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Theoretical and experimental study of second harmonic generation from the surface of the topological insulator Bi_2Se_3

We develop a theoretical model that describes the second harmonic generation of light from the surface of the topological insulator Bi_2Se_3 and experimentally demonstrate that the technique is sensitive to the surface electrons. By performing a crystal symmetry analysis of Bi_2Se_3 (111) we determine the nonlinear electric susceptibility tensor elements that give rise to second harmonic generation. Using these results, we present a phenomenological model that shows that the relative magnitudes of these tensor elements can be determined by measuring the polarization and intensity of the radiated second harmonic light as a function of the in-plane crystal orientation and incident laser polarization. We describe optical techniques capable of isolating second harmonic light and, using these techniques, we measure the first-order linear optical and second-order nonlinear optical responses as a function of crystal orientation and laser polarization on bulk single crystals of Bi_2Se_3 (111). The experimental results are consistent with our theoretical description. By comparing the data to our theoretical model we determine that a portion of the measured second harmonic light originates from the accumulation region of Bi_2Se_3 (111), which we confirm by performing surface doping-dependent studies. Our results show that second harmonic generation is a promising tool for spectroscopic studies of topological surfaces and buried interfaces

Modeling of Algal Productivity and Diel Oxygen Profiles in the Partitioned Aquaculture System

Fish production in pond culture systems is often limited by the dissolved oxygen (DO) concentration. Algal biomass and productivity, which greatly impact the DO concentration, are difficult to control in traditional pond systems. The Partitioned Aquaculture System (PAS) was designed to increase fish production by managing algal productivity and DO through control of algal cell retention time, water depth, and mixing. The objectives of this study were 1) to determine impact of culture conditions on algal productivity, 2) develop a model of algal productivity and diel DO levels, and 3) to predict the maximum fish production attainable with the PAS. For this study, algal cell retention time was controlled through continuous discharge of the culture. Algal productivity was calculated from algal biomass (measured with 0.45 µm filters), retention time and water depth. Oxygen production and respiration rates were measured using in-pond incubations. Algal species composition was dependent on the inorganic carbon content of the culture water. The algal culture was comprised primarily of blue-green algae (Cyanophyta) at 0.7 mmol/1/d inorganic carbon addition rate, and green algae (Chlorophyta) at 1.1 and 1.8 mmol/1/d addition rates. Algal productivity was 1.2 times greater for the 1.1 and 1.8 rates than for the 0.7 mmol/1/d inorganic carbon addition rate; 1.2 times greater for the 0.34 m than the 0.66 m water depth; 1.5 times greater for the 0.125 m/s than for the 0.0313 and 0.0625 m/s water velocity; and 1.3 times greater for the 1.2 day than the 2.5 day retention time. Maximum oxygen production rates of 0.11 and 0.12 mg Oifmg TSS/hr were achieved for blue-green and green algal cultures, respectively. Maximum DO concentrations of 25 -30 mg/1 were routinely observed. The algal productivity and DO model constructed for the PAS incorporates an inhibitory light model for light-limited algal growth and the Monod model for carbon-and nitrogen-limited growth. Steady state and dynamic simulations of the DO profile as a function of the design and environmental conditions closely matched the observed values. Maximum fish production of 10,000 kg/ha is predicted for light-limited cultures ~n the PAS operated at 0.0313 m/s water velocity, 20 hour retention time and 0.6 m water depth

Entanglement of single-photons and chiral phonons in atomically thin WSe2_2

Quantum entanglement is a fundamental phenomenon which, on the one hand, reveals deep connections between quantum mechanics, gravity and the space-time; on the other hand, has practical applications as a key resource in quantum information processing. While it is routinely achieved in photon-atom ensembles, entanglement involving the solid-state or macroscopic objects remains challenging albeit promising for both fundamental physics and technological applications. Here, we report entanglement between collective, chiral vibrations in two-dimensional (2D) WSe$_2$ host --- chiral phonons (CPs) --- and single-photons emitted from quantum dots (QDs) present in it. CPs which carry angular momentum were recently observed in WSe$_2$ and are a distinguishing feature of the underlying honeycomb lattice. The entanglement results from a "which-way" scattering process, involving an optical excitation in a QD and doubly-degenerate CPs, which takes place via two indistinguishable paths. Our unveiling of entanglement involving a macroscopic, collective excitation together with strong interaction between CPs and QDs in 2D materials opens up ways for phonon-driven entanglement of QDs and engineering chiral or non-reciprocal interactions at the single-photon level

Salt Tolerance of Six Switchgrass Cultivars

Panicum virgatum L. (switchgrass) cultivars (‘Alamo’, ‘Cimarron’, ‘Kanlow’, ‘NL 94C2-3’, ‘NSL 2009-1’, and ‘NSL 2009-2’) were evaluated for salt tolerance in two separate greenhouse experiments. In experiment (Expt.) 1, switchgrass seedlings were irrigated with a nutrient solution at an electrical conductivity (EC) of 1.2 dS·m−1 (control) or a saline solution (spiked with salts) at an EC of 5.0 dS·m−1 (EC 5) or 10.0 dS·m−1 (EC 10) for four weeks, once a week. Treatment EC 10 reduced the tiller number by 32% to 37% for all switchgrass cultivars except ‘Kanlow’. All switchgrass cultivars under EC 10 had a significant reduction of 50% to 63% in dry weight. In Expt. 2, switchgrass was seeded in substrates moistened with either a nutrient solution of EC 1.2 dS·m−1 (control) or a saline solution of EC of 5.0, 10.0, or 20.0 dS·m−1 (EC 5, EC 10, or EC 20). Treatment EC 5 did not affect the seedling emergence, regardless of cultivar. Compared to the control, EC 10 reduced the seedling emergence of switchgrass ‘Alamo’, ‘Cimarron’, and ‘NL 94C2-3’ by 44%, 33%, and 82%, respectively. All switchgrass cultivars under EC 10 had a 46% to 88% reduction in the seedling emergence index except ‘NSL 2009-2’. No switchgrass seedlings emerged under EC 20. In summary, high salinity negatively affected switchgrass seedling emergence and growth. Dendrogram and cluster of six switchgrass cultivars indicated that ‘Alamo’ was the most tolerant cultivar, while ‘NSL 2009-2’ was the least tolerant cultivar at both seedling emergence and growth stages. A growth-stage dependent response to salinity was observed for the remaining switchgrass cultivars. ‘NSL 2009-1’ and ‘NL 94C2-3’ were more tolerant to salinity than ‘Cimarron’ and ‘Kanlow’ at the seedling emergence stage; however, ‘Kanlow’ and ‘Cimarron’ were more tolerant to salinity than ‘NSL 2009-1’ and ‘NL 94C2-3’ at the seedling growth stag

Continuous cultivation of photosynthetic microorganisms: approaches, applications and future trends

The possibility of using photosynthetic microorganisms, such as cyanobacteria and microalgae, for converting light and carbon dioxide into valuable biochemical products has raised the need for new cost-efficient processes ensuring a constant product quality. Food, feed, biofuels, cosmetics and pharmaceutics are among the sectors that can profit from the application of photosynthetic microorganisms. Biomass growth in a photobioreactor is a complex process influenced by multiple parameters, such as photosynthetic light capture and attenuation, nutrient uptake, photobioreactor hydrodynamics and gas-liquid mass transfer. In order to optimize productivity while keeping a standard product quality, a permanent control of the main cultivation parameters is necessary, where the continuous cultivation has shown to be the best option. However it is of utmost importance to recognize the singularity of continuous cultivation of cyanobacteria and microalgae due to their dependence on light availability and intensity. In this sense, this review provides comprehensive information on recent breakthroughs and possible future trends regarding technological and process improvements in continuous cultivation systems of microalgae and cyanobacteria, that will directly affect cost-effectiveness and product quality standardization. An overview of the various applications, techniques and equipment (with special emphasis on photobioreactors) in continuous cultivation of microalgae and cyanobacteria are presented. Additionally, mathematical modelling, feasibility, economics as well as the applicability of continuous cultivation into large-scale operation, are discussed.This research work was supported by the grant SFRH/BPD/98694/2013 (Bruno Fernandes) from Fundacao para a Ciencia e a Tecnologia (Portugal). The authors thank the FCT Strategic Project PEst-OE/EQB/LA0023/2013. The authors also thank the Project "BioInd Biotechnology and Bioengineering for improved Industrial and Agro-Food processes, REF. NORTE-07-0124-FEDER-000028" Co-funded by the Programa Operacional Regional do Norte (ON.2-O Novo Norte), QREN, FEDE

The Grizzly, February 1, 2000

A Burning Question: Recent Rash of Fatal College Fires Sparks UC Students to Ask, Are We Safe? • Ursinus Struck Twice More with Ice, Rain and Snow • St. Louis Super in Clash with Titans • Grizzly Library Goes Virtual • Dickinson College Librarian Finally Free from Exile in Chinese Prison • Study Abroad Experience Proves to be a Valuable One • Opinion: After Iowa, Throw out the Rulebook, it\u27s Anybody\u27s Game; Where\u27s the Juice in Juice? • Meistersingers Tour Europe • Career Services Introduces Experience • Swimming Gears Up for Champs • Pair of Losses for UC • Bears Struggle in Conference Play • The Flud Warning • Ursinus Gymnastics Tramples Brockport • Sports Profile: Luther Owenshttps://digitalcommons.ursinus.edu/grizzlynews/1457/thumbnail.jp