Laser-induced periodic surface structures:Fingerprints of light localization

The finite-difference time-domain (FDTD) method is used to study the inhomogeneous absorption of linearly polarized laser radiation below a rough surface. The results are first analyzed in the frequency domain and compared to the efficacy factor theory of Sipe and coworkers. Both approaches show that the absorbed energy shows a periodic nature, not only in the direction orthogonal to the laser polarization, but also in the direction parallel to it. It is shown that the periodicity is not always close to the laser wavelength for the perpendicular direction. In the parallel direction, the periodicity is about lambda/Re((n) over tilde), with (n) over tilde being the complex refractive index of the medium. The space-domain FDTD results show a periodicity in the inhomogeneous energy absorption similar to the periodicity of the low-and high-spatial-frequency laser-induced periodic surface structures depending on the material's excitation

Short and ultrashort pulsed laser processing of zinc : Resolidification morphology of ablated craters

Laser surface texturing of polycrystalline zinc in bulk form and as galvanised coatings is performed with single, as well as multiple nanosecond and picosecond laser pulses at a wavelength of 1030 nm. By varying the laser fluence over a wide range, as well as varying the number of pulses on the same location, thousands of ablated craters can be fabricated and their morphology studied. From the analysis thereof, it can be concluded that, within the boundaries of our experimental (laser) conditions, ablated craters always show molten features. Depending on the pulse fluence and duration, the final solidifed morphology can feature jets with meso-and micro scale spherical endings, micro rims and protrusions, as well as nano-roughness or a rim around the ablated crater. If the laser fluence is high, one observes the evidence of surface waves and finger formation, resulted from different types of hydrodynamic instablity. In contrast to nanosecond laser processing, it is found that craters formed using picosecond pulses deviate significantly from the Gaussian shape of the laser intensity distribution at high laser intensity, due to different temperature gradients in the electronic and lattice subsystems

Crystal structure of laser-induced subsurface modifications in Si

Laser-induced subsurface modification of dielectric materials is a well-known technology. Applications include the production of optical components and selective etching. In addition to dielectric materials, the subsurface modification technology can be applied to silicon, by employing near to mid-infrared radiation. An application of subsurface modifications in silicon is laser-induced subsurface separation, which is a method to separate wafers into individual dies. Other applications for which proofs of concept exist are the formation of waveguides and resistivity tuning. However, limited knowledge is available about the crystal structure of subsurface modifications in silicon. In this work, we investigate the geometry and crystal structure of laser-induced subsurface modifications in monocrystalline silicon wafers. In addition to the generation of lattice defects, we found that transformations to amorphous silicon and Si-iii/Si-xii occur as a result of the laser irradiation

Laser-induced periodic surface structures: fingerprints of light localization

The finite-difference time-domain (FDTD) method is used to study the inhomogeneous absorption of linearly polarized laser radiation below a rough surface. The results are first analyzed in the frequency domain and compared to the efficacy factor theory of Sipe and coworkers. Both approaches show that the absorbed energy shows a periodic nature, not only in the direction orthogonal to the laser polarization, but also in the direction parallel to it. It is shown that the periodicity is not always close to the laser wavelength for the perpendicular direction. In the parallel direction, the periodicity is about λ/Re(ñ), with ñ being the complex refractive index of the medium. The space-domain FDTD results show a periodicity in the inhomogeneous energy absorption similar to the periodicity of the low- and high-spatial-frequency laser-induced periodic surface structures depending on the material's excitation

Demographic and life-history variability across the range of a widespread herb: the role of environmental, geographical and genetic factors / Variabilidad demográfica y de historia vital en una planta de amplia distribución: el papel de los factores medioambientales, geográficos y genéticos

[spa] Las especies de amplia distribución han recibido tradicionalmente poca atención, a pesar de su importancia para la estructura y el funcionamiento de los ecosistemas. En esta tesis, se analiza la variabilidad demográfica, de historia vital y genética en una planta de amplia distribución en Europa y el norte de África (Plantago coronopus), en un total de 22 poblaciones a lo largo de gran parte del rango latitudinal de la especie (centro y periferia norte). Se pretende analizar la magnitud y las causas de esta variabilidad intraespecífica en relación con la posición de las poblaciones dentro del rango y con los principales factores medioambientales. Las poblaciones periféricas mostraron una menor diversidad genética, pero no mostraron en general un peor o más variable comportamiento demográfico en cuanto a densidad o tasa de crecimiento poblacional, contradiciendo así las hipótesis clásicas centro-periferia. Se encontró un mismo patrón de diferenciación demográfica dentro de las regiones tanto central como periférica, en relación con la variación en el régimen de precipitaciones. La tasa de reclutamiento de nuevos individuos fue el proceso del ciclo vital con mayor importancia para el funcionamiento de las poblaciones. Se encontró también una gran variación entre poblaciones en las características de las semillas (número, tamaño, mucílago y proporción de dos tipos de semilla dimórficos) en relación con el gradiente de estrés ambiental. Finalmente, la variación fenotípica dentro de las poblaciones se relacionó con la variabilidad ambiental, mientras que la diversidad genética se correlacionó con la posición central vs. periférica de las poblaciones y posiblemente con la historia demográfica de la especie. Globalmente, este estudio muestra la importancia de distinguir entre periferia geográfica y marginalidad ecológica, y sugiere que el éxito de las plantas de amplia distribución reside en una gran variabilidad demográfica y de historia vital a diferentes escalas espaciales.[eng] Widespread species have traditionally received much less attention than rare and endemic ones. However, they are crucial in macroecological patterns and in ecosystem structure and functioning. Thus, understanding the characteristics that allow widespread organisms to extend over large areas has a high interest from both theoretical and applied perspectives. One of the most frequent hypotheses to explain the success of widespread plants is that they show much wider ecological niches, and thus a high life-history and demographic variability. However, studies are often very specific and carried out over small spatio-temporal scales, which hinders a general understanding of intraspecific variation in widespread taxa. In this thesis, we span a large spatio-temporal scale and a large environmental gradient to analyze the magnitude and the possible causes of natural variation in the in the range centre and the northern periphery of the widespread herb Plantago coronopus. More precisely, we analyze variability in population dynamics, life-history traits, and genetic diversity in up to 22 populations in Europe and North Africa. We aim to explore the relation of such variability with the position of populations within the species’ range, since peripheral populations are traditionally expected to show a lower and more variable performance with respect to central populations. Additionally, we aim to analyze the effects of the most relevant environmental factors in population and individual performance at different spatial scales. In the first chapter, we found higher values in central populations in some vital rates, such as fecundity and growth, but recruitment and density were higher in northern peripheral populations, and there were no clear differences between regions in temporal variability of vital rates. Differences in population performance across the species’ range seemed to be correlated with local precipitation and intraspecific competition. In the second chapter, differences in mean values and variability of vital rates between central and peripheral areas led to no differences in stochastic population growth rates. In addition, recruitment was the most influential vital rate for population growth rates at different spatial scales, and we found the same pattern of differentiation in population dynamics in response to environmental conditions within central and peripheral regions. In the third chapter, we reported high variation among populations in seed traits along a steep environmental stress gradient. Moreover, patterns in seed production were opposite at the fruit and the individual scale, as a strategy of populations to maximize fitness in each set of local conditions. Finally, in the fourth chapter, we found no relationship within populations between phenotypic variability and genetic diversity. Phenotypic variation was mainly shaped by precipitation variability, suggesting adaptive variation, whereas genetic diversity was correlated with the central vs. peripheral position, probably in close relation with some random demographic processes experienced by populations in the past. Despite genetic diversity was higher in central populations, our results contradicted classical hypotheses predicting a lower demographic performance towards species’ range edges. In fact, environmental conditions seemed to have a higher influence on plant performance than the position of populations within the species’ range, which calls for the necessity of distinguishing between geographical periphery and ecological marginality in demographic studies. Overall, our study highlights the versatility of P. coronopus at different spatial scales in response to varying environmental conditions, complementing similar findings of previous research on the same taxon at smaller spatial scales. Such life-history variability seems to be a key factor for widespread plants to extend over large and heterogeneous ranges

On the surface topography of ultrashort laser pulse treated steel surfaces

This paper concentrates on observations of the surface topography by scanning electron microscopy (SEM) on alloyed and stainless steels samples treated by ultrashort laser pulses with duration of 210 fs and 6.7 ps. Globular-like and jet-like objects were found depending on the various levels of the fluence applied. It is shown that these features appear due to solid-liquid and liquid-gas transitions within surface layer irradiated by intense laser light. The observations are confronted to the theory of short-pulsed laser light-matter interactions, including interference, excitation of electrons, electron-phonon coupling as well as subsequent ablation. It is shown that the orientation of small ripples does not always depend on the direction of the polarization of laser light. (C) 2011 Elsevier B.V. All rights reserved