Far-field Perfect Imaging with Time Modulated Gratings

We study the capabilities of time-modulated diffraction gratings as imaging devices. It is shown that a time-dependent but transversally homogeneous slab can be used to make a perfect image of an object in the far-field, since all the evanescent modes couple to propagative time-diffracted orders. It is found that, if the image to be obtained is axially symmetric, it can be recovered by measuring the time-signal at a single point, without the need of performing a spatial scan, so that time gratings can act as well as single-pixel imaging devices. In the case of having an object without axial symmetry, the time-grating can be combined with a spatial grating, and then the full image can be recovered again with a measurement at a single point. We apply the theory of compressive sensing to optimize the recovery method and numerical examples are provided. We show therefore that time-modulated gratings can be used to perfectly recover the image of an object in the far field and after measuring at a single point in the space, being therefore a promising approach to superesoloution and ultra-fast imaging

Observation of Two-Dimensional Acoustic Bound States in the Continuum

The design of devices based on acoustic or optical fields requires the fabrication of cavities and structures capable of efficiently trapping these waves. A special type of cavity can be designed to support resonances with a theoretically infinite quality factor, named bound states in the continuum or BICs. The experimental measurement of such modes is still a challenging problem, as they are, by definition, not accessible from external perturbations. Therefore, current reported works rely on indirect measurements that are based on the traces left by these modes on external properties of one-dimensional systems. Here we report on the theoretical design and experimental realization of a two-dimensional, fully open acoustic resonator supporting BICs. This BIC, whose symmetry is chosen during design by properly tailoring the geometrical properties of the system, is completely accessible and allows for the direct measurement of the whole pressure field and properties. We experimentally demonstrate its existence with high quality factor and field enhancement properties

Separation of Students by Levels in 4th of Middle School and Social Complexity. Limits and Possibilities of the Comprehensive Curriculum

La agrupación o separación por niveles de rendimiento en la enseñanza secundaria es un tema muy estudiado en la sociología de la educación por su relación con el origen social de los alumnos, y muy discutido en la política educativa. En este artículo se contribuye al debate con el análisis empírico a partir de una muestra de 27 centros de secundaria y 1.782 alumnos de cuarto de ESO, de la ciudad de Barcelona. Con los datos de los centros se hace un análisis tipológico sobre la comprensividad, y con los datos de los alumnos se hace un análisis de rendimiento y de composición social de los centros. Los resultados muestran que existe mucha variedad y complejidad en la definición de comprensividad en las prácticas y opciones de los centros educativos, y que es mucho más difícil no establecer separaciones por nivel en centros de alta complejidad social. En los centros con menor complejidad, la comprensividad responde más a opciones de cada centro.Ability grouping according to the achievement students in secondary schools is a topic largely investigated in Sociology of Education, because the relationship between ability grouping and the students social background. The topic is also really controversial in educational policy. This paper wants to contribute to the issue with an empirical analysis based on a sample of 27 schools and 1782 tenth grade students from the city of Barcelona. The school data is used to construct typologies about the comprehensiveness of the schools, and the students data is used to analyze the achievement of pupils and the social composition of schools. The results show the enormous complexity and diversity of comprehensiveness among schools, but it seems to be more difficult not to segregate students according their achievement in schools with high degree of complexity. The comprehensiveness in schools with low degree of complexity could be a choice

High-quality resonances in quasi-periodic clusters of scatterers for flexural waves

Multiple scattering theory is applied to the study of clusters of point-like scatterers attached to a thin elastic plate and arranged in quasiperiodic distributions. Two types of structures are specifically considered: the twisted bilayer and the quasi-periodic line. The former consists in a couple of two-dimensional lattices rotated a relative angle, so that the cluster forms a moir. pattern. The latter can be seen as a periodic one-dimensional lattice where an incommensurate modulation is superimposed. Multiple scattering theory allows for the fast and efficient calculation of the resonant modes of these structures as well as for their quality factor, which is thoroughly analyzed in this work. The results show that quasi-periodic structures present a large density of states with high quality factors, being therefore a promising way for the design of high quality wave-localization devices

A protease activity-depleted environment for heterologous proteins migrating towards the leaf cell apoplast

Recombinant proteins face major constraints along the plant cell secretory pathway, including proteolytic processing compromising their structural integrity. Here, we demonstrate the potential of protease inhibitors as in situ stabilizing agents for recombinant proteins migrating towards the leaf apoplast. Genomic data for Arabidopsis, rice and Nicotiana spp. were assessed to determine the relative incidence of protease families in the cell secretory pathway. Transient expression assays with the model platform Nicotiana benthamiana were then performed to test the efficiency of protease inhibitors in stabilizing proteins targeted to the apoplast. Current genomic data suggest the occurrence of proteases from several families along the secretory pathway, including A1 and A22 Asp proteases; C1A and C13 Cys proteases; and S1, S8 and S10 Ser proteases. In vitro protease assays confirmed the presence of various proteases in N. benthamiana leaves, notably pointing to the deposition of A1- and S1-type activities preferentially in the apoplast. Accordingly, transient expression and secretion of the A1/S1 protease inhibitor, tomato cathepsin D inhibitor (SlCDI), negatively altered A1 and S1 protease activities in this cell compartment, while increasing the leaf apoplast protein content by ~45% and improving the accumulation of a murine diagnostic antibody, C5-1, co-secreted in the apoplast. SlCYS9, an inhibitor of C1A and C13 Cys proteases, had no impact on the apoplast proteases and protein content, but stabilized C5-1 in planta, presumably upstream in the secretory pathway. These data confirm, overall, the potential of protease inhibitors for the in situ protection of recombinant proteins along the plant cell secretory pathway. © 2011 The Authors. Plant Biotechnology Journa