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Baskoutas S.,University of Patras | Garoufalis C.,University of Patras | Garoufalis C.,Technological Education Institute of Ionian Islands | Terzis A.F.,University of Patras
European Physical Journal B | Year: 2011

In the present theoretical study, the linear and third-order nonlinear optical absorption coefficients have been calculated in GaAs/Ga 1-x Al x As inverse parabolic quantum wells (single and double) subjected to an external electric field. Our calculations are based on the potential morphing method in the effective mass approximation. The systematic theoretical investigation contains results with all possible combinations of the involved parameters, as quantum well width, quantum barrier width, Al concentration at each well center and magnitude of the external electric field. Our results indicate that in most cases investigated, the increase of the electric field blue-shifts the peak positions of the total absorption coefficient. In all cases studied it became apparent that the incident optical intensity considerably affects the total absorption coefficient. © 2011 EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.


Schindler S.,University of Vienna | Schindler S.,University of Porto | Von Wehrden H.,Luneburg University | Poirazidis K.,WWF Greece | And 3 more authors.
Ecological Indicators | Year: 2013

Landscape metrics are widely used to investigate the spatial structure of landscapes. Numerous metrics are currently available, yet only little empirical research has comparatively examined their indicator value for species richness for several taxa at several scales. Taking a Mediterranean forest landscape - Dadia National Park (Greece) - as a case study area, we explored the performance of 52 landscape level landscape metrics as indicators of species richness for six taxa (woody plants, orchids, orthopterans, amphibians, reptiles, and small terrestrial birds) and for overall species richness. We computed the landscape metrics for circular areas of five different extents around each of 30 sampling plots. We applied linear mixed models to evaluate significant relations between metrics and species richness and to assess the effects of the extent of the considered landscape on the performance of the metrics. Our results showed that landscape metrics were good indicators for overall species richness, woody plants, orthopterans and reptiles. Metrics quantifying patch shape, proximity, texture and landscape diversity resulted often in well-fitted models, while those describing patch area, similarity and edge contrast rarely contributed to significant models. Spatial scale affected the performance of the metrics, since woody plants, orthopterans and small terrestrial birds were usually better predicted at smaller extents of surrounding landscape, and reptiles frequently at larger ones. The revealed pattern of relations and performances will be useful to understand landscape structure as a driver and indicator of biodiversity, and to improve forest and landscape management decisions in Mediterranean and other forest mosaics. © 2012 Elsevier Ltd. All rights reserved.


Zeng Z.,University of Patras | Garoufalis C.S.,University of Patras | Garoufalis C.S.,Technological Education Institute of Ionian Islands | Baskoutas S.,University of Patras | Bester G.,Max Planck Institute for Solid State Research
Physical Review B - Condensed Matter and Materials Physics | Year: 2013

In the present work, we studied the electronic and optical properties of ZnO quantum dots (QDs) subjected to externally applied hydrostatic pressure. Our single-particle calculations are based on the empirical pseudopotential method and the excitonic effects are considered by employing the configuration interaction approach. The optical band gap, Stokes shift, and optical emission polarization have been investigated as a function of the applied pressure. It is found that the applied pressure causes a linear increase in the optical band gap. The pressure coefficient appears to be highly size dependent, exhibiting a monotonic increase with increasing QD size. In contrast to this monotonic behavior, the applied pressure induces a nonmonotonic Stokes shift which presents a minimum value at a critical pressure. For pressures larger than this critical value, the optical emission polarization exhibits a sharp transition from in-plane to out-of-plane polarization. Finally, it is found that the critical pressure at which the crossing takes place strongly depends on the QD size, showing larger values for larger QD sizes. Beyond this crossing point, the lowest optically bright exciton state mainly originates from one Slater determinant, where both the single-particle electron and hole states have an S-type envelope function and the hole state originates mainly from the bulk Bloch C band. © 2013 American Physical Society.


Baskoutas S.,University of Patras | Zeng Z.,University of Patras | Garoufalis C.S.,University of Patras | Garoufalis C.S.,Technological Education Institute of Ionian Islands | Bester G.,Max Planck Institute for Solid State Research
Journal of Physical Chemistry C | Year: 2012

In the present work, we have performed a systematic theoretical study of the electronic and optical properties of ZnO nanorods as a function of an externally applied hydrostatic pressure. Our calculations are based on the empirical pseudopotential method and configuration interaction to consider the excitonic effects. The optical emission polarization is found to exhibit a transition between in-plane and out-of-plane polarization with increasing pressure. The critical value of the hydrostatic pressure at which the crossover takes place appears to depend strongly on the nanorod's radius. In particular, when the nanorod's radius is smaller than the ZnO exciton Bohr radius, the necessary pressure to reach the crossover point is significantly smaller than the pressure required for a nanorod with radius larger than the exciton Bohr radius. For large pressures, the lowest exciton state is a nearly pure state (0,0), where electron and hole have dominant S-orbital character and the hole has a nearly pure parentage from the bulk Bloch C-band. © 2012 American Chemical Society.


Zeng Z.,University of Patras | Garoufalis C.S.,University of Patras | Garoufalis C.S.,Technological Education Institute of Ionian Islands | Terzis A.F.,University of Patras | Baskoutas S.,University of Patras
Journal of Applied Physics | Year: 2013

In the present work, we investigated theoretically the linear, nonlinear, and total absorption coefficients and refractive index changes associated with intersubband transitions in ZnO/ZnS core shell quantum dot (CSQD) and ZnS/ZnO inverted CSQD (ICSQD), emphasizing on the influence of the shell thickness, impurity, and dielectric environment. The effect of the polarization charges due to the possible existence of the dielectric mismatch between the system and its surrounding matrix is considered. The electronic structures are numerically calculated by employing the potential morphing method in the framework of effective mass approximation. We find that in both impurity-free CSQD and ICSQD, increasing the shell thickness red shifts significantly the threshold energy and enhances drastically the nonlinear absorption coefficients and all the refractive index changes, independently on the dielectric environments. Similar behaviour has also been observed in most of the cases studied when the impurity is displaced from the core center to the shell center. In contrast, comparing to a dielectrically homogeneous system, dispersing the systems into a matrix with a lower dielectric constant blue shifts all the peak positions of the absorption coefficients and refractive index changes. However, the corresponding magnitudes (in absolute value) are substantially reduced. Finally, we find that the nonlinear properties are more sensitive to the external perturbations, while at a weak radiation intensity, the variation of the total quantities is generally dominated by that of the corresponding linear terms. © 2013 AIP Publishing LLC.

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