Experimental analysis of novel telecom source materials and devices

Incumbent telecommunication lasers emitting at 1.5 µm are fabricated on InP substrates and consist of multiple strained quantum well layers of the ternary alloy InGaAs, with barriers of InGaAsP or InGaAlAs. These lasers have been seen to exhibit very strong temperature dependence of the threshold current. This strong temperature dependence leads to a situation where external cooling equipment is required to stabilise the optical output power of these lasers. This results in a significant increase in the energy bill associated with telecommunications, as well as a large increase in equipment budgets. If the exponential growth trend of end user bandwidth demand associated with the internet continues, these inefficient lasers could see the telecommunications industry become the dominant consumer of world energy. For this reason there is strong interest in developing new, much more efficient telecommunication lasers. One avenue being investigated is the development of quantum dot lasers on InP. The confinement experienced in these low dimensional structures leads to a strong perturbation of the density of states at the band edge, and has been predicted to result in reduced temperature dependence of the threshold current in these devices. The growth of these structures is difficult due to the large lattice mismatch between InP and InAs; however, recently quantum dots elongated in one dimension, known as quantum dashes, have been demonstrated. Chapter 4 of this thesis provides an experimental analysis of one of these quantum dash lasers emitting at 1.5 µm along with a numerical investigation of threshold dynamics present in this device. Another avenue being explored to increase the efficiency of telecommunications lasers is bandstructure engineering of GaAs-based materials to emit at 1.5 µm. The cause of the strong temperature sensitivity in InP-based quantum well structures has been shown to be CHSH Auger recombination. Calculations have shown and experiments have verified that the addition of bismuth to GaAs strongly reduces the bandgap and increases the spin orbit splitting energy of the alloy GaAs1−xBix. This leads to a bandstructure condition at x = 10 % where not only is 1.5 µm emission achieved on GaAs-based material, but also the bandstructure of the material can naturally suppress the costly CHSH Auger recombination which plagues InP-based quantum-well-based material. It has been predicted that telecommunications lasers based on this material system should operate in the absence of external cooling equipment and offer electrical and optical benefits over the incumbent lasers. Chapters 5, 6, and 7 provide a first analysis of several aspects of this material system relevant to the development of high bismuth content telecommunication lasers.

(Table 8, page 38) Spectrographic analyses of deep sea manganese nodules from the Pacific and Indian Oceans

In an earlier paper by two of the authors the conclusion was reached that the 33 recognized species of oxides of Mn could be separated into 3 groups: 1) those which appeared to be persistently supergene in origin, 2) those which appeared to be persistently hypogene, and 3) those which were supergene in some localities and hypogene in other localities. When that paper was written, there were available about 250 X-ray diffraction analyses of mineral specimens, also 35 complete and about 150 partial chemical analyses. The conclusions of that paper were based upon the interpretation of the geologic conditions under which these specimens occurred. Late in the preparation of that paper, it seemed worthwhile to make numerous semiquantitative analyses of specimens, largely from 9 western [U.S.A] states, selected carefully from 5 groups of geologic environments, in the hope that the frequency and percentages of some elements might be distinctive of the several geologic groups. For this purpose, 95 specimens were selected from the 5 groups, as follows: 19 specimens interpreted as supergene oxides by the geologists who collected them, 35 specimens of hypogene vein oxides, 22 specimens of Mn-bearing hot spring aprons, 9 specimens of stratified oxides, and 10 specimens of deep-sea nodules. The spectrographic analyses here recorded indicate that a group of elements - W, Ba, Sr, Be, As, Sb, Tl, and Ge - are present more commonly, and largely in higher percentages, in the hypogene oxide than in the supergene oxides and thus serve to indicate different sources of the Mn. Also, the frequency and percentages of some of these elements indicate a genetic relation of the manganese oxides in hypogene veins, hot spring aprons, and stratified deposits. The analyses indicate a declining percentage of some elements from depth to the surface in these 3 related groups and increasing percentages of some other elements. It is concluded that some of the elements in deep-sea nodules indicate that sources other than rocks decomposed on the continents, probably vulcanism on the floors of the seas, have contributed to their formation.

San Francisco Coastal System grain size and geochemical data for sand provenance study

Over 150 million cubic meter of sand-sized sediment has disappeared from the central region of the San Francisco Bay Coastal System during the last half century. This enormous loss may reflect numerous anthropogenic influences, such as watershed damming, bay-fill development, aggregate mining, and dredging. The reduction in Bay sediment also appears to be linked to a reduction in sediment supply and recent widespread erosion of adjacent beaches, wetlands, and submarine environments. A unique, multi-faceted provenance study was performed to definitively establish the primary sources, sinks, and transport pathways of beach sized-sand in the region, thereby identifying the activities and processes that directly limit supply to the outer coast. This integrative program is based on comprehensive surficial sediment sampling of the San Francisco Bay Coastal System, including the seabed, Bay floor, area beaches, adjacent rock units, and major drainages. Analyses of sample morphometrics and biological composition (e.g., Foraminifera) were then integrated with a suite of tracers including 87Sr/86Sr and 143Nd/144Nd isotopes, rare earth elements, semi-quantitative X-ray diffraction mineralogy, and heavy minerals, and with process-based numerical modeling, in situ current measurements, and bedform asymmetry to robustly determine the provenance of beach-sized sand in the region.

Table 2. The element content of manganese crust

The terrains in the South China Sea were apart of the Southeast China continent, and their rift-departing process dominated the formation and evolution of the South China Sea. The survey results of topography and paleoenvironment of the northern South China Sea during SO-49 cruise demonstrate that the terrains rift-departed from the South China continent before early Eocene.

Mineral and chemical compositions of a ferromanganese nodule from Station DM41-3911-76

Processes of authigenic manganese ore formation in sediments of the North Equatorial Pacific are considered on the basis of a study of the surface layer (<2 mm) of a ferromanganese nodule and four micronodule size fractions from associated surface sediment (0-7 cm). Inhomogeneity of nodule composition is shown. Mn/Fe ratio is maximal in samples from lateral sectors of the nodule at the water-sediment interface. Compositional differences of nodules are related to preferential accumulation of trace elements in iron oxyhydroxides (P, Sr, Pb, U, Bi, Th, Y, and REE), manganese hydroxides (Co, Ni, Cu, Zn, Cd, Mo, Tl, W), and lithogenic component trapped during nodule growth (Ga, Rb, Ba, and Cs). Ce accumulation in the REE composition is maximal in the upper and lower parts of the nodule characterized by minimal Mn/Fe values. A compositional comparison of manganese micronodules and surface layers of the nodule demonstrates that micronodule material was subjected to more intense reworking during diagenesis of sediments. The micronodules are characterized by higher Mn/Fe and P/Fe, but lower Ni/Cu and Co/Ni ratios. The micronodules and nodules do not differ in terms of contents of Ce and Th that are the least mobile elements during diagenesis. Differences in chemical composition of the micronodules and nodules are related not only to additional input of Mn in the process of diagenesis, but also to transformation of iron oxyhydroxides after removal of Mn from the close association with Fe formed in suspended matter during sedimentation.

Sedimentology of a profile from Etoliko Lagoon, Western Greece

A key feature of Greece is the large amount of historical and archaeological records. The sedimentary record of the Etoliko Lagoon, Aetolia, Western Greece, offers an ideal opportunity to study human-environment interaction and to disentangle natural and anthropogenic imprints in the sedimentary record. By applying an interdisciplinary approach of combining geoscientific methods (XRF, LOI, grain size analysis) with archaeological and historical records, the 8.8 m long sedimentary sequence ETO1C reveals the palaeoenvironmental history of the lagoon and its catchment since 11,670 cal BP. With a thorough chronology based on 14C age-depth-modelling including varve counting, different evolutionary stages were put in a chronological context. These stages include a lake period (11,670-8310 cal BP) followed by a period of sporadic saltwater intrusion (8310-1350 cal BP) as a result of continuing transgression. Phases of limnic predominance associated with freshwater inflow of episodically activated distributaries (around 5230 cal BP) still occurred. By 1350 cal BP, ongoing sea level rise had connected the lagoons of Etoliko and Messolonghi and freshwater influence had ceased. With the onset of settlement activity in the Late Helladic (1700-1100 cal BC) humans took advantage of the prevailing environmental landscape. A sudden increase in coarse sedimentation correlates with the history of human occupation with its peak of prosperity from the Late Helladic until the end of the Hellenistic Period (30 cal BC).

Element concentrations of bulk sediment from the Southern Cape Basin

High-resolution records of Ca and Sr were obtained from shipboard XRF analyses of bulk sediments in five gravity cores from the southern Cape Basin, South Atlantic Ocean. Sr/Ca ratios display regular glacial/interglacial variations of 14-40% and reveal a close correlation with the SPECMAP record, minimum Sr/Ca ratios appearing during glacial (delta18 O) maxima, distinct increases during periods of deglaciation, and highest ratios in interstadials. Shifts in carbonate-producing phytoplankton and/or zooplankton assemblages over glacial/interglacial cycles are suggested to be the main cause for the observed variations in Sr/Ca patterns. Quick assessment of the relationship between Sr/Ca ratios and the SPECMAP record made it possible to easily transfer an age model to the newly collected cores already during the cruise.