A STRUCTURAL AND FUNCTIONAL STUDY OF STOMATA

Substomatal ion-adsorbent bodies are reported on here for the first time. A brief survey of the plant kingdom suggests that the structures are mainly restricted to the Commelinaceae and Filicales although analogous structures way occur in other plant groups. Microscopical studies indicate that the bodies have a solid external aspect, a hollow lumen, and are situated extracellularly. In Polypodium, the bodies are narrowly attached to the lower periclinal walls in the polar regions of the guard cell complex, whilst in Tradescantia they are located in the intercellular space between the poles of the complex and the adjacent subsidiary cells. In both genera, the body is covered by the endocuticle which can be distended into a substomatal sac by the body pressing against it. The endocuticle, in the immediate vicinity of the bodies, is modified into a series of hollow trabeculae which are considered to be important apoplastic pathways. The bodies are formed at an early stage of stomatal ontogeny from the migration of outer elements of the lower periclinal wall of the guard-cell mother-cell to both poles of the eventual guard cell complex. The walls of the body are believed to be highly pectinaceous and capable of adsorbing a wide variety of ions non-selectively. Preliminary X-ray microanalyses suggest that the bodies may be involved in potassium fluxes associated with stomatal movements. Ultrastructural studies of immature Polypodium guard cells resulted in the erection of a hypothetical model for stoma formation. Ontogenetic studies revealed a major anomaly in existing stomatal classifications which is rectified in a proposed new classification of stomatal types which is explicit a.t both ontogenetic and morphological levels. Previously unrecorded ontogenetic and morphological stomatal types are reported from Polypodium

Valley current characterization of high current density resonant tunnelling diodes for terahertz-wave applications

We report valley current characterisation of high current density InGaAs/AlAs/InP resonant tunnelling diodes (RTDs) grown by metal-organic vapour phase epitaxy (MOVPE) for THz emission, with a view to investigate the origin of the valley current and optimize device performance. By applying a dual-pass fabrication technique, we are able to measure the RTD I-V characteristic for different perimeter/area ratios, which uniquely allows us to investigate the contribution of leakage current to the valley current and its effect on the PVCR from a single device. Temperature dependent (20 – 300 K) characteristics for a device are critically analysed and the effect of temperature on the maximum extractable power (PMAX) and the negative differential conductance (NDC) of the device is investigated. By performing theoretical modelling, we are able to explore the effect of typical variations in structural composition during the growth process on the tunnelling properties of the device, and hence the device performance

Naar gerichter woonbeleid

De overheid rekent het tot haar taak het eigenwoningbezit te bevorderen en goede en betaalbare huisvesting te garanderen voor lage inkomens. De beleidsinstrumenten om deze doelstellingen te realiseren zijn daarop echter te weinig toegespitst en creëren verstoringen op de woonmarkt

Van bedrijfssparen naar verlofsparen

De afschaffing van het spaarloon is op inhoudelijke gronden wel te verdedigen. Dat de spaarloonfaciliteit plaats maakt voor een nieuwe ingewikkelde levensloopregeling is moeilijker te onderbouwen. Het is beter om de bestaande regelingen te hervormen. Eventueel kunnen ze worden gekoppeld aan een fiscale levensloopregeling die is gebaseerd op de omkeerregel

Study of electro-absorption effects in 1300nm In(Ga)As/GaAs quantum dot materials

No abstract available

GaAs-based Self-Aligned Stripe Superluminescent Diodes Processed Normal to the Cleaved Facet

We demonstrate GaAs-based superluminescent diodes (SLDs) incorporating a window-like back facet in a self-aligned stripe. SLDs are realised with low spectral modulation depth (SMD) at high power spectral density, without application of anti-reflection coatings. Such application of a window-like facet reduces effective facet reflectivity in a broadband manner. We demonstrate 30mW output power in a narrow bandwidth with only 5% SMD, outline the design criteria for high power and low SMD, and describe the deviation from a linear dependence of SMD on output power as a result of Joule heating in SLDs under continuous wave current injection. Furthermore, SLDs processed normal to the facet demonstrate output powers as high as 20mW, offering improvements in beam quality, ease of packaging and use of real estate. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only

Quantum well and dot self-aligned stripe lasers utilizing an InGaP optoelectronic confinement layer

We demonstrate and study a novel process for fabrication of GaAs-based self-aligned lasers based upon a single over-growth. A lattice-matched n-doped InGaP layer is utilized for both electrical and optical confinements. Single-lateral-mode emission is demonstrated initially from an In0.17Ga0.83 As double quantum well laser emitting similar to 980 nm. We then apply the fabrication technique to a quantum dot laser emitting similar to 1300 nm. Furthermore, we analyze the breakdown mechanism in our devices and discuss the limitations of index guiding in our structures

Simulation of Broad Spectral Bandwidth Emitters at 1060 nm for Optical Coherence Tomography

The simulation of broad spectral bandwidth light sources (semiconductor optical amplifiers (SOA) and superluminescent diodes (SLD)) for application in ophthalmic optical coherence tomography is reported. The device requirements and origin of key device parameters are outlined, and a range of single and double InGaAs/GaAs quantum well (QW) active elements are simulated with a view to application in different OCT embodiments. We confirm that utilising higher order optical transitions is beneficial for single QW SOAs, but may introduce deleterious spectral modulation in SLDs. We show how an addition QW may be introduced to eliminate this spectral modulation, but that this results in a reduction of the gain spectrum width. We go on to explore double QW structures where the roles of the two QWs are reversed, with the narrow QW providing long wavelength emission and gain. We show how this modification in the density of states results in a significant increase in gain-spectrum width for a given current. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only

Isotopic analysis of faunal material from South Uist, Western Isles, Scotland

This paper reports on the results from stable isotope analysis of faunal bone collagen from a number of Iron Age and later sites on the island of South Uist, in the Western Isles, Scotland. This preliminary investigation into the isotopic signatures of the fauna is part of a larger project to model the interaction between humans, animals, and the broader environment in the Western Isles. The results demonstrate that the island fauna data fall within the range of expected results for the UK, with the terrestrial herbivorous diets of cattle and sheep confi rmed. The isotopic composition for pigs suggests that some of these animals had an omnivorous diet, whilst a single red deer value might be suggestive of the consumption of marine foods, such as by grazing on seaweed. However, further analysis is needed in order to verify this anomalous isotopic ratio

Optimization of High Current Density Resonant Tunneling Diodes for Terahertz Emitters

We discuss the numerical simulation of high current density InGaAs/AlAs/InP resonant tunneling diodes with a view to their optimization for application as THz emitters. We introduce a figure of merit based upon the ratio of maximum extractable THz power and the electrical power developed in the chip. The aim being to develop high efficiency emitters as output power is presently limited by catastrophic failure. A description of the interplay of key parameters follows. We propose an optimized structure utilizing thin barriers paired with a comparatively wide quantum well