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1

First-principles calculations to investigate structural, electronic and optical properties of (AlSb)m/(GaSb)n superlattices

Caid M., Rached D.

Materials Science Poland

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2020

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Vol. 38, No. 2

320--327

EN

The structural, electronic and optical properties of (AlSb)m/(GaSb)n (m-n: 1-1, 2-2, 1-3 and 3-1) superlattices are investigated within the density functional theory (DFT) by using the last version of the first principles full potential linear muffin tin orbital method (FP-LMTO) as implemented in LmtART 7.0 code. The exchange and correlation potential is treated by the local density approximation (LDA) for the total energy calculations. Our calculations of the band structure show that the superlattices (n ≠ 1) have a direct band gap Γ-Γ. The optical constants, including the dielectric function ϵ(w), the refractive index n(w) and the reflectivity R(w) are calculated and discussed.

2

Electronic structure and optical properties of (BeTe)(n)/(ZnSe)(m) superlattices

Caid M., Rached H., Rached D., Khenata R., Bin Omran S., Vashney D., Abidri B., Benkhettou N., Chahed A., Benhellal O.

Materials Science Poland

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2016

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Vol. 34, No. 1

115--125

EN

The structural, electronic and optical properties of (BeTe)n/(ZnSe) m superlattices have been computationally evaluated for different conﬁgurations with m = n and m≠n using the full-potential linear mufﬁn-tin method. The exchange and correlation potentials are treated by the local density approximation (LDA). The ground state properties of (BeTe)n/(ZnSe) m binary compounds are determined and compared with the available data. It is found that the superlattice band gaps vary depending on the layers used. The optical constants, including the dielectric function ε(ω), the refractive index n(ω) and the refractivity R(ω), are calculated for radiation energies up to 35 eV.

3

Properties of chirped periodic layered medium with metamaterial layers

Garus J., Garus S., Błoch K., Nabiałek M., Szota M.

Journal of Achievements in Materials and Manufacturing Engineering

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2014

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Vol. 66, nr 1

13--19

EN

Purpose: The purpose of the article was to investigate the absorption and reflectance properties of chirped periodic layered medium with metamaterial layers. Design/methodology/approach: The use of the algorithm TMM (Transfer Matrix Method) allows to determine the absorption and reflectance for the quasi one-dimensional multilayer structures. Can be analysed structure constructed with RHM (right-handed materials) and LHM (left-handed materials) with layers of any thickness and arranged in any way. It is possible to analyse lossy dispersive materials. Findings: In all the cases studied linear shift in the peaks with an increase in the k coefficient was observed. In all cases there was a shift to higher wavelengths. Noted an increase in the intensity and broadening half-width of the peaks. Research limitations/implications: The simulation was carried out only for the binary structure in the visible light range. Practical implications: Computer simulations allow us to design material with specified properties at a lower cost. The use of chirped periodic layered media allows to shift and broadening of the peak in the required range of work for mirrors or filter. Originality/value: Absorption and reflectance for chirped periodic layered systems using metamaterials layers have not yet been thoroughly investigated. Research can contribute to the implementation of mirrors with specific nonlinear properties.

4

The creation and shifts of band gaps in binary superlattices

Garus J., Garus S., Gruszka K., Błoch K., Nabiałek M.

Inżynieria Materiałowa

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2014

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Vol. 35, nr 2

113--116

EN

Multilayers are characterized by the presence of the photonic band gap, so that electromagnetic waves (EMW) at the specific frequencies does not propagate them. Using emulation properties of multilayer systems allows pre-testing of the structure and the design to get the specific characteristics of the electromagnetic wave transmission bands. This reduces the number of samples performed and reduces the cost of examinations. In this study the behaviour of the electromagnetic wave passing through the superlattice ABABAB binary structure composed of lossless materials whose properties do not depend on the frequency of the electromagnetic wave, using the matrix method and the FDTD algorithm. Due to the use multilayers as a filters of visible light the calculation performed for the wavelength range of 300 to 700 nm. In order to examine the behaviour of a monochromatic electromagnetic wave having a wavelength equal to the wavelength of the band gap analysis was performed using the FDTD algorithm, during which were determined by FFT (Fast Fourier Transform) wavelength characteristics. EMW penetrating the superlattice structure, for each boundary layer is partially reflected and partially passes. Another part of the electromagnetic wave reflected inside the structure interfere with each other, which involves a change in the output wavelength and causes the formation of superlattice band gaps. Based on the study, it can be concluded that the matrix method and FDTD very well complement the study of photonic properties of multilayer structures. It has been found that the presence of the photonic band gap depends strongly on the number of superlattice layers. The environment of the multilayer structure does not affect the existence of band gaps, but it strongly influences the interband areas. Increasing the thickness of the superlattice layers shifts in a linear photonic band gaps toward the higher wavelength and the influences of their size. The use of the FDTD method has allowed to illustrate the behaviour of electromagnetic waves in areas of high and low transmission.

PL

Układy wielowarstwowe charakteryzują się występowaniem fotonicznej przerwy wzbronionej, przez co fale elektromagnetyczne (FEM) o pewnych częstotliwościach nie rozchodzą się w nich. Użycie emulacji właściwości układów wielowarstwowych pozwala na wstępne testowanie struktury i taki jej projekt, aby uzyskać konkretną charakterystykę pasm transmisji fali elektromagnetycznej. Pozwala to na ograniczenie liczby wykonywanych próbek i wpływa na zmniejszenie kosztów przeprowadzonych badań. W pracy badano zachowanie się fali elektromagnetycznej przechodzącej przez supersieć o strukturze binarnej ABABAB zbudowaną z materiałów bezstratnych, których własności nie zależą od częstości fali elektromagnetycznej, za pomocą metody macierzowej oraz algorytmu FDTD. Ze względu na zastosowanie sieci wielowarstwowych jako filtrów promieniowania widzialnego obliczenia przeprowadzono dla długości fali z zakresu 300÷700 nm. W celu zbadania zachowania monochromatycznej fali elektromagnetycznej o długości fali równej długości fali pasma wzbronionego przeprowadzono analizę za pomocą algorytmu FDTD, w trakcie której wykonano metodą FFT (Fast Fourier Transform) charakterystyki długości fali. FEM, wnikając w strukturę supersieci, na każdej granicy warstw częściowo się odbija, a częściowo przechodzi. Kolejne odbite części fali elektromagnetycznej wewnątrz struktury interferują ze sobą, co wiąże się ze zmianą długości fali na wyjściu supersieci i powoduje powstawanie pasm wzbronionych. Na podstawie przeprowadzonych badań można stwierdzić, że metoda macierzowa i FDTD bardzo dobrze się uzupełniają przy badaniu własności fotonicznych struktur wielowarstwowych. Stwierdzono, że występowanie fotonicznej przerwy wzbronionej silnie zależy od liczby warstw supersieci. Otoczenie struktury wielowarstwowej nie wpływa na występowanie pasm wzbronionych, ale silnie oddziałuje na obszary międzypasmowe. Zwiększenie grubości warstw supersieci przesuwa w sposób liniowy fotoniczne przerwy wzbronione w stronę wiekszych długości fali oraz wpływa na ich rozmiar. Wykorzystanie metody FDTD pozwoliło zobrazować zachowanie fali elektromagnetycznej w obszarach o wysokiej i niskiej transmisji.

5

Fotoluminescencyjne badania wpływu temperatury na własności emisyjne periodycznych nanostruktur fotonicznych

Urbańczyk D. M., Wójcik-Jedlińska A., Kosiel K., Bugajski M.

Elektronika : konstrukcje, technologie, zastosowania

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2014

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Vol. 55, nr 11

57-59

PL

Praca prezentuje zastosowanie wyzoskorozdzielczej spektroskopii fotoluminescencyjnej do analizy wpływu temperatury na własności emisyjne periodycznych nanostruktur półprzewodnikowych. Przedmiotem badań były supersieci Al0.45GaAs0.55/GaAs zaprojektowane jako obszar czynny laserów kaskadowych. Struktury zostały osadzone metodą epitaksji z wiązek molekularnych na podłożu GaAs. Wykonano pomiary widm fotoluminescencji obszaru supersieci w różnych warunkach termicznych, w zakresie temperatury od 5 do 65°C. Na podstawie zmian spektralnego położenia pików fotoluminescencji wyznaczono parametr przesunięcia temperaturowego wynoszący ok. 0,3 nm/K.

EN

The paper presents photoluminescence study of the temperature effect on the optical properties of semiconductor periodic nanostructures. The research was performed on Al0.45GaAs0.55/GaAs superlattices grown by molecular beam epitaxy on GaAs substrate. The structures have been designed as an active region of quantum cascade lasers. The temperature dependences were determined based on the results of high resolution photoluminescence measurements carried out at different temperature of values ranging from 5 to 65°C. The obtained temperature offset of the emission spectra was ~0.3 nm/K.

6

Transmission and photonic band gaps in Fibonacci superlattices

Garus J., Garus S., Błoch K., Szota M., Nabiałek M., Szlązak K.

Journal of Achievements in Materials and Manufacturing Engineering

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2013

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Vol. 61, nr 2

229--235

EN

Purpose: The purpose of the article was to broaden the knowledge about the behavior of Fibonacci superlattices as filters electromagnetic waves. Design/methodology/approach: Simulations of multi-layer systems is usually carried out by using two complementary methods. The first, matrix method which allows the study of the properties of structures using transmission maps and the second method used is the Finite-Difference Time Domain (FDTD) algorithm allows on the study of electromagnetic wave propagation in the structure. Findings: It can be seen that the lighting of the filter with monochromatic light in the wavelength range of the band gap filter at the output causes propagation of low intensity in the range other than the wavelength of the incident beam. Research limitations/implications: The simulation was not considered the impact of losses in the material. Practical implications: Present clear differences depending on the polarization allow the use of superlattices as polarizers for specific ranges of wavelengths and angles of incidence. Originality/value: Fibonacci superlattices have been pre-tested in. The purpose of the article was to broaden the knowledge about the behavior of these structures as filters electromagnetic waves with a wavelength range from the near infrared, the effect of the material surrounding the transmission and increasing knowledge of the formation of band gaps.

7

The influence of extinction coefficient on transmission in binary multilayer

Garus S., Garus J., Szlązak K., Nabiałek M., Pietrusiewicz P., Błoch K., Gruszka K., Szota M.

Journal of Achievements in Materials and Manufacturing Engineering

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2013

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Vol. 61, nr 2

236--243

EN

Purpose: The influence of the extinction coefficient for transmission, reflectance and absorption in multilayer binary. Reps cluster was equal to 7 It was built of two types of materials. First was a NaCl equivalent with a refractive index described by nA=1.544-ik , and as second was GaAs lossless metamaterial equivalent Design/methodology/approach: Research was carried out using the map of: transmitting, reflectance and absorption of the electromagnetic wave. Maps determined using the matrix method. Findings: It is shown that the higher the extinction coefficient, which is responsible for the absorption of electromagnetic waves, decreases the transmittance and reflectance of the multilayer system at the same time increasing its absorption. Research limitations/implications: Would be a reasonable correlation between the results of the transmission, reflectance and absorption obtained using numerical methods with with experiment. Practical implications: Taking into account the extinction coefficient allows for a better representation of the real multilayer structures. Quasi one-dimensional superlattices can be used as filters or multiplexers electromagnetic waves. Originality/value: In the literature the most frequently analyzed lossless structure. Taking into account the extinction coefficient allows for better study the properties of the tested materials.

8

Transmission in combination of structures

Garus S., Garus J., Szota M., Nabiałek M., Gruszka K., Błoch K.

Archives of Materials Science and Engineering

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2013

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Vol. 64, nr 2

110--117

EN

Purpose: The paper investigated the effect of the combination of two structures on the transmission of electromagnetic waves as a multilayer filter. Examined how the combination of two structures affect the properties of the filter. As a component materials of structures used both right-handed material (RHM) and left-handed (LHM). Design/methodology/approach: Analysis was performed using a matrix method for calculating the superlattice transmission. The influence of combination of two types of multilayer systems: periodic (binary superlattice) and aperiodic (Severin’s and Thue-Morse’s superlattices). Findings: Studies have shown the structure of the transmission band of the structures, which is dependent on the polarization of the incident wave. Combination of various structures are not commutative, and therefore their transmission maps are not equal. Research limitations/implications: The structures analyzed in the work consisted of a lossless material, isotropic and non-dispersive. An important analysis would be lossy dispersive materials. You should also examine the impact would have a separating layer structure and the influence of defects on transmission properties of superlattices. Practical implications: The test structures may be used as filters of electromagnetic radiation. Placing the filter characteristics of the two structures allows pre-filtering an electromagnetic wave, in order to obtain a structure suitable for applications. Originality/value: The paper shows how combination of two periodic and aperiodic structures affect the propagation of electromagnetic waves in a multi-layered system. The analysis was based on the determination of unpolarized transmission maps for complex structures.

9

The effect of polarization plane rotation on binary superlattice transmission

Garus J., Garus S., Szota M., Nabiałek M., Gruszka K.

Archives of Materials Science and Engineering

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2013

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Vol. 64, nr 1

20--27

EN

Purpose: The purpose of this article is to examine what influence on the transmission has a rotation of the vibration plane, which electric field vector of the electromagnetic wave lies, on the transmission properties of the binary superlattice. In the literature, the most common transmission structure are given for the P or S wave polarization. This article aims to verify the nature of the transmission when the polarization is not strictly defined. Design/methodology/approach: In the paper the transmission of quasi one-dimensional binary structures is analized depending on the angle of incidence and wavelength of electromagnetic wave and on torsion angle of the plane of the electric field, using the matrix method. Findings: Changing the angle of rotation of the incident electromagnetic wave electric field vibration plane affects the size of the interband transmission and causes separation of fixed transmission bands locations for specific wavelength and angle of incidence. Research limitations/implications: Quasi one-dimensional binary superlattices composed of lossless, non dispersive isotropic materials were analyzed. It would be important to investigate influence of loss factor and the two- and three-dimensional periodic and aperiodic structures on the electromagnetic wave transmission. Also important would be to compare results with those obtained from the use of finite increments algorithm in the time domain (FDTD) and the correlation with experimental data. Practical implications: The test structures may be used as filters of electromagnetic wave propagation. The structure and thickness of the layers has a significant influence on the characteristics of the transmission, which will allow to design the structure in order to met the conditions of specific applications. Originality/value: In this paper, a method for the analysis of the electromagnetic waves transmission characteristics in the case where the electric field is not polarized in the S, or P directions only.

10

Infrared transmission in aperiodic structures

Szota M., Nabiałek M., Garus S., Garus J., Błoch K.

Archives of Materials Science and Engineering

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2013

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Vol. 64, nr 2

213--218

EN

Purpose: The analysis of the electromagnetic wave transmission having a wave length near infrared propagating in multilayer structures made of materials GaP and CaF2. Analyzed was the influence of periodicity distribution of layers in the material properties and the presence of photonic forbidden gap for selected wavelengths of the electromagnetic wave. Design/methodology/approach: Maps transmission, which was performed by the analysis were obtained using a matrix method. Was investigated wave propagation wavelength range of infrared radiation in periodic binary multilayers and aperiodic Severin and Thue-Morse superlattices. Findings: It has been shown the structure of the transmission band depending on the type of polarization of the multilayer system. Properties of Thue-Morse superlattices were similar to binary superlattices but differed from the behavior of electromagnetic waves in aperiodic Severin superlattices. Research limitations/implications: The simulation was not considered the impact of losses in the material for propagating electromagnetic wave. Practical implications: Multilayer materials, which have photonic band gap, can be used as filters for electromagnetic radiation can improve the performance of night vision or electromagnetic waves multiplexers. Originality/value: Transmission properties of multilayers were examined in visible light but not for infrared light.

11

Determination of energy difference and width of minibands in GaAs/AlGaAs superlattices by using Fourier transform photoreflectance and photoluminescence

Motyka M., Janiak F., Misiewicz J., Wasiak M., Kosiel K., Bugajski M.

Opto-Electronics Review

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2011

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Vol. 19, No. 2

151-154

EN

In this work, Fourier transform photoreflectance (in a form of fast differential reflectance spectroscopy) has been used to study the interband optical transitions in molecular beam epitaxially grown GaAs/AlGaAs superlattices. The dependence of the measured features on the growth parameters (QW and barrier widths) has been studied. The minibands widths and energy differences between them have been obtained and matched to these coming from effective mass calculations. In addition, it has been shown that Fourier transform photoluminescence measurement might be used in the far infrared region (up to ∼15 µm) to the direct detection of the energies of intraband transitions between the electron minibands (subbands) in the superlattice and QW system.

12

Optyczne właściwości supersieci GaAs/AlGaAs badane za pomocą spektroskopii modulacyjnej

Janiak F., Motyka M., Sęk G., Ryczko K., Misiewicz J., Kosiel K., Bugajski M.

Elektronika : konstrukcje, technologie, zastosowania

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2011

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Vol. 52, nr 10

46-48

PL

Spektroskopię modulacyjną- fotoodbicie zastosowano do badania optycznych i strukturalnych właściwości supersieci AlGaAs/GaAs. Otrzymano widma fotoodbiciowe bogate w linie spektralne związane z procesami absorpcji między poszczególnymi stanami kwantowymi w badanych supersieciach. Zmierzone linie spektralne zidentyfikowane zostały na podstawie obliczeń struktury pasmowej w modelu masy efektywnej. Przeprowadzone pomiary pozwoliły potwierdzić założone w procesie wzrostu składy i grubości poszczególnych warstw studni i barier. Ponadto wykazano bardzo dużą jednorodność wytwarzanych 2-calowych płytek jak również wysoką powtarzalność procesów technologicznych.

EN

Modulation spectroscopy - photoreflectance has been applied for optical investigation of AlGaAs/GaAs superlattices. Optical features obtained in photoreflectance spectra associated with the transitions between confined levels in investigated superlattices have been recognized and analyzed by calculation performed in effective mass approximation formalism. In addition, there has been demonstrated a high uniformity of investigated 2 inch wafers and also high repeatability of the growth process.

13

Fourierowska spektroskopia fotoluminescencyjna i fotoodbiciowa struktur półprzewodnikowych przeznaczonych na zakres średniej i długofalowej podczerwieni

Motyka M., Sęk G., Misiewicz J.

Elektronika : konstrukcje, technologie, zastosowania

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2011

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Vol. 52, nr 10

37-39

PL

W pracy przedstawiono wyniki badań spektroskopowych struktur półprzewodnikowych prowadzonych w Zespole Optycznej Spektroskopii Nanostruktur Instytutu Fizyki Politechniki Wrocławskiej w zakresie widmowym średniej i długofalowej podczerwieni. Zaprezentowano możliwości jakie daje stosowanie spektroskopii modulacyjnej, wspartej spektroskopią fotoluminescencyjną, do wyznaczania parametrów fizycznych istotnych z punktu widzenia konstruowaniu urządzeń takich jak np. źródła promieniowania laserowego. W pierwszej części przedstawiono układ pomiarowy do spektroskopii modulacyjnej bazujący na spektrometrze Fouriera jak również wyniki badań zarówno studni kwantowych I rodzaju InGaAsSb/GaSb jak i II rodzaju GaSb/AlSb/ InAs/GaInSb/InAs/AlSb/GaSb do zastosowań emiterowych i detektorowych w zakresie 2-6 μm . W drugiej części przedstawione zostały rezultaty badań warstw HgCdTe o różnej koncentracji atomów kadmu przeznaczonych do zastosowań w czujnikach gazów w zakresie fal 5... 15 μm jak również rezultaty pomiarów fotoluminescencyjnych w obszarze przejść wewnątrzpodpasmowych dla supersieci GaAs/AIGaAs, będących podstawą laserów kaskadowych na zakres emisji 10...15 μm. Ponadto, zaproponowano eksperyment różnicowej spektroskopii odbiciowej pozwalający na szybką charakteryzacja optyczną (w czasie rzędu pojedynczych minut a nawet sekund) struktur półprzewodnikowych w szerokim zakresie spektralnym 1... 15 μm.

EN

In this work, we show the results of optical studies on low-dimensional structures realized in the Laboratory for Optical Spectroscopy of Nanostructures at Institute of Physics Wrocław University of Technology in the spectral range of mid and long wavelength infrared. In the first part there have been shown the advantages and opportunities which gives the modulation spectroscopy gives for the case of structures as InGaAsSb/GaSb type I and GaSb/AlSb/InAs/GaInSb/InAs/AlSb/GaSb type II quantum wells. In the second part, there are presented the results of optical characterization on HgCdTe layers with different Cd atoms content and also on GaAs/AlGaAs superlatticess in the range of intersubband transitions. Additionally, it has been also introduced a fast differential technique which allowed to measure the modulation-like spectra in the very short time scale (single minutes or even seconds) in the broad spectral range (1 ...15 μm).

14

Właściwości cieplne wybranych konstrukcji laserów kaskadowych

Wasiak M., Sarzała R. P., Nakwaski W.

Elektronika : konstrukcje, technologie, zastosowania

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2010

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Vol. 51, nr 1

32-34

PL

W pracy przeanalizowano właściwości cieplne dwóch struktur laserów kaskadowych wykonanych na bazie materiałów GaAs/AIGaAs. Rozważane są czynniki wpływające na możliwość uzyskania akcji laserowej w wyższych temperaturach, związane ze strukturą poziomów kwantowych w obszarze czynnym i transportem ciepła w laserze.

EN

In this paper we present results concerning some thermal properties of two GaAs/AIGaAs quantum cascade lasers (QCLs). Both electronic structers of the active regions and heat transport in the lasers are discussed as factors determining the possibility of the operation of QCLs at elevated temperatures.

15

Propagation of polarized light through optical nanosuperlattices

Tarnowski K., Salejda W., Tyc M.H.

Optica Applicata

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2007

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Vol. 37, nr 4

387-396

EN

We investigate numerically transmittance of polarized electromagnetic wave through periodic and aperiodic binary multilayered nanostructures made of left- and right-handed materials. The transmittance is calculated as a function of light wavelength and incidence angle as well as ordering, refractive indices and thicknesses of superlattice layers. Thanks to extension of the transfer matrix formalism over the case of complex refraction angles, such effects as tunnelling, absorption and strong dispersion in left-handed metamaterials are taken into account. We find that these effects cannot be neglected and cause new transmission bands to appear in the areas where total internal reflection in a single metamaterial layer occurs.

16

Effective characteristic matrix of ultrathin multilayer structures

Haija A.J., Freeman W.L., Roarty T.

Optica Applicata

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2006

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Vol. 36, nr 1

39-50

EN

This paper presents the calculation of the reflectivity, transmissivity and optical constants of ultrathin Cu-Ni multilayer stacks using the characteristic effective medium approximation (CEMA) introduced in an earlier communication. Each of the Cu-Ni multilayer stacks has an identity period of 100 A, Cu - 45 A and Ni - 55 A. Calculations of the reflectivity and transmissivity are executed via the characteristic matrix technique employed in three ways. In the first the characteristic matrix of the Cu-Ni bilayer is calculated and then raised to a power equal to the number of layers in the stack following the characteristic matrix technique. The second is based on the calculation of the characteristic matrix of the bilayer identity period using the effective complex index of refraction of the identity period as derived according to the CEMA. The third is based on the equivalent characteristic matrix of the whole stack represented by one characteristic matrix, also using the CEMA; in this method the equivalent optical constants of the layered structure are also calculated. All calculations are in the visible and for normal incidence. A comparison between results of the first two methods of calculations shows that they are almost identical. However, displaying values using the equivalent effective matrix of the whole structure shows definite quantitative differences throughout the whole studied spectral range. The difference becomes rather noticeable when the number of layers is greater than or equal to six amounting to a minimum total thickness of 600 A for the layered stack as a whole. This establishes a quantitative criterion to the limit beyond which the CEMA cannot be applied.

17

Electrochemical synthesis and structural properties of Cu/Ni multilayers

Tokarz A.

Polish Journal of Applied Chemistry

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2005

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Vol. 49, nr 4

283-289

EN

Multilayers of Cu/Ni were electodeposited on monocrystalline Si (100) and polycrystalline copper foil substrates using a single bath containing nickel sulphamate, copper sulphate and boric acid. Voltammetric studies allow establishing the suitable potentials for Cu pure and Ni rich single layer. Based on these studies lower activity of silicon than cooper substrate was stated. SEM investigations confirm periodical structure of the prepared multilayers when the Λ period exceeds 50 nm. For lower bilayer thickness XRD method allowed determining Λ value after precise establishing diffraction lines S±i and ML. XRD investigations show that all deposited multilayers exhibited [100] texturing.

18

Dispersion Dependencies For Acoustic Longitudinal Waves In Au/V Multilayers

Aleksiejuk M.

Molecular and Quantum Acoustics

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2004

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Vol. 25

9-16

EN

This work presents study of the acoustic longitudinal waves propagation in Au/V multilayers along the perpendicular direction to multiplayer surface. Considered multilayers have the basic bilayer periodicity from 21 A to 80 A. The dispersion dependencies and frequency gaps for longitudinal acoustic phonons were obtained.

19

An approximation method for simulating temperature dependence of Poissonąs ratios of self - expanding auxegens

Wu H., Wei G.

Computational Methods in Science and Technology

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2004

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Vol. 10, nr 2

229-234

EN

A combined molecular dynamics and molecular mechanics method has been developed for estimating Poisson's ratios of certain types of molecular auxetics at various temperatures. The temperature dependence of the auxeticity of a special class of molecular auxetics, namely, a self-expanding supramolecular network of auxegens containing alternating phenyl and acetylene links, is studied with use of this approximation method. The simulation results show that as temperature increases from O to 300 K, the auxeticity of the resulting superlattice or van der Waals network of auxegens decreases from the initial self-expandability to two negatively small Poisson ratios on the xoy piane.

20

Phenomenological model of changes in the speed of a sound wave in crystals caused by dopants

Błachowicz T.

Archives of Acoustics

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2004

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Vol. 29, no. 3

447-459

EN

The paper examines theoretically changes in the speed of an acoustic wave in crystals caused by metallic dopants. It is well known that an acoustic wave has a wavelength larger than the typical dimension of the elementary crystallographic cell, so it is sensitive only to large-scale features. The model bases on the assumption that a crystal with uniform distributed dopants can be transformed into a multiplayer virtual structure form which some conclusions can be drawn. The approach presented is valid for longitudinal acoustic waves, where the atomic displacement can be considered as a continuum field. It provides positive or negative changes in the acoustic wave speeds for different dopants in a hypothetical lithium niobate crystal.