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The Cosmic Background Imager (CBI) is an instrument designed to make images of the cosmic microwave background radiation and to measure its statistical properties on angular scales from about 3 arc minutes to one degree (spherical harmonic scales from l ˜ 4250 down to l ˜ 400). The CBI is a 13-element interferometer mounted on a 6 meter platform operating in ten 1-GHz frequency bands from 26 GHz to 36 GHz. The instantaneous field of view of the instrument is 45 arcmin (FWHM) and its resolution ranges from 3 to 10 arcmin; larger fields can be imaged by mosaicing. At this frequency and resolution, the primary foreground is due to discrete extragalactic sources, which are monitored at the Owens Valley Radio Observatory and subtracted from the CBI visibility measurements.
The instrument has been making observations since late 1999 of both primordial CMB fluctuations and the Sunyaev-Zeldovich effect in clusters of galaxies from its site at an altitude of 5080 meters near San Pedro de Atacama, in northern Chile. Observations will continue until August 2001 or later. We present preliminary results from the first few months of observations.
The presence of piezoelectric fields within p-i-n GaN/InGaN multiple quantum well structures is discussed. Time integrated and time-resolved photoluminescence measurements and theoretical calculations of the effect of these fields is presented. Furthermore, a description of how these fields influence the carrier dynamics and a discussion of how the piezoelectric field effects the design of GaN/InGaN devices is presented.
Excavations at the Easton Down long barrow were part of a wider programme of research into the Neolithic sequence and context of the Avebury area in north Wiltshire. The short barrow, on high chalk downland to the south-west of Avebury and the upper Kennet valley, and containing only a few inhumations according to Thurnam's 19th-century investigation, dates to the later 4th millennium BC. Test pits around the barrow produced very little struck flint, and virtually no colluvium in the adjacent dry valley to the west. The mound covered a thin calcareous turfline above a rubbly soil, probably formerly cultivated. The pre-barrow molluscan fauna, soil micromorphology and other environmental data indicate a clearance adjacent to woodland. In the secondary fill of the flanking ditches there is a succession from renewed woodland to open conditions in the Late Neolithic.
The Easton Down monument falls relatively late in the regional sequence of long barrow construction. Its setting was probably one of scattered, non-permanent clearances in woodland. Woodland was still widespread on the higher downland of the region in the middle of the Neolithic. Renewed and bigger-scale clearance towards the end of the Neolithic may be connected with the construction of very large monuments elsewhere in the region. The later prehistoric landscape became both more open and less diverse.
The University Appointments Boards exist to give advice and information about careers to Cambridge men and women and to act as a link between undergraduates and employers. It is therefore essential that the Boards' officers should know something of what has happened to students after they have started work in their various callings and professions.
In the period immediately preceding the Second World War the men's Board made plans for a ‘follow-up’ survey for this purpose. But before it could be carried out the war intervened and interrupted the careers of those who might have contributed relevant information. The original plans had to be drastically modified, but a different study was made which is embodied in the publication ‘University Education and Business’ issued in 1946.
In the post-war years the Boards were able to elicit piecemeal information from employers and from graduates about the progress of Cambridge recruits in many occupations. But the Boards felt that it would be desirable to arrange for a more systematic survey of what has actually happened to undergraduates who left the University some years ago. Hence the present survey. The Boards have not supervised the project in detail, but have given their approval to the general lines on which it has been conducted.
It cannot, of course, be assumed either that a survey of a different cross-section of undergraduates (i.e. those who left the University at times different from those covered by the survey) would necessarily show the same pattern or that the pattern of the past will necessarily be reproduced in the future. But the outcome of the survey should enable the Boards' officers—and indeed Tutors and other teaching officers who advise undergraduates—to assess new developments more effectively.
The aim of the survey was primarily to help the Appointments Boards in Cambridge If a study had been made of the careers followed by men and women of the same years in other Universities, the findings would not necessarily have been similar. But, if due allowances are made for this factor, the report will, we hope, be of wide interest both to those who educate young people of high intelligence and to those who may employ them.
The biggest threat to the war on malaria is the continued evolution of drug resistance by the parasite. Resistance to almost all currently available antimalarials now exists in Plasmodium falciparum which causes the most suffering among all human malaria parasites. Monitoring of antimalarial efficacy and the development and subsequent spread of resistance has become an important part in the treatment and control of malaria. With recent reports of reduced efficacy of artemisinin, the current recommended treatment for uncomplicated malaria, there is urgent need for better methods to recognize and monitor drug resistance for effective treatment. Molecular markers have become a welcome addition to complement the more laborious and costly in vitro and in vivo methods that have traditionally been used to monitor drug resistance. However, there are currently no molecular markers for resistance to some antimalarials. This review highlights the role of the various genetic and genomic approaches that have been used in identifying the molecular markers that underlie drug resistance in P. falciparum. These approaches include; candidate genes, genetic linkage and genome-wide association studies. We discuss the requirements and limitations of each approach and use various examples to illustrate their contributions in identifying genomic regions of the parasite associated with antimalarial drug responses.
Cameroon has experienced recurrent cholera epidemics with high mortality rates. In September 2009, epidemic cholera was detected in the Far North region of Cameroon and the reported case-fatality rate was 12%. We conducted village-, healthcare facility- and community-level surveys to investigate reasons for excess cholera mortality. Results of this investigation suggest that cholera patients who died were less likely to seek care, receive rehydration therapy and antibiotics at a healthcare facility, and tended to live further from healthcare facilities. Furthermore, use of oral rehydration salts at home was very low in both decedents and survivors. Despite the many challenges inherent to delivering care in Cameroon, practical measures could be taken to reduce cholera mortality in this region, including the timely provision of treatment supplies, training of healthcare workers, establishment of rehydration centres, and promotion of household water treatment and enhanced handwashing with soap.
A comparative study, using time-resolved and CW photoluminescence spectroscopy, of MOVPE grown InGaN/GaN multiple quantum wells deposited on HVPE GaN/Sapphire at different growth temperatures was undertaken. It was found that the PL linewidth increased and the peak emission energy decreased as the growth temperature was reduced. Moreover, the sample grown at an intermediate growth temperature exhibited total integrated luminescence intensity much greater than the samples grown at higher or lower growth temperatures. A phenomenological carrier recombination dynamics model based on the competition of quantum well-like radative recombination, spatially localized radiative recombination in potential minima and non-radiative recombination through defects is presented to provide an explanation of the observed emission dynamics and efficiency. In this model, the emission efficiency is determined by the relative area of defects and the number density of localized states in the potential minima, both of which are influenced by the growth temperature. Furthermore, the photon energy dependent lifetimes are well fitted with this model by assuming a Gaussian shape localized states distribution. The localized potential minima are consistent with nanoscale indium rich regions due to indium aggregation.
This research is comprised of understanding the linear photophysical properties of various dyes to better understand the more complicated nonlinear optical properties. Determining structure property relationships of a series of structurally closely related chromophores is the key in understanding the drivers for the various photophysical properties. In this paper we survey the effect of physically changing the Pt poly-yne structure on the S0-S1 and T1-Tn absorption properties for each of the chromophores. A series of structurally modified platinum poly-ynes have been studied using experimental methods including UV/Vis absorption and nanosecond laser flash photolysis. We found that with extension of the ligand length both the ground and triplet excited state absorption shift to lower energies. Comparing the absorption properties of the ligands and butadiynes with the platinum containing versions reveal that the S1 and Tn exciton is localized on one portion of the ligand with extension and not conjugated through the whole molecule. Changing the phosphine R group results in little effect to the absorption properties except when the R group is conjugated in the case of phenyl. However, changing the R group results in varied materials properties.
This paper reviews of some of the progress made in the development of ZnO-based light emitting diodes (LEDs). n-ZnO/p-AlGaN-based heterostructures have been successfully for the fabrication of UV emitting LEDs that have operated at temperatures up to 650K, suggesting an excitonic origin for the optical transitions. RF-plasma-assisted molecular beam epitaxy has been used to grow epitaxial CdxZn1-xO films on GaN/sapphire structure. These films have a single-crystal wurtzite structure as demonstrated by structural and compositional analysis. High quality CdxZn1-xO films were grown with up to x=0.78 mole fraction as determined by RBS and SIMS techniques. Optical emission ranging from purple (Cd0.05Zn0.95O) to yellow (Cd0.29Zn0.71O) was observed. Compositional fluctuations in a Cd0.16Zn0.84O films were not detected by spatially resolved CL measurements, although intensity fluctuation with features of ∼0.5 μm diameter were seen on the intensity maps. Time resolved photoluminescence shows multi-exponential decay with 21 psec. and 49±3 psec. lifetimes, suggesting that composition micro-fluctuations may be present in Cd0.16Zn0.84O film.
Knowledge of the carrier recombination, relaxation, and transport processes in InN materials is essential for determining the applicability of this material system in photonic and electronic applications. In this article, we provide a review of time-resolved spectroscopy experimental techniques and our recent results using these techniques to measure transient processes in InN. Specifically, subpicosecond differential transmission experiments were used to determine the carrier recombination lifetime and the carrier thermalization time of InN. In those experiments, we observed a fast initial hot carrier cooling followed by a slower recombination process. At short times after pulsed excitation, modeling of the observed relaxation suggests that the dominant energy relaxation process is longitudinal optical phonon scattering modified by a strong hot phonon effect at room temperature. An inverse proportionality between the carrier lifetime and the free electron concentration was found. This suggests that donor-like defects or impurities may stimulate the formation of non-radiative recombination centers. Furthermore, we report the measurements of in-plane carrier transport and hole mobility of an InN epilayer by time-resolved transient grating spectroscopy using subpicosecond pulses at 800 nm and ∼1900 nm for grating writing and probing, respectively. The ambipolar diffusion coefficient Da = 2.0 cm2/s and hole mobility µh = 39 cm2/Vs at 300 K near the InN surface were determined by monitoring the transient grating kinetics at various grating periods.
Time-resolved photoluminescence studies can provide useful information for the development of InGaN/GaN heterostructures for long wavelength visible emitters. In this paper, we present results of time-resolved photoluminescence from samples grown using two different approaches to achieve green emission from InGaN/GaN MQWs. In one approach, samples, with high indium incorporation, were grown on a high quality AlN substrate to achieve green emission. The resulting photoluminescence decay of the green luminescence is long-lived and non-exponential. Quantitative analysis showed that the decay has a stretched-exponential characteristic, typical of InGaN/GaN MQW with potential fluctuation along the growth plane. This carrier localization, in a structure with low defect density, proves to be an effective means to achieve green emission. In another approach, a piezoelectric Stark-like ladder effect is used. In this case, a methodical layer-by-layer growth homogeneity optimization process was adopted to achieve an optical transition below the electron to heavy-hole (e1hh1) transition when the quantum well is subjected to the strong piezoelectric polarization dipole. This approach has proven to be successful in achieving green luminescence on conventional sapphire substrates. The resulting photoluminescence decay at 14 K, of a sample grown by this approach, is single exponential and shorter in duration than the decay observed in the first approach. This exponential decay is consistent with previous AFM studies that revealed a homogeneous active region.
Pattern size and pitch are two major factors that directly affect the capacity of a data storage unit. Electron beam lithography, because it can be used for direct writing at the nanometer scale, is one of the potential candidates to fabricate ultra-fine and ultra-compact patterns for the next generation of data storage media.
In the paper, we present methods to reduce the pattern size and pitch written by electron beam on positive tone resist Poly(methylmethacrylate) (PMMA). The first method uses a writing dose which is much lower than the critical dose of the PMMA. However, in this technique, the distance between the adjacent writing spots (which are approximately Gaussian in spatial extent) is finely adjusted until the partially overlapping Gaussian functions form a contour with periodic dose peaks higher than the critical dose. This overlapping exposure results in a periodic exposure of the PMMA. Most importantly, this writing strategy, using the overlapping of low dose Gaussian beams, results in patterns with smaller size and pitch compared to the conventional methods that use a higher writing dose than the critical dose. To further improve the results, we use ultrasonic development in combination with cold development to greatly increase the resist contrast. The increased contrast results in smaller pattern sizes, and enables smaller pattern pitches, as well as improving pattern uniformity and reproducibility.
Finally, as a demonstration of the technique using a conventional beam voltage of 15kV, we achieve ultra-fine and high density patterns with ∼10nm linewidths and 50nm in pitch, which corresponds to a storage capacity near 200 billion bits/inch2. In comparison, standard techniques (using a dose slightly higher than the critical dose and normal development strategy), we can only obtain patterns with linewidths of about 20-30nm and pitch of about 100nm.
The need for efficient UV emitting semiconductor sources has prompted the study of a number of heterostructures of III-N materials. In this work, the temperature dependence of the photoluminescence (PL) properties of UV-emitting GaN/AlN multiple quantum well (MQW) heterostructures were investigated in detail. In all samples studied, the structure consisted of 20 GaN quantum wells, with well widths varying between 7 and 15 Å, clad by 6nm AlN barriers, grown on top of a thick AlN buffer that was deposited on sapphire by molecular beam epitaxy. The observed energy corresponding to the peak of the emission spectrum is in agreement with a model that includes the strong confinement present in these structures and the existence of the large built-in piezoelectric field and spontaneous polarization present inside the wells. The observed emission varies from 3.5 eV (15 Å well) to 4.4 eV (7 Å well). Two activation energies associated with the photoluminescence quenching are extracted from the temperature dependence of the time-integrated PL intensity. These activation energies are consistent with donor and acceptor binding energies and the PL is dominated by recombination involving carriers localized on donor and/or acceptor states.
Moreover, the temperature dependence of the full width at half-maximum (FWHM) of the PL feature indicates that inhomogeneous broadening dominates the spectrum at all temperatures. For the 15 and 13 Å wells, we estimate that the electron-phonon interaction is responsible for less than 30% of the broadening at room temperature. This broadening is negligible in the 9 Å wells over the entire temperature range studied. Well width fluctuations are primarily responsible for the inhomogeneous broadening, estimated to be of the order of 250meV for one monolayer fluctuation in well width.
A new family of organic/inorganic hybrid silicate materials, bridged polysilsesquioxanes, was designed and synthesized through a molecular-level mixing technique. Since hybrid materials in the molecular-composite level, whose domain sizes are in the nanometer-scale, and whose constituents often lose individual identities and thus create new properties, we obtained a set of improved properties from those organically modified glasses. By modifying the Si-O-Si polymeric network, in this study, we produced controllable, porous hybrid glasses for facile and uniform doping of various ions, metals or semiconductor particles. By taking advantage of void volume created in those molecularly modified silicate systems, novel optical materials with designed properties can thus be achieved. Via a chemical strategy, we designed hexylene- or fluoroalkylene-bridged hybrid glasses doped with both Er+3 ions and CdSe nano-particles for the development of new laser amplifier materials. In photoluminescence experiments, a significant enhancement in fluorescence intensity at 1540 nm has been obtained from the fluoroalkylene-bridged glass. The presence of CdSe nano-particles, by virtue of their low phonon energy, also appears to significantly influence the nature of the surrounding environment of Er+3 ions in those modified silicate systems, resulting in the increased fluorescence intensity.
Polymer-metal composites offer the possibility of strongly enhanced nonlinear optical properties, which can be used for ultrasmall photonic devices. In this paper, we investigate numerically, by means of the finite-difference time-domain (FDTD) method, the propagation characteristics of surface plasmon polariton (SPP) modes excited in an optical nanowire consisting of a chain of either metallic cylinders or metallic spheres embedded in dielectric shells made of polymers (or other material) with optical Kerr nonlinearity. Our FDTD calculations incorporate both the nonlinear optical response of the dielectrics as well as the frequency dispersion of the metals, which is considered to obey a Drude-like model. It is demonstrated that, in the linear limit, the nanowire supports two SPP modes, a transverse and a longitudinal one, separated by Δλ = 20 nm. Furthermore, the dependence of the transmission of these SPP modes, on both the pulse peak power and Kerr coefficient of the dielectric shell, is investigated. Nonlinear optical phenomena, such as power-dependent mode frequency, switching, or optical limiting, are observed.
We report on detailed temperature dependent, time-resolved photoluminescence (TRPL) studies of Si-doped AlGaN epilayers. In these samples, the Al concentration varies from 25% to 66%. The samples were found to exhibit metallic-like temperature-independent conductivity. The deep level “yellow” emission, whose presence would indicate the existence of a large number of defects associated with growth, Si incorporation, and/or alloy formation, is absent. In addition to emission corresponding to the donor-bound exciton, the PL spectrum exhibits features associated with transitions involving localized carriers. This assignment of the emission mechanisms is based on the activation energies extracted from the temperature dependent photoluminescence (PL) quenching. Specifically, at room temperature the PL spectrum is dominated by transitions involving localized states. The localization energy varied from sample to sample and was observed to be between 115 meV to 200 meV. The PL intensity decay in the lower Al mole fraction epilayers exhibits a slow component associated with the presence of donor-bound excitons.