Far-infrared photo-conductivity of electrons in an array of nano-structured antidots

We present far-infrared (FIR) photo-conductivity measurements for a two-dimensional electron gas in an array of nano-structured antidots. We detect, resistively and spectrally resolved, both the magnetoplasmon and the edge-magnetoplasmon modes. Temperature-dependent measurements demonstrates that both modes contribute to the photo resistance by heating the electron gas via resonant absorption of the FIR radiation. Influences of spin effect and phonon bands on the collective excitations in the antidot lattice are observed.Comment: 5 pages, 3 figure

Geographic genetic structure of Iberian columbines (gen. Aquilegia)

Southern European columbines (genus Aquilegia)are involved in active processes of diversification, and the Iberian Peninsula offers a privileged observatory to witness the process. Studies on Iberian columbines have provided significant advances on species diversification,but we still lack a complete perspective of the genetic diversification in the Iberian scenario. This work explores how genetic diversity of the genus Aquilegia is geographically structured across the Iberian Peninsula. We used Bayesian clustering methods, principal coordinates analyses, and NJ phenograms to assess the genetic relationships among 285 individuals from 62 locations and detect the main lineages. Genetic diversity of Iberian columbines consists of five geographically structured lineages, corresponding to different Iberian taxa. Differentiation among lineages shows particularly complex admixture patterns at Northeast and highly homogeneous toward Northwest and Southeast. This geographic genetic structure suggests the existence of incomplete lineage sorting and interspecific hybridization as could be expected in recent processes of diversification under the influence of quaternary postglacial migrations. This scenario is consistent with what is proposed by the most recent studies on European and Iberian columbines, which point to geographic isolation and divergent selection by habitat specialization as the main diversification drivers of the Iberian Aquilegia complex

Microbial diversity in waters, sediments and microbial mats evaluated using fatty acid-based methods

The review summarises recent advances towards a greater comprehensive assessment of microbial diversity in aquatic environments using the fatty acid methyl esters and phospholipid fatty acids approaches. These methods are commonly used in microbial ecology because they do not require the culturing of micro-organisms, are quantitative and reproducible and provide valuable information regarding the structure of entire microbial communities. Because some fatty acids are associated with taxonomic and functional groups of micro-organisms, they allow particular groups of micro-organisms to be distinguished. The integration of fatty acid-based methods with stable isotopes, RNA and DNA analyses enhances our knowledge of the role of micro-organisms in global nutrient cycles, functional activity and phylogenetic lineages within microbial communities. Additionally, the analysis of fatty acid profiles enables the shifts in the microbial diversity in pristine and contaminated environments to be monitored. The main objective of this review is to present the use of lipid-based approaches for the characterisation of microbial communities in water columns, sediments and biomats

Optical simulation of photonic random textures for thin-film solar cells

We investigate light-scattering textures for the application in thin-film solar cells which consist of a random texture, as commonly applied in thin-film solar cells, that are superimposed with a two-dimensional grating structure. Those textures are called photonic random texture. A scalar optical model is applied to describe the light-scattering properties of those textures. With this model, we calculate the angular resolved light scattering into silicon in transmission at the front contact and for reflection at the back contact of a microcrystalline silicon solar cell. A quantity to describe the lighttrapping efficiency is derived and verified by rigorous diffraction theory. We show that this quantity is well suitable to predict the short-circuit current density in the light-trapping regime, where the absorptance is low. By varying the period, height and shape of the unit cell, we optimize the grating structure with respect to the total generated current density. The maximal predicted improvement in the spectral range from 600-900 nm is found to be about 3 mA/cm2 compared to the standard random texture and about 6 mA/cm2 compared to a flat solar cell

Observations of a mass occurrene of Macoma balthica larvae in midsummer

In 1995 the seasonal development of concentrations of both phytoplankton and larvae of the bivalve Macoma balthica was studied in the coastal zone behind the back-barrier island of Spiekeroog (German Wadden Sea). In July=August larvaereached maximum concentrations of about 1000 to 4200 ind.

Investigation of local light scattering properties of thin-film silicon solar cells with subwavelength resolution

ABSTRACTIn order to obtain efficient light trapping within a thin-film silicon solar cell, randomly textured interfaces are used. The texture can be introduced by wet-chemical etching in diluted hyrdofluoric acid (HF). By varying of the HF concentration, a continuous transition to smaller surface structures can be achieved. Near-field scanning optical microscopy is applied to measure scattered light with sub-wavelength resolution. On those different surfaces, using Fourier high-pass filters on the measured near-field images, surface features with a high light trapping potential are identified. Finally, criteria for optimized scattering surfaces are obtained.</jats:p

Influence of defects in opal photonic crystals on the optical transmission imaged by near-field scanning optical microscopy

The electric field intensity above the surface of opal photonic crystals (PCs) and its alteration due to 'crystallographic' defects is investigated by using nearfield scanning optical microscopy (NSOM). The photonic crystals are developed by dip coating in a liquid solution with PMMA opals. Highly regular hexagonal planes with lattice constants of about 260 nm grow on the glass substrate. During the drying process several crack lines are formed that correspond to defects in the crystal structure. The transmitted light intensity at wavelengths inside and outside of the stop band of the PC is studied with NSOM using a tapered fiber tip scanning in all three dimensions. By this technique, a 3D image of the electric field intensity can be measured with a resolution better than 100 nm. The results show that the local optical field distribution is strongly dominated by the defect states in all directions in space over a length scale of several mu m. Above the crack lines, the intensity of light is strongly reduced. Beams of light are observed emerging from the edges of the crack lines and propagate in air with heights of more than 3 mu m. In between two different crack lines, periodic repetitions of the beams are observed. These results are interpreted as light diffraction on a microscopic scale