Piecewise-constant approximation of the potential profile of multiple quantum well intrinsic heterostructures

Knowledge of the energy band diagram is very important in semiconductor physics due to the fact that the band diagram influences almost all parts of the physics of a semiconductor device. In this paper we examine a piecewise-constant approximation of the potential profile through a comparison with a comprehensive self-consistent model, with regard to the active regions of QW semiconductor lasers and amplifiers. The validity of this approximation is then defined, thus giving an insight into the physics of QW structures.     

Tuning of PEDOT:PSS Properties Through Covalent Surface Modification

Conductive polymer (poly(3,4‐ethylenedioxythiophene)‐poly(styrenesulfonate) (PEDOT:PSS) is an attractive platform for the design of flexible electronic, optoelectronic, and (bio)sensor devices. Practical application of PEDOT:PSS often requires an incorporation of specific molecules or moieties for tailoring of its physical–chemical properties. In this article, a method for covalent modification of PEDOT:PSS using arenediazonium tosylates was proposed. The procedure includes two steps: chemisorption of diazo‐cations on the PEDOT:PSS surface followed by thermal decomposition of the diazonium salt and the covalent bond formation. Structural and surface properties of the samples were evaluated by XPS, SEM‐EDX, AFM, goniometry, and a range of electric and optical measurements. The developed modification procedure enables tuning of the PEDOT:PSS surface properties such as conductivity and optical absorption. The possibility to introduce various organic functional groups (from hydrophilic to hydrophobic) and to create new groups for further functionalization makes the developed procedure multipurpose. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 378–387     

STM Study of Self-Organization of 10CB Molecules on Au(111) Surface

We report the scanning tunnelling microscopy (STM) observation of a self-organization of 4-cyano-4′-n-decylbiphenyl (10CB) molecules on the reconstructed Au(111) surface. The self-organized monolayers were obtained by the deposition from a liquid. We revealed a row-like dimer structure with regularly arranged kinks within each dimer row. The separation between neighbor kinks is equal to 4 or 5 intermolecular distances. STM-images with intramolecular resolution showed that the association of molecules in dimers is due to the interaction between cyano groups. The obtained results are discussed in terms of the balance between the molecule-molecule and molecule-substrate interactions and are supported by the results of DFT calculations.     

Investigation of properties of the solutions of hyperbolic stochastic PDEs

In this paper we investigate the compact support property of the solutions of hyperbolic Stochastic PDE (SPDE) providing that initial condition function is deterministic and has compact support property. First, to approach this problem, we consider semi-SPDE. It turns out that in the semi-SPDE case solution u (t, x) preserve compact support property. When we consider SPDE, we use the stochastic differential-difference equations (SDDE) approach. It turns out that in SPDE case solution u (t, x) does not preserve compact support property. So, if we compare the semi-SPDE and SPDE then it becomes obvious that differentiation in space in SPDE plays crucial role and influence the behavior of the solution. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Singular and Hypersingular Integral Equations Techniques for Gyrotron Coaxial Resonators with a Corrugated Insert

For the first time rigorous theory is developed for eigen traveling TM modes in the resonator of the coaxial cavity gyrotron with a corrugated insert. This mathematical model can be applied for any corrugation parameters and wavelengths. Gyrotron simulation software is developed and allows to calculate mode eigenvalues, electromagnetic field components and Ohmic losses for eigen TE and TM modes. Results of the numerical investigations are presented for the ITER relevant 170 GHz coaxial cavity gyrotron developed in Forschungszentrum Karlsruhe, Germany.     

Ultrafast interband pumping of quantum‐cascade structures: A feasibility study of a THz pulse amplifier

Gain spectra of a three‐well GaAs/AlGaAs quantum‐cascade structure under interband pumping by ultrashort optical pulses are studied within the framework of the density matrix theory. A mathematical model for intersubband kinetics is derived taking into account many‐body interactions and ultrafast interband optical pumping. It is found that pulsed interband pumping leads to an increase in intersubband gain by an order of magnitude, which is characterised by 1 ps rise time and 8 ps recovery time under pumping by 100 fs pulses with a peak intensity of 100 MW/cm². Possible implementations of the concept are highlighted. The results uncover a new possibility for ultrafast switching of the material gain in quantum‐cascade structures.     

Beam combining of lasers with high spectral density using volume Bragg gratings

Incoherent combining of multiple laser beams with offset wavelengths into a single near-diffraction-limited beam is an effective solution to increasing energy brightness and scaling output power of high-power lasers. Volume Bragg gratings (VBGs) recorded in photo-thermo-refractive (PTR) glass allow spectral beam combining with a remarkably high spectral density of channels. Spectral beam combining (SBC) of five channels within 1-2 nm bandwidth around 1064 and 1550 nm into a single near-diffraction-limited beam with absolute efficiency 92-94% is demonstrated. Scaling of this technique to multi-kW power level is discussed.     

Controlling cooperativity of a metastable open system coupled weakly to a noisy environment

The notion of cooperativity comprises a specific characteristic of a multipartite system concerning its ability to demonstrate a sigmoidal-type response of varying sensitivities to input stimuli in transitions between states under controlled conditions.From a statistical physics viewpoint,in this work we attempt to describe the cooperativity by the stability of a metastable open system with respect to irreversibility.To treat the evolution of a system weakly coupled to the environment in a kinetic framework,we consider two fluctuating energy levels of different dimensionalities,initial population of one level,reversible transitions of population between the levels,and irreversible depopulation of another level.An average is made over level fluctuations and environment vibrations so that an inter-level transition rate can be obtained accounting for the influences of external control on level position and dimensionality.It is found that the cooperativity of the two-level system is bounded approximately between 0.736 and unity,with the lower bound indicating worsening system stability.