Polarization features of optically pumped CdS nanowire lasers

High quality CdS nanowires suspended in air were optically pumped both below and above the lasing threshold. The polarization of the pump laser was varied while emission out of the end facet of the nanowire was monitored in a 'head-on' measurement geometry. Highest pump-efficiency and most efficient absorption of the pump radiation are demonstrated for an incident electric field being polarized parallel to the nanowire axis. This polarization dependence, which was observed both above the lasing threshold and in the regime of amplified spontaneous emission, is caused by an enhanced absorption for parallel polarized optical pumping. Measured Stokes parameters of the nanowire emission reveal that due to the onset of lasing the degree of polarization rapidly increases from approximately 15% to 85%. Both, Stokes parameters and degree of polarization of the nanowire lasing emission are independent of the excitation polarization. The transversal lasing mode is therefore not notably affected by the polarization of the pumping beam, although the supply with optical gain is significantly enhanced for an excitation polarization parallel to the nanowire axis

Determination of secondary ion mass spectrometry relative sensitivity factors for polar and non-polar ZnO

Zinc oxide (ZnO) is regarded as a promising material for optoelectronic devices, due to its electronic properties. Solely, the difficulty in obtaining p-type ZnO impedes further progress. In this connection, the identification and quantification of impurities is a major demand. For quantitative information using secondary ion mass spectrometry (SIMS), so-called relative sensitivity factors (RSF) are mandatory. Such conversion factors did not yet exist for ZnO. In this work, we present the determined RSF values for ZnO using primary (ion implanted) as well as secondary (bulk doped) standards. These RSFs have been applied to commercially available ZnO substrates of different surface termination (a-plane, Zn-face, and O-face) to quantify the contained impurities. Although these ZnO substrates originate from the same single-crystal, we observe discrepancies in the impurity concentrations. These results cannot be attributed to surface termination dependent RSF values for ZnO

Lasing and ion beam doping of semiconductor nanowires

Semiconductor nanowires exhibit extraordinary optical properties like highly localized light emission, efficient waveguiding and light amplification. Even the stimulation of laser oscillations can be achieved at optical pumping, making nanowires promising for optoelectronic applications. For successful integration into future devices, three major key challenges have to be faced: 1) the understanding of the fundamental properties, 2) the modification of the emission characteristics and 3) the investigation of the efficiency-limiting factors. All key challenges are addressed in this thesis: 1) The fundamental properties of CdS nanowire have been investigated to uncover the size limits for photonic nanowire lasers. Laser oscillations were observed at room temperature and the emission characteristics were correlated to the morphology, which allowed the determination of a minimum diameter and length necessary for lasing. 2) The emission characteristics of ZnO nanowires have been successfully modified by ion beam doping with Co. The structural investigations revealed a good recovery of the ion induced damage in the crystal lattice. Optical activation of the implanted Co ions was achieved and an intense intra-3d-emission confirmed successful modification. 3) The temporal decay of excited luminescence centers strongly depends on the interplay of luminescent ions and defects, thus offering an approach to investigate the efficiency-limiting processes. Mn implanted ZnS nanowires were investigated, as the temporal decay of the incorporated Mn ions can be described by a Förster energy transfer model modified for nanostructures. The defect concentration was varied systematically by several approaches and the model could successfully fit the transients in all cases. The emission properties of Tb implanted ZnS nanowires were investigated and the temporal decay of the intra-4f-emission could also be fitted by the model, proving its accuracy for an additional element.Halbleiter-Nanodrähte besitzen außergewöhnliche Eigenschaften wie hochlokalisierte Lichtemission, exzellente Wellenleitung und Lichtverstärkung. Sogar Laseroszillationen können unter optischer Anregung stimuliert werden, wodurch Nanodrähte vielversprechend für optoelektronische Anwendungen sind. Für eine zukünftige Integration als Bauteile sind jedoch drei Herausforderungen zu überwinden: 1) Verständnis der fundamentalen Eigenschaften, 2) Modifikation der Emissionscharakteristik und 3) Untersuchung der Effizienz-limitierenden Faktoren. Alle Herausforderungen werden in dieser Arbeit angesprochen: 1) Die fundamentalen Eigenschaften anhand von CdS Nanodrähten untersucht. Laseroszillationen konnten bei Raumtemperatur beobachtet werden. Durch die Korrelation der Emissionscharakteristik mit der Morphologie konnten das Minimum des Durchmesser und der Länge für photonische Nanolaser bestimmt werden. 2) Die Emissionseigenschaften von ZnO Nanodrähten wurden gezielt über Ionenstrahl-Dotierung mit Co modifiziert. Strukturelle Untersuchungen zeigten ein gutes Ausheilen des strahleninduzierten Schaden im Kristallgitter. Die eingebrachten Co-Ionen konnten erfolgreich optisch aktiviert werden und zeigen eine intensive intra-3d-Emission. 3) Das zeitliche Abklingen der angeregten Leuchtzentren hängt stark von der Interaktion dieser mit Defekten ab, so dass darüber die Untersuchung der Effizienz-limitierenden Faktoren ermöglicht wird. Das zeitliche Abklingen von Mn in ZnS Nanodrähten wurde untersucht und die Transienten können mit einem modifizierten Förster-Energietransfer-Modell beschrieben werden. Die Defektkonzentration wurde systematisch durch mehrere Ansätze variiert. Das Modell konnte die Messdaten in allen Fällen gut wiedergeben. Die Emissionseigenschaften Tb implantierter ZnS Nanodrähte wurden charakterisiert und das zeitliche Abklingen der intra-4f-Emission kann ebenfalls mithilfe des Modells beschrieben werden, wodurch dieses für ein weiteres Element bestätigt wird

Defect induced changes on the excitation transfer dynamics in ZnS/Mn nanowires

Transients of Mn internal 3d5 luminescence in ZnS/Mn nanowires are strongly non-exponential. This non-exponential decay arises from an excitation transfer from the Mn ions to so-called killer centers, i.e., non-radiative defects in the nanostructures and is strongly related to the interplay of the characteristic length scales of the sample such as the spatial extensions, the distance between killer centers, and the distance between Mn ions. The transients of the Mn-related luminescence can be quantitatively described on the basis of a modified Förster model accounting for reduced dimensionality. Here, we confirm this modified Förster model by varying the number of killer centers systematically. Additional defects were introduced into the ZnS/Mn nanowire samples by irradiation with neon ions and by varying the Mn implantation or the annealing temperature. The temporal behavior of the internal Mn2+ (3d5) luminescence is recorded on a time scale covering almost four orders of magnitude. A correlation between defect concentration and decay behavior of the internal Mn2+ (3d5) luminescence is established and the energy transfer processes in the system of localized Mn ions and the killer centers within ZnS/Mn nanostructures is confirmed. If the excitation transfer between Mn ions and killer centers as well as migration effects between Mn ions are accounted for, and the correct effective dimensionality of the system is used in the model, one is able to describe the decay curves of ZnS/Mn nanostructures in the entire time window

Carbon and Environmental Labelling of Food Products: Insights into the Data on Display

The food system has been in focus as one of the major drivers behind the environmental and climate crisis. In this context, there is a growing need for more transparent and reliable information on the environmental impacts of food production and consumption as part of the transition process towards more sustainable food systems. Stakeholders along the food supply chain are confronted with multiple requirements and systems as the demand for environmental reporting at the product, company, and country level increases all at the same time. Simultaneously, consumers are often more interested in the sustainability of the food products they consume. While there is currently a lack of coherent supranational or even national legislation regulating methodological procedures, private initiatives for the environmental and carbon labelling of food products have developed rapidly. This article finds that most labels are characterised by a lack of transparency, clarity, and comprehensibility. Examining 14 labels, mainly from the German food retail market, we found a puzzling variety of data sources and methodologies used to calculate the values and claims displayed. We highlight this variety in data sources and footprint values by looking at milk and beef as case studies

Ultrafast Dynamics of Lasing Semiconductor Nanowires

Semiconductor nanowire lasers operate at ultrafast timescales; here we report their temporal dynamics, including laser onset time and pulse width, using a double pump approach. Wide bandgap gallium nitride GaN , zinc oxide ZnO , and cadmium sul amp; 64257;de CdS nanowires reveal laser onset times of a few picoseconds, driven by carrier thermalization within the optically excited semiconductor. Strong carrier amp; 8722;phonon coupling in ZnO leads to the fastest laser onset time of amp; 8764;1 ps in comparison to CdS and GaN exhibiting values of amp; 8764;2.5 and amp; 8764;3.5 ps, respectively. These values are constant between nanowires of di amp; 64256;erent sizes implying independence from any optical in amp; 64258;uences. However, we demonstrate that the lasing onset times vary with excitation wavelength relative to the semiconductor band gap. Meanwhile, the laser pulse widths are dependent on the optical system. While the fastest ultrashort pulses are attained using the thinnest possible nanowires, a sudden change in pulse width from amp; 8764;5 to amp; 8764;15 ps occurs at a critical nanowire diameter. We attribute this to the transition from single to multimode waveguiding, as it is accompanied by a change in laser polarization