombination dynamics and the related polarization of emission in highly in-plane asymmetric nanostructures. Considering general asymmetry- and size-driven effects, we illustrate them with a detailed analysis of InAs/AlGaInAs/InP elongated quantum dots. These offer a widely varied confinement characteristics tuned by size and geometry that are tailored during the growth process, which leads to emission in the application-relevant spectral range of 1.25-1.65 {\mu}m. By exploring the interplay of the very shallow hole confining potential and widely varying structural asymmetry, we show that a transition from the strong through intermediate to even weak confinement regime is possible in nanostructures of this kind. This has a significant impact on exciton recombination dynamics and the polarization of emission, which are shown to depend not only on details of the calculated excitonic states but also on excitation conditions in the photoluminescence experiments. We estimate the impact of the latter and propose a way to determine the intrinsic polarization-dependent exciton light-matter coupling based on kinetic characteristics.Comment: 11 pages, 8 figure

Excitonic fine structure and binding energies of excitonic complexes in single InAs quantum dashes

P.M., J.M. and G. S. acknowledge support from the grant of National Science Centre of Poland No. 2011/02/A/ST3/00152 (Maestro), whereas M.Z. acknowledges support from the Polish National Science Centre under grant No. 2015/18/E/ST3/005 (Sonata Bis). The experiments have partially been performed within the Wroclaw University of Science and Technology laboratory infrastructure financed by the Polish Ministry of Science and Higher Education Grant No. 6167/IA/119/2012.The fundamental electronic and optical properties of elongated InAs nanostructures embedded in quaternary InGaAlAs barrier are investigated by means of high-resolution optical spectroscopy and many-body atomistic tight-binding theory. These wire-like shaped self-assembled nanostructures are known as quantum dashes and are typically formed during the molecular beam epitaxial growth on InP substrates. In this work we study properties of excitonic complexes confined in quantum dashes emitting in a broad spectral range from below 1.2 to 1.55 μm. We find peculiar trends for the biexciton and negative trion binding energies, with pronounced trion binding in smaller size quantum dashes. These experimental findings are then compared and qualitatively explained by atomistic theory. The theoretical analysis shows a fundamental role of correlation effects for the absolute values of excitonic binding energies. Eventually, we determine the bright exciton fine structure splitting (FSS), where both the experiment and theory predict a broad distribution of the splitting varying from below 50 to almost 180 μeV. We identify several key factors determining the FSS values in such nanostructures including quantum dash size variation and composition fluctuations.PostprintPeer reviewe

Exciton lifetime and emission polarization dispersion in strongly in-plane asymmetric nanostructures

The work was supported by the Grant No. 2011/02/A/ST3/00152 from the Polish National Science Centre (Narodowe Centrum Nauki). K. G. acknowledges support by the Grant No. 2014/12/B/ST3/04603 from the Polish National Science Centre (Narodowe Centrum Nauki). S. H. acknowledges support from the State of Bavaria in Germany.We present experimental and theoretical investigation of exciton recombination dynamics and the related polarization of emission in highly in-plane asymmetric nanostructures. Considering general asymmetry- and size-driven effects, we illustrate them with a detailed analysis of InAs/AlGaInAs/InP elongated quantum dots. These offer a widely varied confinement characteristics tuned by size and geometry that are tailored during the growth process, which leads to emission in the application-relevant spectral range of 1.25-1.65 μm. By exploring the interplay of the very shallow hole confining potential and widely varying structural asymmetry, we show that a transition from the strong through intermediate to even weak confinement regime is possible in nanostructures of this kind. This has a significant impact on exciton recombination dynamics and the polarization of emission, which are shown to depend not only on details of the calculated excitonic states but also on excitation conditions in the photoluminescence experiments. We estimate the impact of the latter and propose a way to determine the intrinsic polarization-dependent exciton light-matter coupling based on kinetic characteristics.PostprintPeer reviewe

Confinement regime in self-assembled InAs/InAlGaAs/InP quantum dashes determined from exciton and biexciton recombination kinetics

This research was supported by the National Science Center of Poland within Grant No. 2011/02/A/ST3/00152. Ł.D. acknowledges the financial support from the Foundation for Polish Science within the START fellowship. The experiments have partially been performed within the Wrocław University of Science and Technology laboratory infrastructure financed by the Polish Ministry of Science and Higher Education Grant No. 6167/IA/119/2012.The exciton and biexciton confinement regimes in strongly anisotropic epitaxial InAs nanostructures called quantum dashes (QDashes) embedded in an In0.53Ga0.23Al0.24As matrix, which is lattice-matched to InP(001) substrate, have been investigated. For that purpose, we have performed low-temperature spatially and polarization-resolved photoluminescence and time-resolved photoluminescence measurements on a set of single QDashes. The main conclusions are drawn based on the experimentally obtained distribution of the ratio between the exciton and biexciton lifetimes. We have found that a majority of QDashes for which the abovementioned ratio falls into the range of 1.2 ± 0.1-1.6 ± 0.1 corresponds to the so called intermediate confinement regime, whereas for several cases, it is close to 1 or 2, suggesting reaching the conditions of weak and strong confinement, respectively. Eventually, we support this data with dependence of the lifetimes' ratio on the biexciton binding energy, implying importance of Coulomb correlations, which change significantly with the confinement regime.Publisher PDFPeer reviewe

Directional control of weakly localized Raman from a random network of fractal nanowires

Disordered optical media are an emerging class of materials capable of strongly scattering light. Their study is relevant to investigate transport phenomena and for applications in imaging, sensing and energy storage. While such materials can be used to generate coherent light, their directional emission is typically hampered by their very multiple scattering nature. Here, we tune the out-of-plane directionality of coherent Raman light scattered by a fractal network of silicon nanowires. By visualizing Rayleigh scattering, photoluminescence and weakly localized Raman light from the random network of nanowires via real-space microscopy and Fourier imaging, we gain insight on the light transport mechanisms responsible for the material's inelastic coherent signal and for its directionality. The possibility of visualizing and manipulating directional coherent light in such networks of nanowires opens venues for fundamental studies of light propagation in disordered media as well as for the development of next generation optical devices based on disordered structures, inclusive of sensors, light sources and optical switches

Multi-dimensional modeling and simulation of semiconductor nanophotonic devices

Self-consistent modeling and multi-dimensional simulation of semiconductor nanophotonic devices is an important tool in the development of future integrated light sources and quantum devices. Simulations can guide important technological decisions by revealing performance bottlenecks in new device concepts, contribute to their understanding and help to theoretically explore their optimization potential. The efficient implementation of multi-dimensional numerical simulations for computer-aided design tasks requires sophisticated numerical methods and modeling techniques. We review recent advances in device-scale modeling of quantum dot based single-photon sources and laser diodes by self-consistently coupling the optical Maxwell equations with semiclassical carrier transport models using semi-classical and fully quantum mechanical descriptions of the optically active region, respectively. For the simulation of realistic devices with complex, multi-dimensional geometries, we have developed a novel hp-adaptive finite element approach for the optical Maxwell equations, using mixed meshes adapted to the multi-scale properties of the photonic structures. For electrically driven devices, we introduced novel discretization and parameter-embedding techniques to solve the drift-diffusion system for strongly degenerate semiconductors at cryogenic temperature. Our methodical advances are demonstrated on various applications, including vertical-cavity surface-emitting lasers, grating couplers and single-photon sources

Magnetic field control of the neutral and charged exciton fine structure in single quantum dashes emitting at 1.55 μ m

This research was supported by the Polish Ministry of Science and Higher Education/the National Science Center Grant No. 2011/02/A/ST3/00152. S.H. gratefully acknowledges support by the Royal Society and the Wolfson Foundation.We investigated the neutral and charged exciton fine structure in single InAs/InGaAlAs/InP quantum dashes emitting at 1.55 μm using polarization-resolved microphotoluminescence in a magnetic field. Inverted spin configuration of horizontally [1-10] and vertically [110] polarized transitions has been observed. An in-plane magnetic field of up to 5 Tesla has been applied to tailor the fine structure, and eventually to reduce the splitting of the bright exciton states down to zero. This inverted structure has been observed for all the investigated excitons, making it a characteristic feature for this class of nanostructures with the largest splitting reduction of 170 μeV.Publisher PDFPeer reviewe