Large tunable photonic band gaps in nanostructured doped semiconductors

A plasmonic nanostructure conceived with periodic layers of a doped semiconductor and passive semiconductor is shown to generate spontaneously surface plasmon polaritons thanks to its periodic nature. The nanostructure is demonstrated to behave as an effective material modeled by a simple dielectric function of ionic-crystal type, and possesses a fully tunable photonic band gap, with widths exceeding 50%, in the region extending from mid-infra-red to Tera-Hertz.Comment: 6 pages, 4 figures, publishe

Universal metamaterial absorbe

We propose a design for an universal absorber, characterized by a resonance frequency that can be tuned from visible to microwave frequencies independently of the choice of the metal and the dielectrics involved. An almost resonant perfect absorption up to 99.8 % is demonstrated at resonance for all polarization states of light and for a very wide angular aperture. These properties originate from a magnetic Fabry-Perot mode that is confined in a dielectric spacer of $\lambda/100$ thickness by a metamaterial layer and a mirror. An extraordinary large funneling through nano-slits explains how light can be trapped in the structure. Simple scaling laws can be used as a recipe to design ultra-thin perfect absorbers whatever the materials and the desired resonance wavelength, making our design truly universal

Unlocking Capacity and Revisiting Political Will: Cambodia’s Public Financial Management Reforms

Over the past several years the Royal Government of Cambodia has successfully and consistently been implementing its Public Financial Management Reform Program (PFMRP), which has focused on improving the credibility of the budget while reducing fiduciary risk. This outcome is surprising not only because of the well known difficulties of implementing ambitious PFM reforms in low income, post-conflict countries, but also because most other reform programs in Cambodia have either failed or stalled, including an earlier effort at PFM reform (2001-2004). The paper develops a case study of the PFMRP (using the methodology in Barzelay et al., 2003) and argues that the success of the PFMRP is due to the way in which it was developed. The central hypothesis probed is that the public management processes and techniques that led to the development of the PFMRP are the same ones that explain its successful implementation. These include: a joint government-donor analytical process to define the problem and build consensus, an agreed reform vision and action plan, a pilot civil service reform in the Ministry of Finance to address capacity constraints, and formal coordination mechanisms for government and donors. The paper disputes the dominant hypothesis that the change was related to 'political will,' instead focusing on how public management solved the problem. The conclusion offers lessons on designing reform programs (in terms of public management processes and strategies) that may be applicable to other countries

Large tunable photonic band gaps in nanostructured doped semiconductors

A plasmonic nanostructure conceived with periodic layers of a doped semiconductor and passive semiconductor is shown to generate spontaneously surface plasmon polaritons thanks to its periodic nature. The nanostructure is demonstrated to behave as an effective material modeled by a simple dielectric function of ionic-crystal type, and possesses a fully tunable photonic band gap, with widths exceeding 50%, in the region extending from mid-infra-red to Tera-Hertz.Comment: 6 pages, 4 figures, publishe

Optimal design of the fiber-reinforcement to strengthen existing structures

An original approach is proposed to define the optimal design of any unidirectional fiber–reinforcement to improve the structural performance of existing structural elements. A problem of topology optimization is formulated, simultaneously searching for the regions to be strengthened and the optimal local fiber orientation. The maximum equivalent stress in the underlying material is minimized, for a given amount of reinforcement. The Tsai–Wu strength criterion is employed, to take into account the different strength properties of the material in tension and compression and the possible material anisotropy. Tensile stresses along the fiber direction are not allowed in the reinforcement. The resulting multi–constrained min–max problem is solved by mathematical programming. A numerical example is presented to discuss the features of the achieved optimal layouts, along with their possible application to the preliminary design of any fiber reinforcement

Analysis of masonry vaults as a topology optimization problem

Congreso celebrado en la Escuela de Arquitectura de la Universidad de Sevilla desde el 24 hasta el 26 de junio de 2015.An innovative approach is proposed to analyze 3D masonry vaults, assuming masonry to behave as a linear elastic no‐tension material. Masonry is replaced by a suitable equivalent orthotropic material with spatially varying elastic properties and negligible stiffness in any direction along which tensile stresses must be prevented. An energy‐based algorithm is implemented to define the distribution and the orientation of the equivalent material for a given load, minimizing the potential energy so as to achieve a purely compressive state of stress. The algorithm is embedded within a numerical procedure that performs a non‐incremental analysis under given loads. The collapse load of masonry structural elements can also be predicted running a sequence of independent analyses. The capabilities of the approach in predicting the crack pattern in typical masonry vaults are also shown

Optimal strengthening of masonry arch bridges with externally bonded reinforcing layers

Strengthening is a natural step following a failed bridge assessment. Referring to masonry bridges, a numerical tool is presented to find the optimal distribution of reinforcement to be externally bonded to two-dimensional elastic no-tension structural elements, with the aim of maximizing their overall stiffness. Notwithstanding the non-linearity of the adopted material model, no incremental procedure is needed to prescribe equilibrium of the strengthened element. Indeed, the same minimization procedure handles both the energy-based solution of the no-tension elastic body and the topology optimization problem that distributes the optimal reinforcement. A few numerical simulations are presented to assess the capabilities of the proposed procedure in defining the optimal reinforcement layouts for masonry arches and arch bridges, subjected to gravity loads and resting on fixed or elastic foundations. Designers can exploit the tool to sketch a preliminary layout of the FRP strengthening, which should be subsequently detailed according to technical codes

Analysis of no-tension structures under monotonic loading through an energy-based method

An approach is proposed to estimate the collapse load of linear elastic isotropic no-tension 2D solids. The material is replaced by a suitable equivalent orthotropic material with spatially varying local properties. A non-incremental energy-based algorithm is implemented to define the distribution and the orientation of the equivalent material, minimizing the potential energy so as to achieve a compression-only state of stress. The algorithm is embedded within a numerical procedure that evaluates the collapse mechanisms of structural elements under monotonic loading. The accuracy of the method is assessed through comparisons with the “exact” results predicted by limit analysi

Spin-Exchange Interaction in ZnO-based Quantum Wells

Wurtzitic ZnO/(Zn,Mg)O quantum wells grown along the (0001) direction permit unprecedented tunability of the short-range spin exchange interaction. In the context of large exciton binding energies and electron-hole exchange interaction in ZnO, this tunability results from the competition between quantum confinement and giant quantum confined Stark effect. By using time-resolved photoluminescence we identify, for well widths under 3 nm, the redistribution of oscillator strengths between the A and B excitonic transitions, due to the enhancement of the exchange interaction. Conversely, for wider wells, the redistribution is cancelled by the dominant effect of internal electric fields, which dramatically reduce the exchange energy.Comment: 14 pages, 3 figure