1,797 research outputs found
Polarisation-sensitive terahertz detection by multicontact photoconductive receivers
We have developed a terahertz radiation detector that measures both the
amplitude and polarization of the electric field as a function of time. The
device is a three-contact photoconductive receiver designed so that two
orthogonal electric-field components of an arbitrary polarized electromagnetic
wave may be detected simultaneously. The detector was fabricated on Fe+
ion-implanted InP. Polarization-sensitive detection is demonstrated with an
extinction ratio better than 100:1. This type of device will have immediate
application in studies of birefringent and optically active materials in the
far-infrared region of the spectrum.Comment: 3 pages, 3 figure
Influence of surface passivation on ultrafast carrier dynamics and terahertz radiation generation in GaAs
The carrier dynamics of photoexcited electrons in the vicinity of the surface
of (NH4)2S-passivated GaAs were studied via terahertz (THz) emission
spectroscopy and optical-pump THz-probe spectroscopy. THz emission spectroscopy
measurements, coupled with Monte Carlo simulations of THz emission, revealed
that the surface electric field of GaAs reverses after passivation. The
conductivity of photoexcited electrons was determined via optical-pump
THz-probe spectroscopy, and was found to double after passivation. These
experiments demonstrate that passivation significantly reduces the surface
state density and surface recombination velocity of GaAs. Finally, we have
demonstrated that passivation leads to an enhancement in the power radiated by
photoconductive switch THz emitters, thereby showing the important influence of
surface chemistry on the performance of ultrafast THz photonic devices.Comment: 4 pages, 3 figures, to appear in Applied Physics Letter
Charge trapping in polymer transistors probed by terahertz spectroscopy and scanning probe potentiometry
Terahertz time-domain spectroscopy and scanning probe potentiometry were used
to investigate charge trapping in polymer field-effect transistors fabricated
on a silicon gate. The hole density in the transistor channel was determined
from the reduction in the transmitted terahertz radiation under an applied gate
voltage. Prolonged device operation creates an exponential decay in the
differential terahertz transmission, compatible with an increase in the density
of trapped holes in the polymer channel. Taken in combination with scanning
probe potentionmetry measurements, these results indicate that device
degradation is largely a consequence of hole trapping, rather than of changes
to the mobility of free holes in the polymer.Comment: 4 pages, 3 figure
Experimental evidence for Wigner's tunneling time
Tunneling of a particle through a potential barrier remains one of the most
remarkable quantum phenomena. Owing to advances in laser technology, electric
fields comparable to those electrons experience in atoms are readily generated
and open opportunities to dynamically investigate the process of electron
tunneling through the potential barrier formed by the superposition of both
laser and atomic fields. Attosecond-time and angstrom-space resolution of the
strong laser-field technique allow to address fundamental questions related to
tunneling, which are still open and debated: Which time is spent under the
barrier and what momentum is picked up by the particle in the meantime? In this
combined experimental and theoretical study we demonstrate that for
strong-field ionization the leading quantum mechanical Wigner treatment for the
time resolved description of tunneling is valid. We achieve a high sensitivity
on the tunneling barrier and unambiguously isolate its effects by performing a
differential study of two systems with almost identical tunneling geometry.
Moreover, working with a low frequency laser, we essentially limit the
non-adiabaticity of the process as a major source of uncertainty. The agreement
between experiment and theory implies two substantial corrections with respect
to the widely employed quasiclassical treatment: In addition to a non-vanishing
longitudinal momentum along the laser field-direction we provide clear evidence
for a non-zero tunneling time delay. This addresses also the fundamental
question how the transition occurs from the tunnel barrier to free space
classical evolution of the ejected electron.Comment: 31 pages, 15 figures including appendi
Efficient microbial bioconversion of brown macroalgae obtained through profitable high-density sea cultivation using modified microbial strains to produce commodity and specialty chemicals: A developing blue chemical industry in Chile
Plant biomass is considered a promising feedstock for large scale sustainable bio-based green chemistry. However, only the use of agricultural or forestry residues is viable, since they do not compete for land with feed crops and have competitive costs. Moreover, carbohydrate recovery from these sources is always difficult due to their high lignin content. Alternatively, macroalgae are competitive sources of carbohydrate-rich biomass not requiring land or fresh water for its production. Macrocystis pyrifera is one of the fastest-growing macroalgal species with high CO2 fixation efficiency, highly-abundant and accessible carbohydrates. We demonstrated that it can be cultured in temperate seas, yielding 124 ton/Ha/yr, and can be economically profitable at a 10-hectare scale 1,2. Microbial and enzymatic algal biomass bioprocessing has been also undertaken by our group. We demonstrated the technical feasibility of producing ethanol at a pilot industrial scale by fermenting algal carbohydrates with a genetically modified Escherichia coli 3. However, ethanol production, even with high productivities, was not commercially viable. To make algal biomass bioconversion profitable, we performed a large metabolic engineering and synthetic biology project to discover combinations of metabolic pathways, regulation, carbohydrate sources –algal or not– and alternative bioproducts that maximize microbial efficiency and commercial viability. Using a genome-scale reconstruction of Saccharomyces cerevisiae’s metabolism, we demonstrated that redox ratio constraints and the preferential use of NADH or NADPH for alginate metabolism were key for S. cerevisiae conversion of alginate:mannitol carbohydrate sources 4. However, yeast use makes chemical processes technically and economically unfeasible for low value products due to their inability to produce extracellular enzymes for alginate lysis. By means of dynamic metabolic models developed for E. coli, we demonstrated that the main metabolic process bottleneck is microbial carbohydrate metabolization and that algal carbohydrate composition is a key determinant of fermentation efficiency. Using a multi-objective optimization strategy focused on microorganism growth, energy levels and redox ratio conservation, we also showed that ethanol production from algal biomass is incompatible with E. coli’s metabolism, due to low energetic and redox efficiencies obtained from alginate using host microorganism metabolic pathways. We then used high-performance parallel computing to develop a metabolic potentiality map for E. coli in which we explored more than 10.000 possible combinations of metabolic pathways that could be built in our strain to convert brown macroalgae carbohydrates with high efficiency, considering the best combinations of knock-outs and overexpressions to be introduced in E. coli’s central metabolic pathways. With this technique, we identified other valuable chemicals, such as succinic, aspartic, gluconic and levulinic acids, and complex aromatic and aliphatic biomolecules can be efficiently produced from Macrocystis with specifically modified strains for each product. The bulk of our research fostering algal feedstock production and industrial bioconversion in Chile will be presented in this work. 1. Buschmann, A. H. et al. The Status of Kelp Exploitation and Marine Agronomy, with Emphasis on Macrocystis pyrifera, in Chile. Advances in Botanical Research 71, 161–188 (2014). 2. Camus, C., Infante, J. & Buschmann, A. H. Overview of 3 year precommercial seafarming of Macrocystis pyrifera along the Chilean coast. Reviews in Aquaculture 10, 543–559 (2018). 3. Camus, C. et al. Scaling up bioethanol production from the farmed brown macroalga Macrocystis pyrifera in Chile. Biofuels, Bioproducts and Biorefining 10, 673–685 (2016). 4. Contador, C. A. et al. Analyzing redox balance in a synthetic yeast platform to improve utilization of brown macroalgae as feedstock. Metabolic Engineering Communications 2, 76–84 (2015)
Politics in the Classroom
Nursing and midwifery is, in the UK, regulated by the Nursing and Midwifery Council (NMC). Regulatory duties include establishing standards for education, and from January 2019, new educational programmes will be approved against standards detailed in the document Future nurse: Standards of proficiency for registered nurses (NMC, 2019 – hereafter ‘the standards’). This publication lists “the knowledge and skills that registered nurses must demonstrate when caring for people” (ibid, p.3); and from September 2020, registration (licence) will require the successful completion of programmes that have been ratified against these standards. The importance of this document in a UK context cannot be understated. However, less parochially, learning outcomes contained in section 7 of the standards raise questions that require educator attention whenever politically sensitive topics (broadly conceived) are discussed. This study explores these questions insofar as they relate to the stance (neutrality or partisanship) that educators adopt in politicised discussion, and the management of student speech/expression. Pratt, Boll and Collins’ (2007) paper Towards a plurality of perspectives for nurse educators is recruited to structure argument
Measuring the effects of fractionated radiation therapy in a 3D prostate cancer model system using SERS nanosensors.
Multicellular tumour spheroids (MTS) are three-dimensional cell cultures that possess their own microenvironments and provide a more meaningful model of tumour biology than monolayer cultures. As a result, MTS are becoming increasingly used as tumor models when measuring the efficiency of therapies. Monitoring the viability of live MTS is complicated by their 3D nature and conventional approaches such as fluorescence often require fixation and sectioning. In this paper we detail the use of Surface Enhanced Raman Spectroscopy (SERS) to measure the viability of MTS grown from prostate cancer (PC3) cells. Our results show that we can monitor loss of viability by measuring pH and redox potential in MTS and furthermore we demonstrate that SERS can be used to measure the effects of fractionation of a dose of radiotherapy in a way that has potential to inform treatment planning.EaStCHEM, NHS Lothian, Jamie King Cancer Research FundThis is the final version of the article. It first appeared from the Royal Society of Chemistry via http://dx.doi.org/10.1039/C6AN01032
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