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Reactive oxygen species (ROS) and reactive nitrogen species (RNS) play a fundamental role in regulating endothelial function and vascular tone in the physiological conditions of a vascular system. However, oxidative stress has detrimental effects on human health, and numerous studies confirmed that high ROS/RNS production contributes to the initiation and progression of cardiovascular diseases. The antioxidant defense has an essential role in the homeostatic functioning of the vascular endothelial system. Endogenous antioxidative defense includes various molecules and enzymes such as superoxide dismutase, catalase, glutathione reductase, and glutathione peroxidase. Together all these antioxidative enzymes are essential for defense against harmful ROS features. ROS are mainly generated from redox-active compounds involved in the mitochondrial respiratory chain. Thus, targeting antioxidative enzymes and mitochondria oxidative balance may be a promising approach for vascular diseases occurrence and treatment. This review summarized the most recent research on the regulation of antioxidative enzymes in vascular diseases.

ZnO based heterostructures have recently received increased research attention, related to the development of room temperature THz/MiR semiconductor devices. The potential for these applications stems from the combination of wide direct bandgap and high exciton binding energy. In this work, we focus on the intersubband transition between bound states in the conduction band, and apply self-consistent numerical modelling to a system of Schrödinger–Poisson equations to evaluate the electronics structure of coupled semiconductor quantum wells. We subsequently analyse the fractional optical absorption at room temperature, as it varies with layers’ thicknesses, doping density and external electric field magnitude.

Background: Gentiana lutea (GL), commonly known as yellow gentian, bitter root, and bitterwort, belongs to family Gentianaceae. GL belongs to genus Gentiana, which is a rich natural source of iridoids, secoiridoids, xantones, flavonoids, triterpenoids, and carbohydrates. Medicinal plants from Gentiana species have anti-oxidant, anti-inflammatory, anti-mitogenic, anti-proliferative, and lipidlowering effects, as well as a cardioprotective, hypotensive, vasodilator and anti-platelet activities. Objective: We reviewed the recent literature related to the effects of Gentiana species, and their active components on vascular diseases. Methods: Data used for this review were obtained by searching the electronic database [PUBMED/ MEDLINE 1973 - February 2020]. The primary data search terms of interest were: Gentiana lutea, Gentienacea family, phytochemistry, vascular diseases, treatment of vascular diseases, antioxidant, anti-inflammatory, anti-atherogenic. Conclusion: Gentiana species and their constituents affect many different factors related to vascular disease development and progression. Therefore, Gentiana-based therapeutics represent potentially useful drugs for the management of vascular diseases.

This study aimed to investigate the effect of 150 mg/L sodium fluoride (NaF) on redox status parameters and essential metals [copper (Cu), iron (Fe), and zinc (Zn)] in the blood, liver, kidney, brain, and spleen of Wistar rats and to determine the protective potential of selenium (Se) against fluoride (F-) toxicity. Male Wistar rats were randomly distributed in groups of five (n=5) receiving tap water (control) or water with NaF 150 mg/L, NaF 150 mg/L + Se 1.5 mg/L, and Se 1.5 mg/L solutions ad libitum for 28 days. Fluorides caused an imbalance in the redox and biometal (Cu, Fe, and Zn) status, leading to high superoxide anion (O2.-) and malondialdehyde (MDA) levels in the blood and brain and a drop in superoxide dismutase (SOD1) activity in the liver and its increase in the brain and kidneys. Se givenwith NaF improved MDA, SOD1, and O2.- in the blood, brain, and kidneys, while alone it decreased SH group levels in the liver and kidney. Biometals both reduced and increased F- toxicity. Further research is needed before Se should be considered as a promising strategy for mitigating F- toxicity., Cilj ovog istraživanja bio je utvrditi djelovanje 150 mg/L natrijevog fluorida (NaF) na redoks-status i koncentracije esencijalnih elemenata [bakar (Cu), željezo (Fe) i cink (Zn)] u krvi, jetri, bubrezima, mozgu i slezeni Wistar štakora te mogući zaštitni učinak selena (Se) od toksičnosti prouzročene fluoridom (F-). Mužjaci Wistar štakora nasumično su razvrstani u četiri skupine (n=5), nakon čega su 28 dana konzumirali običnu vodu ili vodu s otopinom NaF 150 mg/L, NaF 150 mg/L + Se 1,5 mg/L ili Se 1,5 mg/L. Izloženost fluoridu dovela je do poremećaja redoks-parametara i koncentracija istraživanih biometala. Utvrđene su povišene razine superoksid aniona (O2.-) i malondialdehida (MDA) u krvi i mozgu, smanjena aktivnost superoksid dismutaze (SOD1) u jetri te njezin porast u mozgu i bubrezima. Nadomjesni Se u kombinaciji s NaF pozitivno je utjecao na razine MDA, SOD1 i O2.- u krvi, mozgu i bubrezima, a sâm Se smanjio je razine SH skupina u jetri i bubrezima. Izloženost fluoridu uzrokovala je sniženje, ali i porast koncentracija biometala. Nužna su dodatna istraživanja kako bi se ispitali antioksidacijski učinci Se na toksičnost izazvanu F-.

In this work, we have systematically studied the impact of geometric parameters on chiral effects which occur in metamaterial made of Ω elements. The analysis of group delays and electric field enhancements indicate stronger interaction between the structure and right circularly polarized waves. Calculations of optical activity and circular dichroism for different values of geometric parameters, allow us to deduce that the thickness of the wire and the gap size have large influence on these effects. On the other hand, the radius of the wire affects mainly the resonant frequency position which enables frequency tunability. The results indicate that multi-functional design of this chiral metamaterial offers great flexibility for various devices: enantiomeric sensors, circular polarizers and light modulators. © 2019 Elsevier B.V.

Linkage between bis(2-ethylhexyl) phthalate (DEHP), dibutyl phthalate (DBP), and bisphenol A (BPA) co-exposure and type 2 diabetes mellitus (T2DM), as well as ability of multi-strained probiotic to reduce DEHP, DBP and BPA mixture-induced oxidative damage in rat pancreas were investigated. The Comparative Toxicogenomics Database, Cytoscape software and ToppGene Suite were used for data-mining. Animals were sorted into seven groups (n = 6): (1) Control group: corn oil, (2) P: probiotic: Saccharomyces boulardii + Lactobacillus rhamnosus + Lactobacillus plantarum LP 6595 + Lactobacillus plantarum HEAL9; (3) DEHP: 50 mg/kg b.w./day, (4) DBP: 50 mg/kg b.w./day, (5) BPA: 25 mg/kg b.w./day, and (6) MIX: 50 mg/kg b.w./day DEHP + 50 mg/kg b.w/day DBP + 25 mg/kg b.w./day BPA; (7) MIX + P. Rats were sacrificed after 28 days of oral exposure. In silico investigation highlighted 44 DEHP, DBP and BPA mutual genes linked to the T2DM, while apoptosis and oxidative stress were highlighted as the main mechanisms of DEHP, DBP and BPA mixture-linked T2DM. In vivo experiment confirmed the presence of significant changes in redox status parameters (TOS, SOD and SH groups) only in the MIX group, indicating possible additive effects, while probiotic ameliorated mixture-induced redox status changes in rat pancreatic tissue.

In this Chapter we explore the possibilities of using advanced optimization methods to obtain structural parameters of gain-maximized quantum cascade lasers emitting at specified wavelengths, suitable for detection of targeted chemicals. This involves the application of specialized techniques for the so-called “quantum engineering”, including sophisticated methods of quantum mechanics, such as supersymmetry and inverse spectral theory, that have been successfully transferred into applied research areas. Particular attention is devoted to modern methods for global optimization: simulated annealing algorithm, multi-objective evolutionary algorithms, procedures based on applying the optimal control theory, which eliminate the unnecessary constraints from the optimization process.

The aims of this study were to: (i) examine the toxic effects of sodium fluoride (NaF) in blood, liver, spleen, and brain cells of Wistar rats after the subacute exposure; (ii) explore the potential protective properties of selenium (Se) against fluoride toxicity after the simultaneous administration. Twenty male Wistar rats, eight weeks old, weighing approximately 140–190 g, were divided into four experimental groups (n = 5) as follows: I control-tap water; II NaF 150 ppm; III NaF 150 ppm and Se 1.5 mg/L; IV Se 1.5 mg/L, and had available water with solutions ad libitum for 28 days. DNA damage detected by comet assay was confirmed in the liver, spleen, and brain cells, but not in blood. Selenium supplementation together with NaF decreased DNA damage in liver and spleen cells. According to the histological findings, no changes were observed in spleen and brain tissues after NaF administration. Unlike the observed Se protective effect on the DNA level, no significant reduction of liver tissue injury was observed after the NaF and Se treatment, resulting in mild inflammation. Data of this study suggest that DNA damage after NaF subacute exposure at moderately high concentration was reduced in liver and spleen cells due to Se supplementation, but a similar change was not seen in the brain.

We review our recent theoretical studies on multimode instabilities in Fabry–Perot cavity mid-infrared quantum cascade lasers (QCLs) caused by parametric excitation of Rabi flopping oscillations. Numerical simulations are based on the semiclassical traveling wave Maxwell–Bloch equations. QCLs with a few mm cavity without an absorber exhibit intermittent RNGH self-pulsations, while regular self-pulsations are possible in short-cavity QCLs, with the cavity length of 100 μm or smaller. However, the second threshold in short-cavity QCLs is significantly increased compared to the values for a few mm long devices. We provide here a new insight on RNGH instability via bifurcation analysis of the output waveform and studies of the recurrence period density entropy. We propose an interpretation of the broadening/narrowing of the optical spectrum of a QCL i.e. switching the RNGH instability on and off observed in experiment.

Vascular complications in patients with diabetes mellitus (DM) are common. Since impaired oxygen balance in plasma plays an important role in the pathogenesis of chronic DM-associated complications, the administration of hyperbaric oxygen therapy (HBOT) has been recommended to influence development of vascular complications. Hyperbaric oxygen therapy involves inhalation of 100% oxygen under elevated pressure from 1.6 to 2.8 absolute atmospheres in hyperbaric chambers. Hyperbaric oxygen therapy increases plasma oxygen solubility, contributing to better oxygen diffusion to distant tissues and preservation of the viability of tissues reversibly damaged by atherosclerosis-induced ischemia, along with microcirculation restoration. Hyperbaric oxygen therapy exerts antiatherogenic, antioxidant, and cardioprotective effects by altering the level and composition of plasma fatty acids and also by promoting signal transduction through membranes, which are impaired by hyperglycemia and hypoxia. In addition, HBOT affects molecules involved in the regulation of nitric oxide synthesis and in that way exerts anti-inflammatory and angiogenic effects in patients with DM. In this review, we explore the recent literature related to the effects of HBOT on DM-related vascular complications.

By the application of processes of stabilization through drying, separation of seed and grinding of remaining placenta dietary fiber-rich powdered product was obtained from pepper (Capsicum annum L.) processing waste. Effects of dosing of pepper placenta-based supplement (1, 2 and 3%) on farinograph, extensograph and amylograph parameters, as well as test baking, were conducted in this study. Addition of pepper placenta-based supplement resulted in an increase of farinograph water absorption in obtaining of darker and more intensively colored bread crust and in smaller and more evenly distributed pores of breadcrumb. It also inhibited the firming of breadcrumb during bread shelf life. The main negative effects of the application of placenta-based supplement were the increase of dough resistance and the decrease of breadcrumb cohesiveness. Keywords: Bakery, fiber, pepper, products, waste

Tunneling times in complex potentials are investigated. Analytical expressions for dwell time, self-interference time and group delay are obtained for the case of complex double delta potentials. It is shown that we can always find a set of parameters of the potential so that the tunneling times achieve very large values and even approach infinity for the case of resonance. The phenomenon of infinite tunneling times occurs for only one particular positive value of the imaginary part of the potential, if all other parameters are given.

We have theoretically explored the influence of the magnetic field on supporting the multimode Risken-Nummendal-Graham-Haken (RNGH) self-pulsations seen in the optical spectrum as two broad modulations sidebands at the Rabi flopping frequency [25] , [26] , [27] . On the example of microcavity quantum cascade lasers (μ-QCLs), we demonstrate that Landau quantization in an external magnetic field slows down the effective decoherence and diffusion rates. The pump current required to reach the broadband multimode RNGH self-pulsations is lowered with the magnetic field strength, while the Rabi flopping frequency and the overall optical spectrum width remain practically unchanged. Our theoretical results indicate that an external magnetic field can be a valuable tool for achieving high-power broadband QCL self-pulsations in practice.

Tunneling times in complex bianisotropic materials have been examined in detail, with absorption and dispersion taken into account. Tunneling is characterized by the dwell and the phase tunneling time. In this paper, we have developed a theoretical model and derived the appropriate expressions for each of these quantities, as well as a relationship between them and the corresponding expression for the energy density. The model has been verified through numerical calculations based on experimental data. We have distinguished cases in which the phases of transmitted and incident wave match each other, and showed that for small angles of incidence, the time that the wave spends inside the barrier can be approximated as a linear function of the barrier width. The Hartman effect has been detected, although for very thick layers of metamaterial.

We present a method for modeling nonparabolicity effects (NPE) in quantum nanostructures in presence of external electric and magnetic field by using second order perturbation theory. The method is applied to analysis of quantum well structure and active region of a quantum cascade laser (QCL). This model will allow us to examine the influence of magnetic field on dipole matrix element in QCL structures, which will provide a better insight to how NPE can affect the gain of QCL structures.

We consider the influence of additional carrier confinement, achieved by application of strong perpendicular magnetic field, on inter Landau levels electron relaxation rates and the optical gain, of two different GaAs quantum cascade laser structures operating in the terahertz spectral range. Breaking of the in-plane energy dispersion and the formation of discrete energy levels is an efficient mechanism for eventual quenching of optical phonon emission and obtaining very long electronic lifetime in the relevant laser state. We employ our detailed model for calculating the electron relaxation rates (due to interface roughness and electron-longitudinal optical phonon scattering), and solve a full set of rate equations to evaluate the carrier distribution over Landau levels. The numerical simulations are performed for three- and four-well (per period) based structures that operate at 3.9 THz and 1.9 THz, respectively, both implemented in GaAs/Al0.15Ga0.85As. Numerical results are presented for magnetic field values from 1.5 T up to 20 T, while the band nonparabolicity is accounted for. (C) 2016 Elsevier B.V. All rights reserved.

Photovoltaics is amongst the most important technologies for renewable energy sources, and plays a key role in the development of a society with a smaller environmental footprint. Key parameters for solar cells are their energy conversion efficiency, their operating lifetime, and the cost of the energy obtained from a photovoltaic system compared to other sources. The optimization of these aspects involves the exploitation of new materials and development of novel solar cell concepts and designs. Both theoretical modeling and characterization of such devices require a comprehensive view including all scales from the atomic to the macroscopic and industrial scale. The different length scales of the electronic and optical degrees of freedoms specifically lead to an intrinsic need for multiscale simulation, which is accentuated in many advanced photovoltaics concepts including nanostructured regions. Therefore, multiscale modeling has found particular interest in the photovoltaics community, as a tool to advance the field beyond its current limits. In this article, we review the field of multiscale techniques applied to photovoltaics, and we discuss opportunities and remaining challenges. © T. Abu Hamed et al., published by EDP Sciences, 2018., European Cooperation in Science and Technology: MP1406, The authors are grateful for the financial support by the COST Action MP1406 “MultiscaleSolar.”

We demonstrate several mechanisms for switching QCL frequency comb on and off in double-resonance mid-infrared InGaAs/InAlAs Quantum Cascade Lasers using nonstationary pump current and unstable part of the I-V curve. All of them are based on Risken-Nummedal-Graham-Haken instability.

By the application of processes of stabilization through drying, separation of seed and grinding of remaining placenta dietary fiber-rich powdered product was obtained from pepper (Capsicum annum L.) processing waste. Effects of dosing of pepper placenta-based supplement (1, 2 and 3%) on farinograph, extensograph and amylograph parameters, as well as test baking, were conducted in this study. Addition of pepper placenta-based supplement resulted in an increase of farinograph water absorption in obtaining of darker and more intensively colored bread crust and in smaller and more evenly distributed pores of breadcrumb. It also inhibited the firming of breadcrumb during bread shelf life. The main negative effects of the application of placenta-based supplement were the increase of dough resistance and the decrease of breadcrumb cohesiveness. Keywords: Bakery, fiber, pepper, products, waste

We considered electromagnetic wave propagating through double semiconductor layered structure with anisotropic permittivity. A model which allows the analysis of the permittivity is provided. Transmission, reflection and absorption characteristics of the structure were analyzed in frequency span of interest. Expressions for the dwell time and group delay were obtained for the considered barrier system and their values discussed. A relation which links dwell time with group delay is derived. The appearance of Hartman effect was confirmed.

In this work we theoretically investigate a possibility to use cubic nitride based multi-layer periodic nanostructure as a semiconductor metamaterial. The structure design is based on an active region of a quantum cascade laser optimized to achieve optical gain in the Terahertz (THz) spectral range. In particular, we test the GaN/AlGaN quantum well configurations, which should exhibit important advantages compared to GaAs-based structures, namely room temperature operation without the assistance of magnetic field and lower doping densities. Our numerical rate-equations model is solved self-consistently and it takes into account electron-longitudinal optical phonon scattering between all the relevant states among the adjacent periods of the structure. A global optimization routine, specifically genetic algorithm is then used to generate new gain-optimized structures. This work confirms the advantages of cubic GaN designs over GaAs ones, namely feasibility of negative refraction at room temperature without the assistance of magnetic field while keeping the doping densities of the same order of magnitude. © 2018, Springer Science+Business Media, LLC, part of Springer Nature.

Supersymmetric quantum mechanics (SUSYQM) is a method that can be used for generating complex potentials with entirely real spectrum with bound states in the continuum (BIC). These complex potentials are isospectral with the initial one, but SUSYQM method adds discrete BIC's at selected energies. Corresponding wavefunctions created by SUSYQM are biorthogonal and complex, hence we can discuss their phase rigidity and illustrate the application of SUSYQM on the examples of three specific potential profiles (free electron, negative Dirac potential and quantum well with infinite walls).

Tunneling of electrons through a barrier with complex potential is investigated. We focus on two cases, symmetric double rectangular barrier and double delta potential barrier, and give expressions for resonant transmission probability for both cases. Expressions for reflection amplitude and absorption are also obtained in the case of delta potential. It will be shown that for given incident particle energy, parameters of the potential structure can be obtained so that the resonant transmission probability approaches infinity. The phenomenon of infinite transmission occurs only for potentials which have positive imaginary part.

One of the challenges in the design of metamaterials’ unit cells is the reduction of losses caused by the metallic inclusions. In order to overcome this obstacle, it has been proposed to use the active medium as the unit cell. Quantum cascade lasers are great candidates for the active medium materials since they are able to provide high values of optical gain. In this paper we investigate and compare two quantum cascade structures optimized for emission frequencies lower than 2 THz and simulate the effect of a strong magnetic field applied perpendicularly to the layers. Comprehensive description of conduction-band nonparabolicity is used to calculate the electronic structure, and subsequently evaluate the longitudinal optical phonon and interface roughness scattering rates and solve the system of rate equations which govern the distribution of carriers among the Landau levels. Once we assess the degree of population inversion, we have all the necessary information about the permittivity component along the growth direction of the structure and may determine the conditions under which the structure displays negative refraction.

Influence of the Rashba effect on electronic properties of resonant tunneling diode in an external magnetic field is analyzed in this paper. Wave functions and energies, as well as expressions for currents densities, are determined for electrons of both spins. Appearances of many modes due to the external magnetic field induce irregularities in the current–voltage characteristics, which are observable in case when the thermal energy is lower than, or comparable to, the energy difference of two consecutive Landau levels. Current density through the heterostructure is investigated with emphasis on the degree of spin polarization; further, spin transfer is shown to depend on the direction of external magnetic field.

Applied research, carried out to examine the possibility of improving students’ understanding of heat phenomena through multi-stage homework and teacher-supported classwork assignments, is described. These assignments, designed within the theoretical framework of self-regulated learning, enable students to overcome errors and doubts from the individual problem solving stage by peer interaction and teacher feedback and to reflect on individual and group learning experiences. They also promote the development of students’ self-efficacy, critical consideration of information and error detection, making individual and group learning processes more visible to students and teacher. This applied research was conducted in an intact grade VII primary school physics class in Serbia. Analysis shows significant progress in students’ understanding of the anomalous behavior of water and their ability to interpret a non-linear graph. The level of self-reflection outcomes depends mostly on the quality of teacher-supported group classwork. It is noted that students should improve expressive and critical thinking abilities. Key words: graph reading in school physics, home physics experiments, inquiry-based homework, self-regulated physics learning, textbook error detection.

We analyze the influence of nonparabolicity on the bound electronic states in the conduction-band of quantum wells in external electric field. Numerical results, obtained by transfer matrix method are presented for active region of GaAs/Al0.3Ga0.7As quantum cascade laser. The structure was initially optimized by genetic algorithm, using Kane's model of nonparabolicity, with emission wavelength set to λ ≈ 15.1 μm . However, our numerical results indicate the change in lasing wavelength to 14.04 μm when using a more comprehensive description of nonparabolicity.

A method is proposed for the optimization of structural parameters of GaN/AlGaN quantum wells and Bragg-confined structures, with respect to peak intersubband absorption from the ground to the first excited electronic state in the mid and near infrared spectral range. It is based on the application of the Genetic Algorithm and delivers globally optimal structures with a preset number of embedded layers. Simple rectangular quantum well profile is investigated for the maximal Stark effect and applications to tunable mid-infrared photodetectors. In case of Bragg-confined structures, an above the barrier bound state is used to extend the range of transition energies above the values available in conventional quantum wells. The effects of band nonparabolicity are taken into account. (C) 2013 Elsevier Ltd. All rights reserved

A strong magnetic field applied along the growth direction of a quantum cascade laser (QCL) active region gives rise to a spectrum of discrete energy states, the Landau levels. By combining quantum engineering of a QCL with a static magnetic field, we can selectively inhibit/enhance non-radiative electron relaxation process between the relevant Landau levels of a triple quantum well and realize a tunable surface emitting device. An efficient numerical algorithm implementation is presented of optimization of GaAs/AlGaAs QCL region parameters and calculation of output properties in the magnetic field. Both theoretical analysis and MATLAB implementation are given for LO-phonon and interface roughness scattering mechanisms on the operation of QCL. At elevated temperatures, electrons in the relevant laser states absorb/emit more LO-phonons which results in reduction of the optical gain. The decrease in the optical gain is moderated by the occurrence of interface roughness scattering, which remains unchanged with increasing temperature. Using the calculated scattering rates as input data, rate equations can be solved and population inversion and the optical gain obtained. Incorporation of the interface roughness scattering mechanism into the model did not create new resonant peaks of the optical gain. However, it resulted in shifting the existing peaks positions and overall reduction of the optical gain. Program summary Program title: QCL Catalogue identifier: AERL_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AERL_v1_0.html Program obtainable from: CPC Program Library, Queen’s University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 37763 No. of bytes in distributed program, including test data, etc.: 2757956 Distribution format: tar.gz Programming language: MATLAB. Computer: Any capable of running MATLAB version R2010a or higher. Operating system: Any platform supporting MATLAB version R2010a or higher. RAM: Minimum required is 1 GB. Memory usage increases for less intense magnetic fields. Classification: 15. Nature of problem: The nature of the problem is to provide an efficient numerical algorithm implementation for optimization of GaAs/AlGaAs QCL active region parameters and calculation of output properties in the magnetic field. Solution method: The optimization of the QCL laser performance at selected wavelength is performed at entire free-parameters space using simulated annealing algorithm. The scattering rates are calculated in the presence and without magnetic field and used as coefficients in rate equations. The standard MATLAB procedures were used to solve iteratively this system of equations and obtain distribution of electron densities over electronic states. Restrictions: The machine must provide the necessary main memory which decreases roughly quadratically with the increase of the magnetic field intensity. Running time: Optimization time on Intel 3 GHz processor is about 2×10 4 s. The calculation time of laser output properties for values set automatically in GUI is 5×10 4 s.

Solar cells are unable to absorb photons with energies lower than the constituent material's band-gap, which eliminates a significant part of the spectrum. The efficiency of the process may be increased by converting low-energy photon pairs, via nonlinear optical effects, into photons optimal for the solar cell. The converter structures consist of GaN/AlGaN super lattice series perturbed by asymmetric quantum wells which form the Bragg confined structures (BSCs). BCSs support additional bound electron states in the super lattice minigaps, including states above the barrier. Parameters of the structure are optimized for each photon pair, by using the genetic algorithm, to obtain a continual converter.

Optical and structural properties of materials can be characterized by spectroscopic ellipsometry which represents an experimental technique that mea sures polarized light reflected from a material surface. Since the most complicated part of this me thod is data analysis and modeling, we present the calculation of the ellipsometric parameters for bi- isotropic materials. We consider the ellipsometric model for these materials which require not just pe rmittivity like in conventional model, but also magnetic permeability and magnetoelectric coupling.

We present research into the effects of traditional teaching and an applied active learning model to overcome students’ alternative conceptions on floating and sinking with a sample of 153 thirteen-year-old students in Serbia. An overview of the most important properties of the experimental model of active learning based on the IBSE approach is given. The results show significant differences in achievements between the control and the experimental group: the average normalized gain was 0.84±0.21 for experimental group and 0.04±0.25 for the control group, which points to the need for adoption of modern constructivist view on learning within the teaching process.

The fifth International School and Conference on Photonics—Photonica 2015 held on 24 August–28 August 2015 in Belgrade, Serbia, was one of the largest events dedicated to the celebration of the UNESCO proclaimed International year of light and light based technologies (IYL 2015) in southern Europe. In a year in which many magnificent scientific anniversaries related to great achievements in light science from the last thousand years coincided, the Vinca Institute of Nuclear Sciences (Belgrade, Serbia), the Serbian Academy of Sciences and Arts, the Optical Society of Serbia and Aston University (Birmingham, UK) tried to contribute to this global initiative which highlighted to the mankind the immense importance of light and related technologies in everyday life by organizing this respectable scientific and educational meeting. The conference was attended by a record number of 222 registered participants where, for the first time, domestic researchers have been outnumbered by foreign ones from 30 countries and all continents. Almost eighty PhD and master students had the luck and privilege to get special postal envelopes dedicated to the IYL 2015 and to listen to six illuminating tutorial talks, seven red-hot keynote talks and 21 invited lectures from

Recently, group delay has been proven as a very convenient tool for control of terahertz electromagnetic signals used for filters, waveguides and polarization components. Here, we investigate the propagation of circularly polarised terahertz pulse via analysis of group delays in X particle chiral metamaterial. When varying the geometry of the X particle chiral metamaterial, significant modification of chiroptical effects- optical activity and circular dichroism can be observed. Through the analysis of group delays in the frequency region between 1 and 3 THz we conclude that the right circularly polarised wave is more sensitive to changes in the geometry than the left circularly polarised wave. This result implies that the right circularly polarised wave is responsible for variation of chiroptical effects in these structures and opens up a possibility for potential applications.

We investigate the propagation of circularly polarized electromagnetic waves through a free-standing $\mathrm{\ensuremath{\Omega}}$ particle chiral metamaterial slab and study the associated delay times: dwell time and group delay. Through this analysis, we observe different interactions of right- and left-circularly polarized waves in the terahertz frequency range (1--2.5 THz) with the resonant elements. As a consequence, the resonant-frequency group delay and dwell time of the right-circularly polarized wave are one or more orders of magnitude longer than those of the left. In addition, we compare the delay times obtained from rigorous numerical simulations with those calculated using the retrieved effective parameters. We find that the dwell time cannot be evaluated correctly from the effective medium approximation, while no such problems exist in the case of group delay.

We present a method for modeling nonparabolicity effects (NPE) in quantum nanostructures by using second order perturbation theory. We will analyze application of this model on a quantum well without external electric field and a quantum cascade laser (QCL). This model will allow us to examine the influence of magnetic field on dipole matrix element in QCL structures which will give better insight how NPE can disrupt gain of QCL structures. [Projekat Ministarstva nauke Republike Srbije, br. III45010]

In order to avoid losses in metamaterial unit cells at frequencies of interest, caused by metallic inclusions, an active medium design has been proposed. As candidate structures for this active medium, we have chosen quantum cascade lasers because of their high output gain. Here we analyze and compare two quantum cascade structures that emit at 4.6 THz and 3.9 THz, respectively, placed under the influence of a strong magnetic field. We first solve the full system of rate equations for all relevant Landau levels, and obtain the necessary information about carrier distribution among the levels, after which we are able to evaluate the permittivity component along the growth direction of the structure. With these data one can determine the conditions under which negative refraction occurs, and calculate the values of the refractive index of the structure, as well as the range of frequencies at which the structure exhibits negative refraction for a predefined total electron sheet density.

A theoretical study on low-threshold multimode instabilities in quantum cascade lasers (QCLs) and laser diodes (LDs) is presented. Previously, low threshold Risken-Nummedal-Graham-Haken (RNGH) instabilities were reported in several experimental studies of QCLs. They were attributed so far to the combined effect of the induced grating of carrier population and of a built-in saturable absorption feature that may be present in the monolithic single-section cavity of these Fabry-P\'erot lasers. Here we show that low-threshold RNGH instabilities in QCLs occur due to a carrier coherence grating induced in the gain medium and not due to intracavity saturable absorption. We find that QCLs with a few mm long cavity exhibit intermittent RNGH self-pulsations while regular self-pulsations are possible in short-cavity QCLs, with the cavity length of 100 um or smaller. We examine a transient behavior to RNGH self-pulsations in short-cavity QCLs and find features that resemble cooperative superradiance. Our findings open a practical way of achieving ultra-short pulse production regimes in the mid-infrared spectral range. Applying same approach to semiconductor laser diodes (LDs) we explain the absence of RNGH self-pulsation in LDs based on a quantum well gain media, while practically established method for reaching the ultrafast coherent emission regimes in LDs is to incorporate a separately contacted saturable absorber section in the LD cavity.

We present a comparative analysis of the tunneling times of electromagnetic (EM) waves propagating in isotropic and anisotropic media. First, suitable expressions for the tunneling times in a layered periodic material, with anisotropic properties originating from its structure, are derived, followed by numerical calculations performed for a new type of anisotropic semiconductor metamaterial. In the first case, we have considered a layered structure which contains two differently doped In0.53Ga0.47As semiconductor layers. The second structure under investigation is made of alternately placed layers of doped In0.53Ga0.47As and undoped Al0.48Ga0.52As. The investigation of the dwell time as a function of incident wave frequency has revealed the existence of two peaks, one of which may be interpreted as a consequence of anisotropy, while the other one corresponds to the peak related to the absorption and the group delay. Both of these two peaks are affected by variations of layers’ doping densities. Furthermore, at increased incident angles of incoming EM waves, the dwell time peak occurs at the upper boundary of the frequency interval, for which the structure exhibits negative refractive index.

The aspects of light propagation through a semiconductor metamaterial (SMM) which consists of two alternating differently doped In0.53Ga0.47As layers are studied analytically and numerically. A general analytical expression for the effective index of refraction of the Poynting vector is derived for periodic structure composed of two alternating layers of absorptive and dispersive semiconductors and light propagation is simulated by finite-difference-time-domain method. Negative values of the effective refractive index with low absorption are observed in the mid-infrared frequency range. The main advantage of this design is the possibility to control the frequency region with negative effective refractive index by changing the doping densities of the layers.

In this paper we provide a detailed analysis of the energy position and type of transmission maxima in rectangular quantum wells (QWs), taking into consideration the difference of electron effective masses in the barrier and well layers. Particular attention is given to transmission maxima that are less than unity and the implications of effective mass variation. It is shown that, within theoretical considerations, there exist an infinite number of intervals of the strength of the QW in which such specific resonances occur, and that those intervals have finite asymptotic value for high QW strengths.

We present a detailed analysis of GaAs/AlGaAs terahertz quantum cascade laser in the presence of an intense external magnetic field. One of the objectives in further development of THz quantum cascade laser is the realization of structures operating at higher temperatures. This is difficult to obtain as the operating photon emission energy is smaller than the longitudinal-optical phonon energy in the semiconductor material. With increased temperature, electrons in the upper radiative state gain sufficient in-plane energy to emit an longitudinal-optical phonon, which represents a non-radiative scattering and reduces the optical gain. By applying strong magnetic field, two-dimensional continuous energy subbands become split into series of discrete Landau levels, and at particular values of B it is possible to quench these non-radiative channels. Numerical simulations are performed on two-well design quantum cascade laser operating at 4.6 THz, implemented in CaAs/Al(0.15)Ga(0.85)As, and the magnetic field is perpendicular to the epitaxial layers. Strong oscillations of carrier lifetimes for the upper state of the laser transition, as a function of magnetic field are observed, which can be attributed to interface roughness scattering and longitudinal-optical phonon scattering between Landau levels. 12th Annual YUCOMAT Conference, Sep 06-10, 2010, Herceg Novi, Montenegro

We present an analysis of the influence of the Goos-Hanchen effect on tunneling times, group delay and dwell time, of electromagnetic waves propagating through an obstacle made of left-handed metamaterial embedded in a dielectric which exhibits saturable type of nonlinearity. The derived equations show that only the group delay, is affected by the Goos-Hanchen shift without any impact on the dwell time. Besides the reduction of the group delay, the most remarkable result is the possibility for total reduction of the Goos-Hanchen shift for finite incident angles. These phenomena are observable in the frequency region for which metamaterial exhibits negative index of refraction. (C) 2011 Elsevier B.V. All rights reserved.

in generating optimal structures which emit radiation at specified wavelengths corresponding to absorption fingerprints of particular harmful pollutants found in the atmosphere. We also illustrate another interesting prospective application of quantum cascade laser-type structures: the design of metamaterials with tunable complex permittivity, based on amplification via intersubband transitions. In this case, the role of the magnetic field is to assist the attainment of sucient optical gain (population inversion), necessary to eectively manipulate the permittivity and fulfill the conditions for negative refraction (left-handedness).