Change search
Refine search result
59606162636465 3051 - 3100 of 122069
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Rows per page
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sort
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
Select
The maximal number of hits you can export is 250. When you want to export more records please use the Create feeds function.
  • 3051.
    Akpe, Victor
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Cell Physics.
    Ogunsipe, Abimbola
    Madu, Christian
    Brismar, Hjalmar
    KTH, School of Engineering Sciences (SCI), Applied Physics, Cell Physics.
    Red-Shift Index Concept in Solvent Effects of Chromophore-Substituted Metallophthalocyanines: A Look at the Empirical Relationship of the Macroscopic Properties of the Solute-Solvent Interactions2015In: Journal of Solution Chemistry, ISSN 0095-9782, E-ISSN 1572-8927, Vol. 44, no 2, p. 307-326Article in journal (Refereed)
    Abstract [en]

    Solvent effects on the UV/vis spectra of metallopthalocyanines (MPcs) have been interpreted using the red-shift index concept (R (s) I). The concept connects empirically, direct, experimental, easily accessible optical spectral data, which are explained by considering the differential behavior of the solute-solvent interactions at the ground state and excited state using the spectral values of MPcs along with the derived concept, called the associated solvation energy (ASE). R (s) I is formulated from three fundamental parameters, which are: ground state electronic absorption spectrum, polarization red-shift and a scaling factor of MPc (N (dye)) in the respective solvents. The R (s) I is a reflection of the index value of the chromophore substituent of MPc in the solvent; thus, the concept can be used as a solvatochromic parameter to study a wide range of supramolecular and heterocyclic compounds that can be modified at their periphery or 'handles'. Particularly, in this study, the concept has been used to rank MPc candidates by using the statistical mean performance of the solvatochromic parameters, which are red shift index, polarizability efficiency and ASE. We hereby review the solvent effects on the UV/vis spectra of substituted and unsubstituted MPcs.

  • 3052.
    Akpe, Victor
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Cell Physics.
    Vernet, Erik
    KTH, School of Biotechnology (BIO), Molecular Biotechnology.
    Gräslund, Torbjörn
    KTH, School of Biotechnology (BIO), Molecular Biotechnology.
    Brismar, Hjalmar
    KTH, School of Engineering Sciences (SCI), Applied Physics, Cell Physics.
    Characterization studies of aluminum phthalocyanine binding to antibodies from SKBR 3 cell line2008Report (Other academic)
  • 3053.
    Akpe, Victor
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Vernet, Erik
    KTH, School of Biotechnology (BIO), Protein Technology.
    Madu, Christian
    Obirai, Joseph C.
    Brismar, Hjalmar
    KTH, School of Engineering Sciences (SCI), Applied Physics, Cell Physics.
    Understanding the Photochemical Pathway of In Vitro Target Delivery of Aluminium Phthalocyanine: A Mechanistic Approach Using Radical Reaction Chemistry2014In: ChemPlusChem, ISSN 2192-6506, Vol. 79, no 5, p. 671-679Article in journal (Refereed)
    Abstract [en]

    A classical dye, aluminium phthalocyanine (AlPc), is used to study the photochemical processes involved in the chromophore-assisted laser inactivation technique. Both cell-free and cell-based systems are investigated by novel methods and radical reaction chemistry. Findings on the photochemical pathways in two models representing cell-free and a cell-based systems are reported. In the cell-free system, the unsubstituted, free, fluorescence-active photosensitiser AlPc recovers its fluorescence signal by means of phosphorescence through a reversible photobleaching process. In the cell-based system, photoactivation of substituted AlPc conjugated to an antibody results in the loss of fluorescence signal at the area examined. Reinjection of the AlPc-conjugated antibodies restores the fluorescence signal.

  • 3054. Akram, M. N.
    et al.
    Schatz, Richard
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Optics and Photonics, OFO.
    Marcinkevicius, Saulius
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Kjebon, Olle
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Berggren, J.
    Experimental evaluation of carrier transport, gain, T0 and chirp of 1.55 mm MQW structures with different barrier compositions2005In: 31st European Conference on Optical Communications (ECOC 2005), 2005, Institution of Engineering and Technology, 2005, Vol. 2005, no CP502, p. 297-298Conference paper (Refereed)
    Abstract [en]

    Direct carrier transport measurements were performed for different InGaAsP/InGaAlAs MQW test structures. Shallow InGaAlAs barrier QW showed faster carrier transport. Semi-insulating regrown FP lasers with InGaAlAs barrier QW showed improved high temperature operation, modal gain, differential modal gain and chirp.

  • 3055.
    Akram, Muhammad
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Land and Water Resources Engineering.
    A Freundlich-based model for prediction of pH-dependent sulfate adsorption in forest soil.2015Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    The period of industrialization after the second World War in Europe released SO2 and NOx by combustion of fossil fuels and contributed the formation of S and N compounds in the forest ecosystem. The Swedish forest soil systems were influenced by emissions of SO2 followed by H2SO4 deposition, consequently the pool of SO42- had increased in the forest ecosystem. This thesis studied SO42- adsorption in a podzolic Bs horizon soils taken from a Swedish forest soil system. The soil samples from five different sampling sites were collected and the results revealed different amounts of adsorbed SO42- in response to changes in equilibrium concentration and pH. This study found that the amount of adsorbed SO42- (mmol/kg) increased with an added equilibrium concentration of SO42- (mmol/l) and with a decreasing pH. This was determined by equilibration experiments. Based on the results a Freundlich-based model was developed to predict the pool of adsorbed SO42- in the soil samples. The model predicted the pool of adsorbed SO42- (mmol/kg) as a function of pH and the equilibrium concentration of SO42- (mmol/l) in the soil solution system. The extended Freundlich model was optimized in three different ways: by use of unconstrained, constrained and simplified two-point calibration. The results showed that the adsorption of sulfate in the Kloten Bs1 and Risbergshöjden B soils was higher as compared to the Tärnsjo B, Österström B, and Risfallet B soils. The coefficient of determination (R2) determined from an unconstrained fit of the extended Freundlich model (with three adjustable parameters) for Risbergshöjden B and Kloten Bs1 were R2 =0.998 and R2=0.993. Nearly as good fits were found in a constrained fit with two adjustable parameters when it was assumed that nearly 2 protons (2 H+) are co-adsorbed with one SO42- ion (Risbergshöjden B; R2=0.997 and Kloten Bs; R2=0.992). The simplified two-point calibration with two adjustable parameters showed similar parameter values for all most soils and was considered the best optimization method of extended Freundlich model, especially as it requires only limited input data.

  • 3056. Akram, Muhammad Nadeem
    et al.
    Xiang, Yu
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Yu, Xingang
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Zabel, Thomas
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Hammar, Mattias
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Influence of base-region thickness on the performance of Pnp transistor-VCSEL2014In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 22, no 22, p. 27398-27414Article in journal (Refereed)
    Abstract [en]

    We have recently reported a 980nm GaAs-based three terminal Pnp transistor-vertical-cavity surface-emitting laser (TVCSEL) operating at room temperature with optical power up to 1.8mW. However, the current gain beta = Delta I-c/Delta I-b was near zero just before lasing and became negative after the lasing threshold. The main cause of the negative current gain was found to be a gradual and position-dependent forward-biasing (saturation) of the base-collector junction with increasing bias even before lasing threshold. In this article, detailed multi-physics device simulations are performed to better understand the device physics, and find ways to avoid the premature saturation of the base-collector junction. We have optimized the thickness of the base region as well as its doping concentration and the location of the quantum wells to ensure that the T-VCSEL is in the active mode throughout its range of operation. That is, the emitter-base junction is forward biased and base-collector junction is reversed biased for sweeping the excess charges out of the base region.

  • 3057. Akram, N. M.
    et al.
    Schatz, Richard
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Marcinkevicius, Saulius
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Kjebon, Olle
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Berggren, Jesper
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Experimental evaluation of carrier transport, gain, T0 and chirp of 1.55 mu;m MQW structures with different barrier compositions2005In: Optical Communication, 2005. ECOC 2005. 31st European Conference on, 2005, Vol. 2, p. 297-298Conference paper (Refereed)
    Abstract [en]

    Direct carrier transport measurements were performed for different InGaAsP/InGaAlAs MQW test structures. Shallow InGaAlAs barrier QW showed faster carrier transport. Semi-insulating regrown FP lasers with InGaAlAs barrier QW showed improved high temperature operation, modal gain, differential modal gain and chirp.

  • 3058.
    Akram, Nadeem
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Lateral current injection (LCI) multiple quantum-well 1.55 mu m laser with improved gain uniformity across the active region2004In: Optical and quantum electronics, ISSN 0306-8919, E-ISSN 1572-817X, Vol. 36, no 9, p. 827-846Article in journal (Refereed)
    Abstract [en]

    A simulation study of lateral current injection 1.55 mum laser with strain-compensated multiple quantum-well (MQW) active region (InGaAsP well, InGaAlAs barrier) is presented using self-consistent 2D numerical simulations. The effects of different mesa width and p-doping in the QWs on the carrier and gain uniformity across the active region are explored. A high p-doping in the quantum wells is found to increases the carrier and gain non-uniformity across the active region. The QW region close to the n-contact side does not provide much gain at high optical powers. An asymmetric optical waveguide design is proposed to help reduce the gain non-uniformity across the active region. By shifting the optical modal peak toward the p-side, the modal overlap between the gain region and the optical mode is improved and a more even carrier and gain distribution is obtained. However, due to reduced bandgap of the quaternary InGaAsP p-cladding, an enhanced electron leakage out of the QWs into the p-cladding degrades the laser efficiency and increases the threshold current. Transient time - domain simulations are also performed to determine the small-signal modulation response of the laser promising a simulated high modulation bandwidth suitable for direct-modulation applications.

  • 3059.
    Akram, Nadeem
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Photonic devices with MQW active material and waveguide gratings: modelling and characterisation2005Doctoral thesis, comprehensive summary (Other scientific)
    Abstract [en]

    The research work presented in this thesis deals with modelling, design and characterisation of passive and active optical waveguide devices. The rst part of the thesis is related to algorithm development and numerical modelling of planar optical waveguides and gratings using the Method of Lines (MoL). The basic three-point central-di erence approximation of the δ2=δx2 operator used in the Helmholtz equation is extended to a new ve-point and seven-point approximation with appropriate interface conditions for the TE and TM elds. Di erent structures such as a high-contrast waveguide and a TM surface plasmon mode waveguide are simulated, and improved numerical accuracy for calculating the optical mode and propagation constant is demonstrated. A new fast and stable non-paraxial bi-directional beam propagation method, called Cascading and Doubling algorithm, is derived to model deep gratings with many periods. This algorithm is applied to model a quasi-guided multi-layer anti-resonant reecting optical waveguide (ARROW) grating polarizing structure.

    In the second part of the thesis, our focus is on active optical devices such as vertical-cavity and edge-emitting lasers. With a view to improve the bandwidth of directly modulated laser, an InGaAsP quantum well with InGaAlAs barrier is studied due to its favorable band o set for hole injection as well as for electron con nement. Quantum wells with di erent barrier bandgap are grown and direct carrier transport measurements are done using time and wavelength resolved photoluminescence upconversion. Semi-insulating regrown Fabry-Perot lasers are manufactured and experimentally evaluated for light-current, optical gain, chirp and small-signal performance. It is shown that the lasers having MQW with shallow bandgap InGaAlAs barrier have improved carrier transport properties, better T0, higher di erential gain and lower chirp. For lateral current injection laser scheme, it is shown that a narrow mesa is important for gain uniformity across the active region. High speed directly modulated DBR lasers are evaluated for analog performance and a record high spurious free dynamic range of 103 dB Hz2=3 for frequencies in the range of 1-19 GHz is demonstrated. Large signal transmission experiment is performed at 40 Gb/s and error free transmission for back-to-back and through 1 km standard single mode ber is achieved.

  • 3060.
    Akram, Nadeem
    et al.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Photonics.
    Kjebon, Olle
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Photonics.
    Chacinski, Marek
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP. KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Photonics.
    Schatz, Richard
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP. KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Photonics.
    Berggren, Jesper
    KTH, School of Information and Communication Technology (ICT), Material Physics, Semiconductor Materials, HMA.
    Olsson, Fredrik
    KTH, School of Information and Communication Technology (ICT), Material Physics, Semiconductor Materials, HMA.
    Lourdudoss, Sebastian
    KTH, School of Information and Communication Technology (ICT), Material Physics, Semiconductor Materials, HMA.
    Berrier, Audrey
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Experimental characterization of high-speed 1.55 mu m buried heterostructure InGaAsP/InGaAlAs quantum-well lasers2009In: Journal of the Optical Society of America. B, Optical physics, ISSN 0740-3224, E-ISSN 1520-8540, Vol. 26, no 2, p. 318-327Article in journal (Refereed)
    Abstract [en]

    Detailed experimental characterization is performed for 1550 nm semi-insulating regrown buried heterostructure Fabry-Perot (FP) lasers having 20 InGaAsP/InGaAlAs strain-balanced quantum wells (QWs) in the active region. Light-current-voltage performance, electrical impedance, small-signal response below and above threshold, amplified spontaneous emission spectrum below threshold and relative intensity noise spectrum are measured. Different laser parameters such as external differential quantum efficiency eta(d), background optical loss alpha(i), K-factor, D-factor, characteristic temperature T-0, differential gain dg/dn, gain-compression factor epsilon, carrier density versus current, differential carrier lifetime tau(d), optical gain spectrum below threshold, and chirp parameter alpha are extracted from these measurements. The FP lasers exhibited a high T-0 (78-86.5 degrees C) and very high-resonance frequency (23.7 GHz). The results indicate that appropriately designed lasers having a large number of InGaAsP well/InGaAlAs barrier QWs with shallow valence-band discontinuity can be useful for un-cooled high-speed direct-modulated laser applications.

  • 3061.
    Akram, Nadeem
    et al.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Photonics.
    Kjebon, Olle
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Photonics.
    Chacinski, Marek
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Photonics.
    Schatz, Richard
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Photonics.
    Berggren, Jesper
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Olsson, Fredrik
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Lourdudoss, Sebastian
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Berrier, Audrey
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    High-Speed Performance of 1.55 µm Buried Hetero-Structure Lasers with 20 InGaAsP/InGaAlAs Quantum-Wells2006In: 2006 European Conference on Optical Communications Proceedings, ECOC 2006, IEEE , 2006, p. 1-2Conference paper (Refereed)
    Abstract [en]

    1550 nm re-grown FP lasers having 20 InGaAsP/InGaAlAs strain-balanced QWs exhibit low threshold current density, high T0 (78.0 #x000B0;C) and high resonance frequency (24 GHz) indicating that a large number of shallow barrier QWs are attractive for un-cooled high-speed direct-modulation applications.

  • 3062.
    Akram, Nadeem
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Kjebon, Olle
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Marcinkevičius, Saulius
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Schatz, Richard
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Berggren, Jesper
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Olsson, Fredrik
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Lourdudoss, Sebastian
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    The effect of barrier composition on the vertical carrier transport and lasing properties of 1.55-mu m multiple quantum-well structures2006In: IEEE Journal of Quantum Electronics, ISSN 0018-9197, E-ISSN 1558-1713, Vol. 42, no 7, p. 713-714Article in journal (Refereed)
    Abstract [en]

    In this paper, the effect of barrier bandgap and composition on the optical performance of 1.55-mu m InGaAsP/In-GaAsP and InGaAsP/InGaAlAs multiple quantum-well structures and Fabry-Perot lasers is evaluated experimentally. Direct vertical carrier transport measurements were performed through strain-compensated multiple quantum-well (MQW) test structures using femto-second laser pulse excitation and time-resolved photoluminescence up-conversion method. MQW test structures were grown with different barrier composition (InGaAsP and InGaAlAs) and barrier bandgap (varied from lambda(g) = 1440 to 1260 nm) having different conduction band Delta E-c and valence band discontinuity Delta E-v, while keeping the same InGaAsP well composition for all the structures. The ambipolar carrier transport was found to be faster in the structures with lower valence band discontinuity Delta E-v. Regrown semi-insulating buried heterostructure Fabry-Perot (SIBH-FP) lasers were fabricated from similar QWs and their static light-current-voltage characteristics (including optical gain and chirp spectra below threshold) and thermal characteristics were measured. Lasers with InGaAlAs barrier showed improved high-temperature operation, higher optical gain, higher differential gain, and lower chirp, making them suitable candidates for high-bandwidth directly modulated uncooled laser applications.

  • 3063.
    Akram, Nadeem
    et al.
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Schatz, Richard
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Kjebon, Olle
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Influence of electrical parasitics and drive impedance on the laser modulation response2004In: IEEE Photonics Technology Letters, ISSN 1041-1135, E-ISSN 1941-0174, Vol. 16, no 1, p. 21-23Article in journal (Refereed)
    Abstract [en]

    In this letter, we analyze the effects of electrical parasitics and drive impedance on the laser modulation response. It is found that for lasers with small active-region volume, e.g., vertical-cavity surface-emitting lasers, the finite drive impedance and/or the laser parasitic capacitance can significantly enhance the damping of the laser resonance peak at low bias. This is due to the voltage fluctuations across the laser diode active layer during modulation. It is also shown that the real pole of the small-signal response transfer function corresponding to the laser parasitic cutoff frequency is not fixed but decreases with the increased bias level.

  • 3064.
    Akram, Nadeem
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Silfvenius, Christofer
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Berggren, Jesper
    KTH, School of Information and Communication Technology (ICT), Material Physics (Closed 20120101), Semiconductor Materials, HMA (Closed 20120101).
    Kjebon, Olle
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Schatz, Richard
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Design optimization of InGaAsP-InGaAlAs 1.55 mu;m strain-compensated MQW lasers for direct modulation applications2004In: Indium Phosphide and Related Materials, 2004. 16th IPRM. 2004 International Conference on, IEEE , 2004, p. 418-421Conference paper (Refereed)
    Abstract [en]

    A comprehensive simulation study of InGaAsP (well)/InGaAlAs(barrier) 1.55 mu;m strain-compensated MQW lasers is presented. For MQWs, a uniform vertical distribution of holes is achieved due to a reduced effective hole confinement energy by optimizing the bandgap and strain of the barriers and p-doping in the active region. Some preliminary results are also presented for the manufactured lasers using these QWs indicating a good material platform.

  • 3065.
    Akram, Nadeem
    et al.
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Silfvenius, Christofer
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Kjebon, Olle
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Schatz, Richard
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Design optimization of InGaAsP-InGaAlAs 1.55 mu m strain-compensated MQW lasers for direct modulation applications2004In: Semiconductor Science and Technology, ISSN 0268-1242, E-ISSN 1361-6641, Vol. 19, no 5, p. 615-625Article in journal (Refereed)
    Abstract [en]

    In this paper, a simulation study of InGaAsP(well)/InGaAlAs(barrier) 1.55 mum strain-compensated multi-quantum well (MQW) lasers is presented. Due to a large conduction band discontinuity in this material system, a higher material gain and differential gain can be obtained from such a quantum well (QW) as compared to a traditional InGaAsP/InGaAsP quantum well. The deeper electron well should also improve elevated temperature operating characteristics and reduce the electron spillover from QWs. For MQWs, a uniform vertical distribution of holes is achieved due to a reduced effective hole confinement energy by optimizing the bandgap and the strain in the barriers. A large number of quantum wells can be uniformly pumped, reducing the carrier density in each individual well. A uniform and low carrier density in all the wells help reduce the total Auger recombination current. High p-doping in the active region is shown to enhance the carrier and gain non-uniformity in the MQWs. A simulated high modulation bandwidth has been demonstrated, promising directly modulated lasers as a low-cost source for short to medium distance (1-10 km) high speed optical links.

  • 3066.
    Akram, Saad Ullah
    KTH, School of Computer Science and Communication (CSC).
    Visual Recognition of Isolated Swedish Sign Language Signs.2012Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Visual Recognition of Isolated Swedish Sign Language Signs

    Saad Ullah Akram saadua@kth.se

    Abstract:

    Deaf people face serious challenges when communicating with people who have hearing capability, vast majority of whom do not have any sign language skills. Deaf people in these situations have to rely on written language which is usually not their primary language. Most of the deaf children are born to hearing parents who usually have very rudimentary sign language skills, these children do not get enough opportunity to practice and improve their signing skills outside the school environment. This puts these children at a disadvantage as they take more time to be able to communicate complex ideas with others.

    This project proposes a method for recognition of Isolated Swedish Sign Language signs in a natural environment using Kinect sensor with only one important restriction that signer must wear a full-sleeved non-skin colored clothing. An adaptive histogram based skin color model and motion information are used to segment hands. 3D position of the hands relative to the torso are used as a cue together with the hand shape, and HMMs (Hidden Markov Models) trained with this input are used for classification. This project also presents a method which is able to solve one (hand over face) of the two common (other being hand over hand) occlusions encountered in sign language.

    The results obtained show that Sign Language Recognition problem for very challenging medium sized (94 signs) vocabulary can be solved with very good performance (92% recognition accuracy) in signer dependent case and with reasonable performance (42% recognition accuracy) in signer independent case.

  • 3067. Akrami, Yashar
    et al.
    Hassan, S. F.
    Koennig, Frank
    Schmidt-May, Angnis
    Solomon, Adam R.
    Bimetric gravity is cosmologically viable2015In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 748, p. 37-44Article in journal (Refereed)
    Abstract [en]

    Bimetric theory describes gravitational interactions in the presence of an extra spin-2 field. Previous work has suggested that its cosmological solutions are generically plagued by instabilities. We show that by taking the Planck mass for the second metric, M-f, to be small, these instabilities can be pushed back to unobservably early times. In this limit, the theory approaches general relativity with an effective cosmological constant which is, remarkably, determined by the spin-2 interaction scale. This provides a late-time expansion history which is extremely close to Lambda CDM, but with a technically-natural value for the cosmological constant. We find M-f should be no larger than the electroweak scale in order for cosmological perturbations to be stable by big-bang nucleosynthesis. We further show that in this limit the helicity-0 mode is no longer strongly-coupled at low energy scales. (C) 2015 The Authors. Published by Elsevier B.V.

  • 3068. Akrami, Yashar
    et al.
    Koivisto, Tomi S.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Solomon, Adam R.
    The nature of spacetime in bigravity: Two metrics or none?2015In: General Relativity and Gravitation, ISSN 0001-7701, E-ISSN 1572-9532, Vol. 47, no 1, p. 1838-Article in journal (Refereed)
    Abstract [en]

    The possibility of matter coupling to two metrics at once is considered. This appears natural in the most general ghost-free, bimetric theory of gravity, where it unlocks an additional symmetry with respect to the exchange of the metrics. This double coupling, however, raises the problem of identifying the observables of the theory. It is shown that if the two metrics couple minimally to matter, then there is no physical metric to which all matter would universally couple, and that moreover such an effective metric generically does not exist even for an individual matter species. By studying point particle dynamics, a resolution is suggested in the context of Finsler geometry.

  • 3069.
    Akrami, Yashar
    et al.
    Leiden Univ, Lorentz Inst Theoret Phys, POB 9506, NL-2300 RA Leiden, Netherlands..
    Kuhnel, Florian
    Stockholm Univ, AlbaNova, Oskar Klein Ctr Cosmoparticle Phys, Dept Phys, SE-10691 Stockholm, Sweden..
    Sandstad, Marit
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Uncertainties in primordial black-hole constraints on the primordial power spectrum2018In: Physics of the Dark Universe, ISSN 0953-8585, E-ISSN 2212-6864, Vol. 19, p. 124-128Article in journal (Refereed)
    Abstract [en]

    The existence (and abundance) of primordial black holes (PBHs) is governed by the power spectrum of primordial perturbations generated during inflation. So far no PBHs have been observed, and instead, increasingly stringent bounds on their existence at different scales have been obtained. Up until recently, this has been exploited in attempts to constrain parts of the inflationary power spectrum that are unconstrained by cosmological observations. We first point out that the simple translation of the PBH non-observation bounds into constraints on the primordial power spectrum is inaccurate as it fails to include realistic aspects of PBH formation and evolution. We then demonstrate, by studying two examples of uncertainties from the effects of critical and non-spherical collapse, that even though they may seem small, they have important implications for the usefulness of the constraints. In particular, we point out that the uncertainty induced by non-spherical collapse may be much larger than the difference between particular bounds from PBH non-observations and the general maximum cap stemming from the condition Omega <= 1 on the dark-matter density in the form of PBHs. We therefore make the cautious suggestion of applying only the overall maximum dark-matter constraint to models of early Universe, as this requirement seems to currently provide a more reliable constraint, which better reflects our current lack of detailed knowledge of PBH formation. These, and other effects, such as merging, clustering and accretion, may also loosen constraints from non-observations of other primordial compact objects such as ultra-compact minihalos of dark matter. 

  • 3070.
    Akrawi, Kahi
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Technology and Design.
    Halim, Magdalena
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Technology and Design.
    Ombyggnation av befintlig byggnad till hotell2018Independent thesis Basic level (university diploma), 10 credits / 15 HE creditsStudent thesis
  • 3071.
    Akrawi, Kahi
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Technology and Design.
    Sharif, Vina
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Technology and Design.
    Tätningsmetoder av tunnlar för skydd av installationer: En studie baserad på litteratur, intervjuer och tidigare arbeten.2016Independent thesis Basic level (university diploma), 5 credits / 7,5 HE creditsStudent thesis
  • 3072. Akselsen, O. M.
    et al.
    Aune, Ragnhild E.
    Fostervoll, H.
    Harsvoer, A. S.
    Dry hyperbaric welding of subsea pipelines2006In: Welding Journal, ISSN 0043-2296, Vol. 85, no 6, p. 52-55Article in journal (Refereed)
  • 3073. Akselsen, O. M.
    et al.
    Harsvaer, A.
    Fostervoll, H.
    Aune, Ragnhild E.
    Root bead profiles in hyperbaric GTAW of X70 pipeline2006In: International Journal of Offshore and Polar Engineering, ISSN 1053-5381, Vol. 16, no 2, p. 123-127Article in journal (Refereed)
    Abstract [en]

    This investigation began with the goal of studying the root bead penetration profiles in hyperbaric GTAW welding of X70 pipelines. Such profiles may vary substantially depending on the wire and base metal chemical composition. Root beads were deposited with a pressure corresponding to 75-m seawater depth, and with a systematic increase of 10 A in the pulse current until burnthrough took place. The results obtained showed that the penetration profiles were different between the 2 wires included in the welding program. The largest penetration width was found for wire B with the highest sulphur content (0.013% S), with a maximum width of about 8-mm width on the pipe inside for 160 A. At pulse current levels of 120 to 170 A, the difference between the 2 wires was about 1 mm. With current beyond 180 A, the profiles approached similar values, followed by burnthroughs for both wires at 190 A. However, the welding parameters were already too hot at 180 A. These results are probably caused by a Marangoni convection in the weld pool. A high content of surface active elements (e.g., sulphur) is known to shift the How pattern, providing deeper penetration. In practice, small variations in the bead penetration profile may have large consequences during offshore tie-in welding. Cost-increasing repair operations may be required if nondestructive inspection later reveals poor root bead quality. Such actions require mobilisation of huge resources.

  • 3074. Akselsen, Odd M.
    et al.
    Aune, Ragnhild E.
    Olden, Vigdis
    Rorvik, Gisle
    Effects of phase transformations on residual stresses in welding of stainless steels2007In: International Journal of Offshore and Polar Engineering, ISSN 1053-5381, Vol. 17, no 2, p. 145-151Article in journal (Refereed)
    Abstract [en]

    In this investigation, we investigate the effects of solid state phase transformation on residual stresses in welding super-martensitic stainless steels, using the Satoh experimental test approach. The results obtained clearly demonstrate that the austenite-to-martensite phase transformation has remarkable effects on the final tensile residual stress level, and stresses as low as 70 to 170 MPa were found, depending on the weld thermal program. These results may imply that unnecessary conservatism is often used when assuming residual stresses to be of the same order as the base metal yield strength.

  • 3075. Akselsen, Odd M.
    et al.
    Fostervoll, Hans
    Harsvaer, Ansgar
    Aune, Ragnhild E.
    Weld metal mechanical properties in hyperbaric GTAW of X70 pipeline2006In: International Journal of Offshore and Polar Engineering, ISSN 1053-5381, Vol. 16, no 3, p. 233-240Article in journal (Refereed)
    Abstract [en]

    In the present investigation, 2 different wires for hyperbaric (underwater) GTA (gas tungsten arc) welding of X70 pipelines have been tested with respect to their weld metal mechanical properties. Welding of full coupons at different pressures (seawater depths of 16, 75 and 200 msw) was done with subsequent weld metal chemical analyses, hardness measurements, tensile testing and Charpy V notch testing as well as microstructure characterization. It is shown that both wires satisfied strength requirements set to X70 grade, representing a weld metal overmatch situation. Both wires gave sufficient impact toughness, but the toughness of the Ni-Mo containing weld was reduced with increasing seawater depth. This observation was strongly linked to the positioning of the Charpy V notch, and crack growth in a brittle, partially transformed region as a consequence of reheating by subsequent stringer beads. The embrittling microstructure consisted of high carbon MA (martensite-austenite constituents islands) decorating prior austenite grain boundaries. This microstructure was less pronounced when welding with the high Ni wire, which may explain why no similar toughness drop was found.

  • 3076.
    Aksjonova, Jevgenija
    KTH, School of Electrical Engineering and Computer Science (EECS).
    LDD: Learned Detector and Descriptor of Points for Visual Odometry2018Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Simultaneous localization and mapping is an important problem in robotics that can be solved using visual odometry -- the process of estimating ego-motion from subsequent camera images. In turn, visual odometry systems rely on point matching between different frames. This work presents a novel method for matching key-points by applying neural networks to point detection and description. Traditionally, point detectors are used in order to select good key-points (like corners) and then these key-points are matched using features extracted with descriptors. However, in this work a descriptor is trained to match points densely and then a detector is trained to predict, which points are more likely to be matched with the descriptor. This information is further used for selection of good key-points. The results of this project show that this approach can lead to more accurate results compared to model-based methods.

  • 3077. Aktas, A.
    et al.
    Ismail, Mohammed
    Pad de-embedding in RF CMOS2001In: IEEE Circuits & Devices, ISSN 8755-3996, E-ISSN 1558-1888, Vol. 17, no 3, p. 8-11Article in journal (Refereed)
    Abstract [en]

    Welcome to The Chip! We remain committed to bringing you material you can use in your work and research. We solicit your contributions and input on what we present here. Material or short articles on chip design tips, modeling and characterization techniques, yield enhancement, packaging, and test are welcome, as well as news on new chips and start-ups, mergers, acquisitions, partnerships in the microchip business, etc. Please continue to e-mail us at ismail@ee.eng.ohio-state.edu or ntan@globespan.net. In this column, we discuss techniques for RF pad layout and de-embedding, a topic of great interest particularly for implementing radio frequency (RF) circuits in mainstream CMOS technology. Happy reading!

  • 3078. Aktas, Adem
    et al.
    Jonsson, Fredrik
    KTH, School of Information and Communication Technology (ICT), Electronic Systems.
    Ahola, Rami
    Ismail, Mohammed
    A 4 Ghz 0.18um CMOS PLL Frequency Synthesizer withWide-Band VCO for Multi-Standard Wireless Applications2003In: Proc. 22nd Norchip Conference, 2003, p. 248-251Conference paper (Refereed)
  • 3079.
    Aktas, Bagsen
    et al.
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Bria, Aurelian
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Evaluation of User Perceived Performance in Sparce Infrastructure Wireless Systems2004Conference paper (Refereed)
    Abstract [en]

    This work tries to evaluate the user perceived performance in a wireless system based on sparse infrastructure (islands of coverage).We investigate a mobile user in a Manhattan environment and the considered service is web browsing. Data caching in the use terminal is employed in order to hide the discontinuities of coverage. We show that for this particular application, the user's behavior of revisiting pages previously accessed makes caching a way to increase the user perceived coverage. Our results show that at low levels of effective area coverage caching brings a large improvement, while the gain is not significant at high levels. If the sparse coverage network is overlaid over a cellular system the obtained results show potential for significant cost savings.

  • 3080.
    Aktas, Meryem
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Applied Mechanical Engineering (KTH Södertälje).
    Anslin, Anslin
    KTH, School of Industrial Engineering and Management (ITM), Applied Mechanical Engineering (KTH Södertälje).
    Analys och utveckling avverifieringsenhet för säker kontakteringvid kalibrering av RRUS.2014Independent thesis Basic level (university diploma), 10 credits / 15 HE creditsStudent thesis
  • 3081.
    Aktas, O.
    et al.
    Univ Southampton, Optoelect Res Ctr, Southampton SO17 1BJ, Hants, England..
    Ren, H.
    Univ Southampton, Optoelect Res Ctr, Southampton SO17 1BJ, Hants, England..
    Runge, A. F. J.
    Univ Southampton, Optoelect Res Ctr, Southampton SO17 1BJ, Hants, England..
    Peacock, A. C.
    Univ Southampton, Optoelect Res Ctr, Southampton SO17 1BJ, Hants, England..
    Hawkins, T.
    Clemson Univ, COMSET, Clemson, SC 29634 USA.;Clemson Univ, Dept Mat Sci & Engn, Clemson, SC 29634 USA..
    Ballato, J.
    Clemson Univ, COMSET, Clemson, SC 29634 USA.;Clemson Univ, Dept Mat Sci & Engn, Clemson, SC 29634 USA..
    Gibson, U. J.
    KTH, School of Engineering Sciences (SCI), Applied Physics. Norwegian Univ Sci & Technol, Dept Phys, N-7491 Trondheim, Norway.
    Interfacing Telecom Fibers and Silicon Core Fibers with Nano-Spikes for In-Fiber Silicon Devices2018In: 2018 Optical Fiber Communications Conference and Exposition, OFC 2018 - Proceedings, Institute of Electrical and Electronics Engineers (IEEE), 2018Conference paper (Refereed)
    Abstract [en]

    We report fabrication of tapered silicon core fibers with nano-spikes enabling efficient optical coupling into the core, as well as their seamless integration with single mode fibers. A proof-of-concept integrated in-fiber silicon device is demonstrated.

  • 3082. Aktas, O.
    et al.
    Ren, H.
    Runge, A. F. J.
    Peacock, A. C.
    Hawkins, T.
    Ballato, J.
    Gibson, Ursula J.
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Interfacing telecom fibers and silicon core fibers with nano-spikes for in-fiber silicon devices2018In: Optics InfoBase Conference Papers, Optics Info Base, Optical Society of America, 2018, article id u12d3i3mConference paper (Refereed)
    Abstract [en]

    We report fabrication of tapered silicon core fibers with nano-spikes enabling efficient optical coupling into the core, as well as their seamless integration with single mode fibers. A proof-of-concept integrated in-fiber silicon device is demonstrated. © OSA 2018.

  • 3083.
    Aktas, Stefanos
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Integrated Product Development.
    Wennhall, Thomas
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Integrated Product Development.
    AI - an Untapped Opportunity for Innovation Developing a Screening Tool for AI and Innovation2018Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    It is known that innovation enables companies to penetrate new markets and achieve higher margins and that technology can contribute to achieving a competitive ad-vantage and growth for organizations. A technology that has as of recently grown to become relevant for organizations is Artificial Intelligence (AI). Even so, previous studies have expressed the difficulty of implementing AI, which motivated this study.

    The main purpose of this study was to develop and test a screening tool that will work as a support in increasing an organization’s utilization of AI and innovation capability. During the course of the study, a great amount of focus was also put into conducting a preliminary analysis in preparation for a larger study that will be dependent on gathering large amounts of quantitative data.

    The research took on a three-phase-process. The first phase focused on gaining basic knowledge in regards to AI, innovation, technology management and model development. The findings in the first phase helped to formulate proper research questions that were applicable to the study.

    After that, the study moved on to the second phase which focused on a more in-depth literature study. This then led on to the development of an appropriate questionnaire for investigating factors that are relevant for AI and innovation, and an assessment model that would be connected to the questionnaire. The questionnaire was used for gathering responses that would be beneficial for the preliminary analysis in the form of a pilot study. The questionnaire and the assessment model together form a screening tool that gives a visual output of an organization’s position in regards to AI and innovation.

    The third and final phase included testing of the created screening tool, analyzing the findings from the pilot study and drawing conclusions from both the developed screening tool, and the results from the pilot study.

    The result from the literature study was the screening tool which takes five di-mensions into consideration that shows relevance to AI and innovation. These di-mensions are Structures, Resources, Methods, Action and Business,eachcontaining areas that exist in organizations that can be adjusted for the sake of the implemen-tation of AI and improvement of innovation management. The screening tool was tested on two separate organizations and managed to reflect these organizations’ AI progress through the assessment model. The screening tool was also applied to the pilot study which resulted in giving indications of what to expect when conducting a larger quantitative study.

    Despite the results gained from this study, it showed that further tests and studies need to be made in order to obtain more viable results. This study will act as a guideline for future studies to attain those results.

  • 3084. Akter, Farhima
    et al.
    Mie, Masayasu
    Grimm, Sebastian
    KTH, School of Biotechnology (BIO), Molecular Biotechnology.
    Nygren, Per-Åke
    KTH, School of Biotechnology (BIO), Molecular Biotechnology.
    Kobatake, Eiry
    Detection of Antigens Using a Protein-DNA Chimera Developed by Enzymatic Covalent Bonding with phiX Gene A2012In: Analytical Chemistry, ISSN 0003-2700, E-ISSN 1520-6882, Vol. 84, no 11, p. 5040-5046Article in journal (Refereed)
    Abstract [en]

    The chemical reactions used to make antibody DNA conjugates in many immunoassays diminish antigen-binding activity and yield heterogeneous products. Here, we address these issues by developing an antibody-based rolling circle amplification (RCA) strategy using a fusion of phi X174 gene A* protein and Z(mab2s) (A*-Zmab). The phi X174 gene A* protein is an enzyme that can covalently link with DNA, while the Z(mab2s) protein moiety can bind to specific species of antibodies. The DNA in an A*-Zmab conjugate was attached to the A* protein at a site chosen to not interfere with protein function, as determined by enzyme-linked immunosorbent assay (ELISA) and gel mobility shift analysis. The novel A*-Zmab-DNA conjugate retained its binding capabilities to a specific class of murine immunoglobulin gamma 1 (IgG1) but not to rabbit IgG. This indicates the generality of the A*-Zmab-based immuno-RCA assay that can be used in-sandwich ELISA format. Moreover, the enzymatic covalent method dramatically increased the yields of A*-Zmab-DNA conjugates up to 80% after a 15 min reaction. Finally, sensitive detection of human interferon-gamma (IFN-gamma) was achieved by immuno-RCA using our fusion protein in sandwich ELISA format. This new approach of the use of site-specific enzymatic DNA conjugation to proteins should be applicable to fabrication of novel immunoassays for biosensing.

  • 3085.
    Akter, Nasrin
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Energy and Climate Studies, ECS.
    Energy Need Assessment and Preferential Choice Survey o fMatipukur Village in Bangladesh: Energy Need Assessment and Preferential Choice Survey of Rural People in Bangladesh2015Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    The aim of this study is to perform a baseline energy survey to understand the existing energy demand and usepattern and to verify the feasibility of a small scale poly-generation project supported by renewable sources ofenergy (biogas based) in a rural area of Bangladesh. A poly-generation solution shall provide multiple outputservices of clean gas, electricity and arsenic free water supply. The project requires using animal dung oragricultural waste to produce biogas and electric energy. The study has analyzed the demand of domesticenergy and water of the village named ‘Matipukur’ in the Jessore district in Bangladesh. The study alsoconsidered available biomass feedstock and energy potential surrounding the village area, as well as the socioeconomicstatus of villagers. The case study included a door to door survey to collect relevant information.Three different economic groups in terms of income scale were investigated throughout the study to obtainbetter insight of the energy-water access situation, requirements and related problems in the village.

    Almost 98% household of this village relies on biomass for energy due to limited access of modern fuel. Thevillage has various biomass potential in the form of animal dung, fuel wood and agricultural waste which can beused for cooking or serve as the basis for other energy carriers. Kerosene is used for lighting. Among thedifferent fuels, dung meets about 44% of the total demand. The contribution of other fuels for domestic use is24% firewood, 22% agricultural waste, and 2% kerosene. The analytical observation found that the annualaverage energy demand of the village is 8.45 GJ per capita. The share of average demand for cooking and3lighting energy is 8.24 GJ per person/year and 0.21 GJ per person/year respectively. The energy consumptionvaries within different income groups. This study has examined the income per capita, family size, education,agricultural land holding per capita, priorities of their annual expenditure etc. which have direct influence onthe fuel consumption pattern of the household. It could be observed that expenses on energy changes as theincome level increases. About awareness of biogas opportunities and willingness to provide feedstock for apoly generation project, the majority of households answered positively, that is, indicating that they wouldcontribute. Educated respondents showed more positive attitude. Regarding changing of traditional cooking,about 95% of the respondents want to change to a more efficient and reliable cooking system to avoid healthand environmental problems associated with indoor biomass cooking.

    The study has revealed that only cow dung is not enough to produce clean energy according to demand so theco-digestion method is considered to producing biogas from various energy potentials (animal manure &agricultural waste). The poly-generation system could work with the scenario providing electricity and watersupply for the entire household and cooking gas is limited only for 2/3rd household. Rest 1/3rd household fromlow income group then could be supplied with improve cook stove to meet their daily cooking demand and tominimize indoor pollutions. It is observed that, majority of household has expressed their willingness toprovide raw materials for poly-generation plant though they are using.

  • 3086.
    Aktug, Irem
    KTH, School of Computer Science and Communication (CSC), Theoretical Computer Science, TCS.
    Algorithmic Verification Techniques for Mobile Code2008Doctoral thesis, monograph (Other scientific)
    Abstract [en]

    Modern computing platforms strive to support mobile code without putting system security at stake. These platforms can be viewed as open systems, as the mobile code adds new components to the running system. Establishing that such platforms function correctly can  be divided into two steps. First, it is shown that the system functions correctly regardless of the mobile components that join it, provided that they satisfy certain assumptions. These assumptions can, for instance, restrict the behavior of the component to ensure that the security policy of the platform is not violated. Second, the mobile component is checked to satisfy its assumptions, before it is allowed to join the system. This thesis presents algorithmic verification techniques to support this methodology. In the first two parts, we present techniques for the verification of open systems relative to the given component assumptions. In the third part, a technique for the  quick certification of mobile code is presented for the case where a particular type of program rewriting is used as a means of enforcing the component assumptions.In the first part of this study, we present a framework for the verification of open systems based on explicit state space representation. We propose Extended Modal Transition Systems (EMTS) as a suitable structure for representing the state space of open systems when assumptions on components are written in the modal μ-calculus. EMTSs are based on the Modal Transition Systems (MTS) of Larsen and provide a formalism for graphical specification and facilitate a thorough understanding of the system by visualization. In interactive verification, this state space representation enables proof reuse and aids the user guiding the verification process. We present a construction of state space representations from process algebraic open system descriptions based on a maximal model construction for the modal μ-calculus. The construction is sound and complete for systems with a single unknown component and sound for those without dynamic process reation. We also suggest a tableau-based proof system for establishing temporal properties of open systems represented as EMTS. The proof system is sound in general and complete for prime formulae.The problem of open system correctness  also arises in compositional verification, where the problem of showing a global property of a system is reduced to showing local properties of components. In the second part, we extend an existing  compositional verification framework for Java bytecode programs. The framework employs control flow graphs with procedures to model component implementations and open systems for the purpose of checking control-flow properties. We generalize these models to capture exceptional and multi-threaded behavior. The resulting control flow graphs are specifically tailored to support the compositional verification principle; however, they are sufficiently intuitive and standard to be useful on their own. We describe how the models can be extracted from program code and give preliminary experimental results for our implementation of the extraction of control flow graphs with exceptions. We also discuss further tool support and practical applications of the method.In the third part of the thesis, we develop a technique for the certification of safe mobile code, by adapting the proof-carrying code scheme of Necula to the case of security policies expressed as security automata. In particular, we describe how proofs of policy compliance can  be automatically generated for  programs that include a monitor for the desired policy. A monitor is an entity that observes the execution of a program and terminates the program if a violation to the property is about to occur. One way to implement such a monitor is by rewriting the program to make it self-monitoring. Given a property, we characterize self-monitoring of Java bytecode programs for this property by an annotation scheme with annotations in the style of Floyd-Hoare logics. The annotations generated by this scheme can be extended in a straightforward way to form a correctness proof in the sense of axiomatic semantics of programs. The proof generated in this manner essentially establishes that the program satisfies the property because it contains a monitor for it. The annotations that comprise the proofs are simple and efficiently checkable, thus facilitate certification of mobile code on devices with restricted computing power such as mobile phones.

  • 3087.
    Aktug, Irem
    KTH, School of Computer Science and Communication (CSC), Numerical Analysis and Computer Science, NADA.
    State space representation for verification of open systems2006Licentiate thesis, monograph (Other scientific)
    Abstract [en]

    When designing an open system, there might be no implementation available for cer- tain components at verification time. For such systems, verification has to be based on assumptions on the underspecified components. In this thesis, we present a framework for the verification of open systems through explicit state space representation.

    We propose Extended Modal Transition Systems (EMTS) as a suitable structure for representing the state space of open systems when assumptions on components are writ- ten in the modal μ-calculus. EMTSs are based on the Modal Transition Systems (MTS) of Larsen. This representation supports state space exploration based verification tech- niques, and provides an alternative formalism for graphical specification. In interactive verification, it enables proof reuse and facilitates visualization for the user guiding the verification process.

    We present a two-phase construction from process algebraic open system descriptions to such state space representations. The first phase deals with component assumptions, and is essentially a maximal model construction for the modal μ-calculus that makes use of a powerset construction for the fixed point cases. In the second phase, the models obtained are combined according to the structure of the open system to form the complete state space. The construction is sound and complete for systems with a single unknown component and sound for those without dynamic process creation. We suggest a tableau-based proof system for establishing open system properties of the state space representation. The proof system is sound and it is complete for modal μ-calculus formulae with only prime subformulae.

    A complete framework based on the state space representation is offered for the auto- matic verification of open systems. The process begins with specifying the open system by a process algebraic term with assumptions. Then, the state space representation is ex- tracted from this description using the construction described above. Finally, open system properties can be checked on this representation using the proof system.

  • 3088.
    Aktug, Irem
    et al.
    KTH, School of Computer Science and Communication (CSC), Theoretical Computer Science, TCS. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Dam, Mads
    KTH, School of Computer Science and Communication (CSC), Theoretical Computer Science, TCS. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Gurov, Dilian
    KTH, School of Computer Science and Communication (CSC), Theoretical Computer Science, TCS.
    Provably Correct Runtime Monitoring2009In: Journal of Logic and Algebraic Programming, ISSN 1567-8326, E-ISSN 1873-5940, Vol. 78, no 5, p. 304-339Article in journal (Refereed)
    Abstract [en]

    Runtime monitoring is an established technique to enforce a wide range of program safety and security properties. We present a formalization of monitoring and monitor inlining, for the Java Virtual Machine. Monitors are security automata given in a special-purpose monitor specification language, ConSpec. The automata operate on finite or infinite strings of calls to a fixed API, allowing local dependencies on parameter values and heap content. We use a two-level class file annotation scheme to characterize two key properties: (i) that the program is correct with respect to the monitor as a constraint on allowed program behavior, and (ii) that the program has a copy of the given monitor embedded into it. As the main application of these results we sketch a simple inlining algorithm and show how the two-level annotations can be completed to produce a fully annotated program which is valid in the standard sense of Floyd/Hoare logic. This establishes the mediation property that inlined programs are guaranteed to adhere to the intended policy. Furthermore, validity can be checked efficiently using a weakest precondition based annotation checker, thus preparing the ground for on-device checking of policy adherence in a proof-carrying code setting.

  • 3089.
    Aktug, Irem
    et al.
    KTH, School of Computer Science and Communication (CSC), Theoretical Computer Science, TCS.
    Dam, Mads
    KTH, School of Computer Science and Communication (CSC), Theoretical Computer Science, TCS.
    Gurov, Dilian
    KTH, School of Computer Science and Communication (CSC), Theoretical Computer Science, TCS.
    Provably correct runtime monitoring (extended abstract)2008In: Fm 2008: Formal Methods, Proceedings / [ed] Cuellar, J; Maibaum, T; Sere, K, 2008, Vol. 5014, p. 262-277Conference paper (Refereed)
    Abstract [en]

    Runtime monitoring is an established technique for enforcing a wide range of program safety and security properties. We present a formalization of monitoring and monitor inlining, for the Java Virtual Machine. Monitors are security automata given in a special-purpose monitor specification language, ConSpec. The automata operate on finite or infinite strings of calls to a fixed API, allowing local dependencies on parameter values and heap content. We use a two-level class file annotation scheme to characterize two key properties: (i) that the program is correct with respect to the monitor as a constraint on allowed program behavior, and (ii) that the program has an instance of the given monitor embedded into it, which yields state changes at prescribed points according to the monitor's transition function. As our main application of these results we describe a concrete inliner, and use the annotation scheme to characterize its correctness. For this inliner, correctness of the level II annotations can be decided efficiently by a weakest precondition annotation checker, thus allowing on-device checking of inlining correctness in a proof-carrying code setting.

  • 3090.
    Aktug, Irem
    et al.
    KTH, School of Computer Science and Communication (CSC), Numerical Analysis and Computer Science, NADA.
    Gurov, Dilian
    KTH, School of Computer Science and Communication (CSC), Numerical Analysis and Computer Science, NADA.
    State Space Representation for Verification of Open Systems2006In: Algebraic Methodology And Software Technology, Proceedings / [ed] Johnson, M; Vene, V, Berlin: Springer , 2006, p. 5-20Conference paper (Refereed)
    Abstract [en]

    When designing an open system, there might be no implementation available for certain components at verification time. For such systems, verification has to be based on assumptions on the underspecified components. When component assumptions are expressed in Hennessy-Milner logic (HML), the state space of open systems can be naturally represented with modal transition systems (NITS), a graphical specification language equiexpressive with HML. Having an explicit state space representation supports state space exploration based verification techniques, Besides, it enables proof reuse and facilitates visualization for the user guiding the verification process. in interactive verification. As an intuitive representation of system behavior, it aids debugging when proof generation fails in automatic verification.

    However, HML is not expressive enough to capture temporal assumptions. For this purpose, we extend MTSs to represent the state space of open systems where component assumptions are specified in modal mu-calculus. We present a two-phase construction from process algebraic open system descriptions to such state space representations. The first phase deals with component assumptions, and is essentially a maximal model construction for the modal p-calculus. In the second phase, the models obtained are combined according to the structure of the open system to form the complete state space. The construction is sound and complete for systems with a single unknown component and sound for those-without dynamic process creation. For establishing open system properties based on the representation, we present a proof system which is sound and complete for prime formulae.

  • 3091.
    Aktug, Irem
    et al.
    KTH, School of Computer Science and Communication (CSC), Theoretical Computer Science, TCS.
    Gurov, Dilian
    KTH, School of Computer Science and Communication (CSC), Theoretical Computer Science, TCS.
    Towards State Space Exploration Based Verification of Open Systems2005In: 4th International Workshop on Automated Verification of Infinite-State Systems (AVIS’05), April 2005, Edinburgh, Scotland, 2005Conference paper (Refereed)
  • 3092.
    Aktug, Irem
    et al.
    KTH, School of Computer Science and Communication (CSC), Theoretical Computer Science, TCS.
    Naliuka, Katsiaryna
    University of Trento, Italy.
    ConSpec: A Formal Language for Policy Specification2008In: Science of Computer Programming, ISSN 0167-6423, E-ISSN 1872-7964, Vol. 74, no 1-2, p. 2-12Article in journal (Refereed)
    Abstract [en]

    The paper presents ConSpec, an automata-based policy specification language. The language trades off clean semantics to language expressiveness: a formal semantics for the language is provided as security automata. ConSpec specifications can be used at different stages of the application lifecycle, rendering possible the formalization of various Policy enforcement techniques.

  • 3093.
    Aktug, Irem
    et al.
    KTH, School of Computer Science and Communication (CSC), Theoretical Computer Science, TCS.
    Naliuka, Katsiaryna
    University of Trento, Italy.
    ConSpec: A Formal Language for Policy Speci-fication2007In: Proceedings of The First International Workshop on Run Time Enforcement for Mobile and Distributed Systems (REM’07), 2007, p. 45-58Conference paper (Other academic)
    Abstract [en]

    The paper presents ConSpec, an automata based policy specification language. The language trades off clean semantics to language expressiveness; a formal semantics for the language is provided as security automata. ConSpec specifications can be used at different stages of the application lifecycle, rendering possible the formalization of various policy enforcement techniques.

  • 3094. Akulich, A
    et al.
    Isacsson, Ulf
    KTH, School of Architecture and the Built Environment (ABE), Transport Science, Highway and Railway Engineering.
    Laboratorieundersökningar av åldringsegenskaper hos dränerande asfaltbetong1992Report (Other academic)
  • 3095. Akyol, Z
    et al.
    Arbaugh, B
    Cleveland-Innes, Marta
    KTH, School of Computer Science and Communication (CSC), Media Technology and Graphic Arts, Media (closed 20111231).
    Garrison, R
    Ice, P
    Richardson, J
    Swan, K
    A response to the review of the community of inquiry framework2009In: Journal of distance education = Revue de l'enseignement à distance, ISSN 1916-6818, Vol. 23, no 2, p. 123-136Article in journal (Refereed)
    Abstract [en]

    The Community of Inquiry (CoI) framework has become a prominent model of teaching and learning in online and blended learning environments. Considerable research has been conducted which employs the framework with promising results, resulting in wide use to inform the practice of online and blended teaching and learning. For the CoI model to continue to grow and evolve, constructive critiques and debates are extremely beneficial, in so much as they identify potential problems and weaknesses in the model or its application, as well as provide direction for further research. In this context, the CoI framework was recently reviewed and critiqued by Rourke and Kanuka in their JDE article entitled “Learning in Communities of Inquiry: A Review of the Literature.” This paper is a response to this article and focuses on two main issues. The first issue is the focus of the review and critique on learning outcomes. The second issue concerns the representation, comprehensiveness, and methodology of the review.

  • 3096.
    Akçay, H.
    et al.
    Linköping University.
    Hjalmarsson, Håkan
    Linköping University.
    The Least-Squares Identification of FIR Systems Subject to Worst-Case Noise1994In: 10th IFAC Symposium on System Identification, 1994, Vol. 2, p. 85-90Conference paper (Refereed)
  • 3097.
    Akçay, H.
    et al.
    Linköping University.
    Hjalmarsson, Håkan
    Linköping University.
    The least-squares identification of FIR systems subject to worst-case noise1994In: System & Control Letters, Vol. 23, p. 329-338Article in journal (Refereed)
  • 3098.
    Akçay, H.
    et al.
    Linköping University.
    Hjalmarsson, Håkan
    Linköping University.
    Ljung, Lennart
    Department of Electrical Engineering, Linköping University.
    On the choice of norms i system identification1994In: 10th IFAC Symposium on System Identification, 1994, Vol. 2, p. 103-108Conference paper (Refereed)
  • 3099.
    Al Abassi, Mohammed
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Urban and Regional Studies.
    The Possibilities for Establishing Agricultural Vocational Training Institutions in Kisumu2012Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
  • 3100.
    Al Ahad, Muhammed Abdullah
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Centres, Centre for High Performance Computing, PDC.
    Simmendinger, Christian
    T Syst Solut Res GmbH, D-70563 Stuttgart, Germany..
    Iakymchuk, Roman
    KTH, School of Electrical Engineering and Computer Science (EECS), Centres, Centre for High Performance Computing, PDC.
    Laure, Erwin
    KTH, School of Electrical Engineering and Computer Science (EECS), Centres, Centre for High Performance Computing, PDC.
    Markidis, Stefano
    KTH, School of Electrical Engineering and Computer Science (EECS), Centres, Centre for High Performance Computing, PDC.
    Efficient Algorithms for Collective Operations with Notified Communication in Shared Windows2018In: PROCEEDINGS OF PAW-ATM18: 2018 IEEE/ACM PARALLEL APPLICATIONS WORKSHOP, ALTERNATIVES TO MPI (PAW-ATM), IEEE , 2018, p. 1-10Conference paper (Refereed)
    Abstract [en]

    Collective operations are commonly used in various parts of scientific applications. Especially in strong scaling scenarios collective operations can negatively impact the overall applications performance: while the load per rank here decreases with increasing core counts, time spent in e.g. barrier operations will increase logarithmically with the core count. In this article, we develop novel algorithmic solutions for collective operations such as Allreduce and Allgather(V)-by leveraging notified communication in shared windows. To this end, we have developed an extension of GASPI which enables all ranks participating in a shared window to observe the entire notified communication targeted at the window. By exploring benefits of this extension, we deliver high performing implementations of Allreduce and Allgather(V) on Intel and Cray clusters. These implementations clearly achieve 2x-4x performance improvements compared to the best performing MPI implementations for various data distributions.

59606162636465 3051 - 3100 of 122069
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf