Optical response with threefold symmetry axis on oriented microdomains of opal photonic crystals

The paper deals with three-dimensional photonic crystals known as artificial opals, namely, fcc lattices of dielectric spheres: such systems have been the subject of numerous investigations. Opal photonic crystals viewed along the [111] direction of the fcc structure have a threefold symmetry axis; however this microscopic symmetry is difficult to observe in optical measurements performed on macroscopic areas containing microdomains with different orientations. In this work polarized transmittance measurements on [111]-stacked silica opals with single oriented microdomains, identified by field-emission scanning electron microscopy and laser-scanning confocal microscopy, demonstrate different optical response of twin structures with the two possible vertical stacking sequences. A detailed comparison with theory shows that microtransmittance experiments probe the photonic band structure along the Gamma-L-K and Gamma-L-U orientations of the Brillouin zone, respectively, thus giving conclusive evidence for macroscopic optical response related to the presence of a threefold (instead of a sixfold) symmetry axis in the photonic microstructure. The paper arises from a collaboration between the University of Pavia and the Politecnico di Torino

14 th Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes -2-6

Abstract In regional emission inventories road transport contribution is often evaluated by adopting a top-down methodology, which is based on the choice of large scale variables (for example fuel sold or consumed) then conveyed to smaller scale by using proxy variables (for example resident population or registered fleet). While this approach is the most effective for the compilation of national databases of emissions, since it allows to obtain a geographically complete and methodologically homogeneous set of information, when it comes to the assessment of air quality and to the evaluation of measures in alternative scenarios the bottom-up strategy could prove to be preferable. The amount of pollutants released by traffic networks is in this latter case estimated starting from site-specific data such as traffic flow, vehicles speed, vehicle categories and local fleet technical features (fuel supply, weight, etc.). The bottom-up approach is clearly affected by the amount and quality of information available, but it allows the estimate of emission data with a greater spatial and temporal detail. In this paper, we present a comparison of the results obtained by the application of the two approaches in evaluating road transport emissions in the metropolitan area of Torino, in which 1 350 000 inhabitants on average travel 8.5 km a day. Traffic flow data are available on an hourly basis on a network consisting of 5125 links, part of a larger road monitoring system. On site surveys have been used to differentiate vehicle categories on highway, rural and urban roads. Even if both methodologies are based on Copert IV, the quantitative comparison of estimated traffic emissions enlightens differences which are not negligible for some pollutants. In order to fully assess pros and cons of the two methodologies, emissions have been used to feed an air quality modelling system, based on the Eulerian chemical transport model FARM, on a 1 km horizontal spatial resolution square grid (51 x 51 km 2 ), using a regional simulation on a 4 km grid as one-way boundary conditions. The results emphasize a good description of the pattern distribution of pollution caused by the road network, treated as a linear source in the bottom-up approach, and show the appropriateness of using such an approach when proposed measures for abatement of traffic-related pollution need to be assessed. INTRODUCTION The Directive 2008/50/EC, in accordance with the previous European Directives, requires Member States to develop air quality plans in order to comply with air quality standards in the most effective and timesaving way. A reliable and updated estimation of emission sources is thus a crucial factor in the definition of effective air quality plans and related abatement measures

Carriers reactivation in p+-type porous silicon accompanies hydrogen desorption

FTIR spectra of p+-type mesoporous silicon (m-PS) outgassed in the 300-600 K range show a loss of transparency with increasing temperature, more pronounced at low frequencies. This is evidence of free-carrier formation. Previous work (F. Geobaldo et al., Sensors and Actuators B, in press [1]) concerning the reversible interaction of NO2 and NH3 has shown the presence at the surface of adsorption sites involving Si/B pairs. Thermal treatment of the sample causes desorption of molecular hydrogen, released through the homolytic splitting of Si-H bonds. Besides meeting each other forming a H2 molecule, H atoms may interact with an adsorption site, by creating a new H-Si-B bond. This new bond needs one additional electron to be formed and injection of a hole takes place into the solid. At higher temperatures, surface hydrogen is almost totally removed and the sample transparency recovered

Self-alignment of liquid crystals in three-dimensional photonic crystals

We report on the observation of self-alignment of nematic liquid crystals into colloidal photonic crystals, over distances much larger than the typical size of the voids between the spheres. We observe that the infiltrated structure possesses a unique optical axis that is determined by an intrinsic structural anisotropy of photonic crystal opals. We develop a simple model to describe this self-alignment based on the connectivity of the pores. The resulting structure constitutes a polarization dependent photonic crystal that can be controlled electricall