科研成果 by Year: 2015

In addition to surface plasmon polaritons (SPPs), quasi-cylindrical waves (CWs) are also important components of electromagnetic fields on metal surfaces. In this study, we present a closed-form expression for CW fields on a dielectric-film-coated metal surface. It is found that the effective refractive index of CWs roughly keeps unchanged when the coated dielectric film becomes thicker, while the effective refractive index of SPPs increases rapidly. These different responses are explained by referring to a waveguide perspective, in which the SPP and CW waves are, respectively, identified as a waveguide mode and superposition of radiation modes. The results and related analyses with the waveguide perspective explicitly show the different physical natures of SPPs and CWs. (C) 2015 Optical Society of America

Lakes are widely distributed on the Mongolian Plateau and, as critical water sources, have sustained Mongolian pastures for hundreds of years. However, the plateau has experienced significant lake shrinkage and grassland degradation during the past several decades. To quantify the changes in all of the lakes on the plateau and the associated driving factors, we performed a satellite-based survey using multitemporal Landsat images from the 1970s to 2000s, combined with ground-based censuses. Our results document a rapid loss of lakes on the plateau in the past decades: the number of lakes with a water surface area >1 km2 decreased from 785 in the late 1980s to 577 in 2010, with a greater rate of decrease (34.0%) in Inner Mongolia of China than in Mongolia (17.6%). This decrease has been particularly pronounced since the late 1990s in Inner Mongolia and the number of lakes >10 km2 has declined by 30.0%. The statistical analyses suggested that in Mongolia precipitation was the dominant driver for the lake changes, and in Inner Mongolia coal mining was most important in its grassland area and irrigation was the leading factor in its cultivated area. The deterioration of lakes is expected to continue in the following decades not only because of changing climate but also increasing exploitation of underground mineral and groundwater resources on the plateau. To protect grasslands and the indigenous nomads, effective action is urgently required to save these valuable lakes from further deterioration.

This paper studies an economy inhabited by overlapping generations of households and investors, with the only difference between the two being that households derive utility from housing services, whereas investors do not. Tight collateral constraint limits the borrowing capacity of households and drives the equilibrium interest rate level down to the housing price growth rate, which makes housing attractive as a store of value for investors. A housing bubble arises in an equilibrium in which investors hold houses for resale purposes only and without the expectation of receiving a dividend either in terms of utility or in terms of rent. Pension reform that reduces the contribution rate may increase the supply of credit and create the housing bubble. Empirical findings from China are consistent with theoretical predictions. Copyright Springer-Verlag Berlin Heidelberg 2015

It remains unclear whether dicofol should be defined as a persistent organic pollutant. Its environmental persistence has gained attention. This study focused on its degradation by cellulase. Cellulase was separated using a gel chromatogram, and its degradation activity towards dicofol involved its endoglucanase activity. By analyzing the kinetic parameters of cellulase reacting with mixed substrates, it was shown that cellulase reacted on dicofol and carboxyl methyl cellulose through two different active centers. Thus, the degradation of dicofol was shown to be an oxidative process by cellulase. Next, by comparing the impacts of tert-butyl alcohol (a typical OH free-radical inhibitor) on the removal efficiencies of dicofol under both cellulase and Fenton reagent systems, it was shown that the removal of dicofol was initiated by OH free radicals produced by cellulase. Finally, 4,4′-dichloro-dibenzophenone and chloride were detected using gas chromatography mass spectrometry and ion chromatography analysis, which supported our hypothesis. The reaction mechanism was analyzed and involved an attack by OH free radicals at the orthocarbon of dicofol, resulting in the degradation product 4,4′-dichloro-dibenzophenone.

As a class of brown carbon, organonitrogen compounds originating from the heterogeneous uptake of NH3 by secondary organic aerosol (SOA) have received significant attention recently. In the current work, particulate organonitrogen formation during the ozonolysis of alpha-pinene and the OH oxidation of m-xylene in the presence of ammonia (34-125 ppb) was studied in a smog chamber equipped with a high resolution time-of-flight aerosol mass spectrometer and a quantum cascade laser instrument. A large diversity of nitrogen-containing organic (NOC) fragments was observed which were consistent with the reactions between ammonia and carbonyl-containing SOA. Ammonia uptake coefficients onto SOA which led to organonitrogen compounds were reported for the first time, and were in the range of similar to 10(-3)-10(-2), decreasing significantly to <10(-5) after 6 h of reaction. At the end of experiments (similar to 6 h) the NOC mass contributed 8.9 +/- 1.7 and 31.5 +/- 4.4 wt% to the total alpha-pineneand m-xylene-derived SOA, respectively, and 4-15 wt% of the total nitrogen in the system. Uptake coefficients were also found to be positively correlated with particle acidity and negatively correlated with NH3 concentration, indicating that heterogeneous reactions were responsible for the observed NOC mass, possibly limited by liquid phase diffusion. Under these conditions, the data also indicate that the formation of NOC can compete kinetically with inorganic acid neutralization. The formation of NOC in this study suggests that a significant portion of the ambient particle associated N may be derived from NH3 heterogeneous reactions with SOA. NOC from such a mechanism may be an important and unaccounted for source of PM associated nitrogen. This mechanism may also contribute to the medium or long-range transport and wet/dry deposition of atmospheric nitrogen.

As the fast development of ultra-short ultra-intense laser technology and mechanism research of laser ion acceleration, laser-driven ion accelerators have drawn a lot of attention for the past decades. Thomson spectrometer is one of the critical diagnostics for laser-driven ion acceleration, because of its ability to measure ions' energy spectrum, charge, and charge-to-mass radio parameters simultaneously. This paper presents a compact real-time single-shot Thomson spectrometer, which uses electromagnetic field to disperse ions and scintillator coupled with high performance EMCCD as a record medium. The spectrometer has a pair of inclined electric plates with the advantage of avoiding electromagnetic noise caused by low energy ions' collision with electric plates, improving signal-to-noise ratio accordingly. Experiments were carried out on two conventional accelerators in Peking University to test the spectrometer and measure detection efficiency of ions at the same time. It turns out that the spectrometer can be used for detection of ions driven by 100 TW class femtosecond laser system, and will contribute to our understanding of laser-based high energy density physics.介绍一种使用闪烁体耦合电子倍增电荷耦合器件(EMCCD)的方式对离子进行记录的汤姆逊能谱仪,可实现对离子能谱的实时单发测量。同时,该谱仪利用倾斜 电极板对离子进行偏转,可减少由于离子打在电极板上产生的电磁噪声,能够提高实验结果的信噪比。该谱仪在北京大学4.5 MV静电加速器和2*6 MV串列加速器上进行了标定实验,测量了闪烁体将离子转化成光子后的探测效率,实验结果也验证了该谱仪的可行性和稳定性。该汤姆逊谱仪将用于北京大学激光 加速器CLAPA对离子束流的测量研究。

This letter reports a GaN high-electron mobility transistor (HEMT) with reduced current collapse using a multicycle combined plasma-free ozone oxidation and wet surface treatment before Si3N4 passivation. The surface oxide and decomposed layers could be effectively removed and a perfect AlGaN surface is obtained after the treatment. Pulsed IV and RF power measurement indicate that the current collapse is greatly suppressed due to the removal of imperfect surface layer and damage free nature, providing an effective surface treatment method to improve the effect of passivation in GaN HEMT.

Inversion is a critical and challenging task in geophysical research. Geophysical inversion can be formulated as an optimization problem to find the best parameters whose forward synthesis data most fit the observed data. The inverse problems are usually highly non-linear, multi-modal as well as ill-posed, so conventional optimization algorithms cannot handle it very efficiently. In the past decades, genetic algorithm (GA) and its many variants are widely applied to inverse problems and achieve great success. Swarm intelligence algorithms are a family of global optimizers inspired by swarm phenomena in nature, and have shown better performance than GA for diverse optimization problems. However, swarm intelligence algorithms are not utilized for geophysical inversion problems until recently and only limited number of works are reported. In this paper, we try to apply two swarm intelligence algorithms, Particle Swarm Optimization (PSO) and Fireworks Algorithm (FWA), to the regional seismic waveform inversion. To explore the advantages and disadvantages of swarm intelligence algorithms over GA, synthetic experiments are conducted by using these two swarm intelligence algorithm and several GA variants as well as Differential Evolution (DE). The experimental results show that, both swarm intelligence algorithms outperform the widely used GA, DE, and the models estimated by swarm intelligence algorithms are closer to the true solution. The promising results imply that swarm intelligence algorithms are a potentially more powerful tool for inversion problems.

Water quality management and load reduction are subject to inherent uncertainties in watershed systems and competing decision objectives. Therefore, optimal decision-making modeling in watershed load reduction is suffering due to the following challenges: (a) it is difficult to obtain absolutely ``optimal'' solutions, and (b) decision schemes may be vulnerable to failure. The probability that solutions are feasible under uncertainties is defined as reliability. A reliability-oriented multi-objective (ROMO) decision-making approach was proposed in this study for optimal decision making with stochastic parameters and multiple decision reliability objectives. Lake Dianchi, one of the three most eutrophic lakes in China, was examined as a case study for optimal watershed nutrient load reduction to restore lake water quality. This study aimed to maximize reliability levels from considerations of cost and load reductions. The Pareto solutions of the ROMO optimization model were generated with the multiobjective evolutionary algorithm, demonstrating schemes representing different biases towards reliability. The Pareto fronts of six maximum allowable emission (MAE) scenarios were obtained, which indicated that decisions may be unreliable under unpractical load reduction requirements. A decision scheme identification process was conducted using the back propagation neural network (BPNN) method to provide a shortcut for identifying schemes at specific reliability levels for decision makers. The model results indicated that the ROMO approach can offer decision makers great insights into reliability tradeoffs and can thus help them to avoid ineffective decisions. (C) 2015 Elsevier B.V. All rights reserved.