从理论上分析了以理想磁导体(perfect magnetic conductor,PMC)为边界的手征负折射平板波导的模式特性及其磁场的具体分布.利用PMC边界条件,导出了手征负折射波导的色散方程和磁场各分量表达式.通过波导的色散曲线发现,当手征参数κ>...从理论上分析了以理想磁导体(perfect magnetic conductor,PMC)为边界的手征负折射平板波导的模式特性及其磁场的具体分布.利用PMC边界条件,导出了手征负折射波导的色散方程和磁场各分量表达式.通过波导的色散曲线发现,当手征参数κ>1时,某些区域曲线出现下凹,左旋圆极化(left circularly polarized,LCP)波的折射率为负值,实现了负折射特性,截止频率也不再是传播常数β=0所对应的频率.根据传播常数β和K_+,K__的关系,将色散曲线分为3个区域,给出了各区域一阶奇模和一阶偶模磁场的具体分布,并与一般手征媒质平板波导的磁场分布作了比较.结果发现,切向分量场H_y,H_z在上下边界处为零,满足PMC边界条件.而由于面磁流的存在,法向分量H_x在边界处不为零.展开更多
Several studies have characterized the Yungas as a separate biogeographic unit, mainly based in floristic components. However, these characterizations were mainly qualitative and did not include faunal groups. The Yun...Several studies have characterized the Yungas as a separate biogeographic unit, mainly based in floristic components. However, these characterizations were mainly qualitative and did not include faunal groups. The Yungas have been assumed as a region with rich floral and faunal diversity, but without testing how well they are described by animal distributions. Our study consists of a formal analysis of endemism based on distribution of small mammals in the southernmost portion of the Yungas. This area is biogeographically very interesting because the Yungas are comprised of discontinuous fragments of forests that extend into temperate arid and semiarid habitats. As a first approximation, we contrasted a group of volant species （bats） versus a group of non-volant species （marsupials）. Our results show that small mammals are efficient indicators of endemism in Yungas. Eighty percent of the species of small marsupials included in the analysis supported the identified areas as being zones of endemism. Regarding bats, almost 55 percent of the species supported a designation of endemism. The results also show that the areas we considered are congruent with the botanical definition of the Yungas of northwestern Argentina and their subdivisions, an assumption that had not been previously tested with a formal quantitative method. We also found that non-volant species are better indicators of endemism than volant ones at regional scales, but volant species are better indicators than was previously thought.展开更多
The theoretical mechanism for realizing a negative refractive index material in an optical frequency range with an atomic gas system of electromagnetically induced transparency (EIT) is studied. It is shown that under...The theoretical mechanism for realizing a negative refractive index material in an optical frequency range with an atomic gas system of electromagnetically induced transparency (EIT) is studied. It is shown that under certain conditions such a dense gas can exhibit simultaneously negative permittivity and negative permeability, and negligibly small loss.展开更多
A completely open cavity, which is formed by three 60-degree wedges of a photonic crystal with negative effective index, is introduced. Such a realizable design for a completely open cavity (i.e., without any reflecti...A completely open cavity, which is formed by three 60-degree wedges of a photonic crystal with negative effective index, is introduced. Such a realizable design for a completely open cavity (i.e., without any reflective wall in the radial direction) is the first of its kind ever been reported. Due to the negative effective refraction index of the photonic crystal and the high transmission at the photonic crystal/air interfaces, a closed path with zero optical path is formed for the resonance. The present open cavity is very suitable for use as a biosensor as it has large air wedges into which a liquid measurand can flow easily.展开更多
文摘从理论上分析了以理想磁导体(perfect magnetic conductor,PMC)为边界的手征负折射平板波导的模式特性及其磁场的具体分布.利用PMC边界条件,导出了手征负折射波导的色散方程和磁场各分量表达式.通过波导的色散曲线发现,当手征参数κ>1时,某些区域曲线出现下凹,左旋圆极化(left circularly polarized,LCP)波的折射率为负值,实现了负折射特性,截止频率也不再是传播常数β=0所对应的频率.根据传播常数β和K_+,K__的关系,将色散曲线分为3个区域,给出了各区域一阶奇模和一阶偶模磁场的具体分布,并与一般手征媒质平板波导的磁场分布作了比较.结果发现,切向分量场H_y,H_z在上下边界处为零,满足PMC边界条件.而由于面磁流的存在,法向分量H_x在边界处不为零.
文摘Several studies have characterized the Yungas as a separate biogeographic unit, mainly based in floristic components. However, these characterizations were mainly qualitative and did not include faunal groups. The Yungas have been assumed as a region with rich floral and faunal diversity, but without testing how well they are described by animal distributions. Our study consists of a formal analysis of endemism based on distribution of small mammals in the southernmost portion of the Yungas. This area is biogeographically very interesting because the Yungas are comprised of discontinuous fragments of forests that extend into temperate arid and semiarid habitats. As a first approximation, we contrasted a group of volant species （bats） versus a group of non-volant species （marsupials）. Our results show that small mammals are efficient indicators of endemism in Yungas. Eighty percent of the species of small marsupials included in the analysis supported the identified areas as being zones of endemism. Regarding bats, almost 55 percent of the species supported a designation of endemism. The results also show that the areas we considered are congruent with the botanical definition of the Yungas of northwestern Argentina and their subdivisions, an assumption that had not been previously tested with a formal quantitative method. We also found that non-volant species are better indicators of endemism than volant ones at regional scales, but volant species are better indicators than was previously thought.
基金Project supported by the National Natural Science Foundation(NSF) of China (Nos. 90101024 and 60378037) and the NationalBasic Research Program (973) of China (No. 2004CB719805)
文摘The theoretical mechanism for realizing a negative refractive index material in an optical frequency range with an atomic gas system of electromagnetically induced transparency (EIT) is studied. It is shown that under certain conditions such a dense gas can exhibit simultaneously negative permittivity and negative permeability, and negligibly small loss.
基金Project supported by the National Basic Research Program (973) of China (No. 2004CB719801), and the National Natural Science Foun-dation of China (Nos. 90101024 and 60378037)
文摘A completely open cavity, which is formed by three 60-degree wedges of a photonic crystal with negative effective index, is introduced. Such a realizable design for a completely open cavity (i.e., without any reflective wall in the radial direction) is the first of its kind ever been reported. Due to the negative effective refraction index of the photonic crystal and the high transmission at the photonic crystal/air interfaces, a closed path with zero optical path is formed for the resonance. The present open cavity is very suitable for use as a biosensor as it has large air wedges into which a liquid measurand can flow easily.
