Graphene, the two-dimensional lattice of sp2-hybridized carbon atoms, has a great potential for future electronics, in particular for opto-electronic devices. The carrier relaxation dynamics, which is of key importance for such applications, is in the main focus of this thesis. Besides a short introduction into the most prominent material properties of graphene and the experimental techniques, this thesis is divided into three main parts. The investigation of the carrier relaxation dynamics in the absence of a magnetic field is presented in Chapter 3. In the first experiment, the anisotropy of the carrier excitation and relaxation in momentum space was investigated by pump-probe measurements in the near-infrared range. While this anisotropy was not considered in all previous experiments, our measurements with a temporal resolution of less than 50 fs revealed the polarization dependence of the carrier excitation and the subsequent relaxation. About 150 fs after the electrons are excited, the carrier distribution in momentum space gets isotropic, caused by electron-phonon scattering. In a second set of two-color pump-probe experiments, the temperature of the hot carrier distribution, which was obtained within the duration of the pump pulse (about 200 fs), could be estimated. Furthermore, a change in sign of the pump-probe signal can be used as an indicator for the Fermi energy of different graphene layers. Pump-probe experiments in the far-infrared range in reflection and transmission geometry were performed at high pump power. A strong saturation of the pump-induced transmission was found in previous experiments, which was attributed to the pump-induced change in absorption. Our investigation shows the strong influence of pump-induced reflection at long wavelengths, as well as a lot smaller influence of the saturation of the pump-induced change in absorption. At a high pump power, the increase of the reflection exceeds the change in absorption strongly, which leads to negative pump-probe signals in transmission geometry. In Chapter 4, investigations of the carrier dynamics of graphene in magnetic fields of up to 7T are presented. Even though the optical properties of Landau-quantized graphene are very interesting, the carrier dynamics were nearly unexplored. A low photon energy of 14meV allows the investigation of the intraband Landau-level (LL) transitions. These experiments revealed two main findings: Firstly, the Landau quantization strongly suppresses the carrier relaxation via optical-phonon scattering, resulting in an increased relaxation time. Secondly, a change in sign of the pump-probe signal can be observed when the magnetic field is varied. This change in sign indicates a hot carrier distribution shortly after the pump pulse, which means that carrier-carrier scattering remains very strong in magnetic fields. In a second set of pump-probe measurements, carried out at a photon energy of 75meV, the relaxation dynamics of interband LL transitions was investigated. In particular, experiments on the two energetically degenerate LL transitions LL(−1)->LL(0) and LL(0)->LL(1) showed the influence of extremely strong Auger processes. An ultrafast and extremely broadband terahertz detector, based on a graphene flake, is presented in the last chapter of this thesis. To couple the radiation efficiently to the small flake, the inner part of a logarithmic periodic antenna is connected to it. With a rise time of about 50 ps in a wavelength range of 9 μm to 500 μm, this detector is very interesting to obtain the temporal overlap in two-color pump-probe experiments with the free-electron laser FELBE. Furthermore, the importance of the substrate material, in particular for the high-speed performance, is discussed.

The radio spectrum is becoming increasingly congested everyday with emerging technologies and with the increasing number of wireless devices. Considering the limited bandwidth availability, accommodating the demand for higher capacity and data rates is a challenging task, requiring innovative technologies that can offer new ways of exploiting the available radio spectrum. Cognitive radio arises to be a tempting solution to the spectral crowding problem by introducing the notion of opportunistic spectrum usage. Because of its attractive features, orthogonal frequency division multiplexing (OFDM) has been successfully used in numerous wireless standards and technologies. We believe that OFDM will play an important role in realizing the cognitive radio concept as well by providing a proven, scalable, and adaptive technology for air interface. The goal of this dissertation is to identify and address some of the challenges that arise from the introduction of cognitive radio. Specifically, we propose methods for obtaining awareness about channel, spectrum, and waveform in OFDM-based cognitive radio systems in this dissertation. Parameter estimation for enabling adaptation, spectrum sensing, and OFDM system identification are the three main topics discussed. OFDM technique is investigated as a candidate for cognitive radio systems. Cognitive radio features and requirements are discussed in detail, and OFDM's ability to satisfy these requirements is explained. In addition, we identify the challenges that arise from employing OFDM technology in cognitive radio. Algorithms for estimating various channel related parameters are presented. These parameters are vital for enabling adaptive system design, which is a key requirement for cognitive radio. We develop methods for estimating root-mean-square (RMS) delay spread, Doppler spread, and noise variance. The spectrum opportunity and spectrum sensing concepts are re-evaluated by considering different dimensions of the spectrum which is known as multi-dimensional spectrum space. Spectrum sensing problem in a multi-dimensional space is addressed by developing a new sensing algorithm termed as partial match filtering (PMF). Cognitive radios are expected to recognize different wireless networks and have capability of communicating with them. Algorithms for identification of multi-carrier transmissions are developed. Within the same work, methods for blindly detecting transmission parameters of an OFDM based system are developed. Blind detection is also very helpful in reducing system signaling overhead in the case of adaptive transmission where transmission parameters are changed depending on the environmental characteristics or spectrum availability.

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第19007号 / 工博第4049号 / 新制||工||1623 / 31958 / 京都大学大学院工学研究科高分子化学専攻 / (主査)教授 伊藤 紳三郎, 教授 赤木 和夫, 教授 辻井 敬亘 / 学位規則第4条第1項該当

published_or_final_version / abstract / toc / Medicine / Master / Master of Philosophy

E-handel är en revolutionerande industri som växer ständigt och kommer med all sannolikhet att fortsätta växa i en accelererande fart. Syftet med denna studie är att få en överblick över vilka faktorer som påverkar svenska konsumenters köpbeteende i samband med e-handel. Samt vilka faktorer som spelar en anspelande roll vid val av leveransaktör för e-handelsföretagen. Studien visar hur valet av leveransaktör kan påverka både konsument och företag och vilka krav som ställs i samband med en leverans.  Insamlingen av teori och data genomfördes med hjälp av en litteratur studie, enkätundersökningar och intervjuer. De viktigaste faktorerna som påverkar konsumenternas köpbeteende vid hemsidans utformning visade sig vara websidans pålitlighet (säkerhet) och websidans användbarhet följt av interaktivitet Konsumenterna ansåg att en marknads mix och en estetisk tilltalande hemsida inte påverkar deras köp avsevärt.  Vid val av den logistiska aspekten så som frakt, ses en leverans som ankommer i tid, och en fri frakt vara av störst betydelse bland deltagarna. De angivna faktorerna visade sig även påverka återkommande köp från samma hemsida i framtiden. E-handelsföretagen ser inte leveransalternativet som ett konkurrensmedel bland sina konkurrenter då alla de ledande distributörerna erbjuder samma tjänster så som spårbar frakt, korta leveranstider och enkelhet. Vid val av leveransaktör har samtliga företag rangordnat leverans tid, möjligspårning och snabb respons tillsammans med pris och godsförlust/godshantering som dem två viktigaste faktorerna.

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Very little is known about the relationship between locus of control (LOC) orientation and mild or temporary hearing losses associated with chronic otitis media. Furthermore, it seems this relationship may never have been studied in the unique cultural context of Northern Canadian Native Indian societies. The present study investigated the relationship between LOC orientation and hearing status category among Carrier-Sekani children from Northern British Columbia. The relationship between LOC orientation, chronologic age, and academic achievement was also explored. Demographic data collected for a larger study, provided an opportunity to conduct some post hoc analyses on LOC orientation, place in the family, number of parents in the home and family income. Ninety Carrier-Sekani students from grades four to twelve, received a modified Nowicki-Strickland Locus of Control Scale for Children. Students were divided into two broad categories, normally hearing and those having a history of a hearing loss. The latter category was further divided into students with a pure tone loss, students with a history of chronic otitis media and those with observed otitis media at the time of testing. Students could be members of more than one sub-group. Correlation coefficients and Analyses of Variance were computed to explore the relationship between LOC orientation and the independent variables. No significant relationship was discovered between LOC orientation and category of hearing loss. An internal LOC orientation was positively associated with chronologic age, medium family income, two parents in the home and partially associated with academic achievement. This study indicates that for Carrier-Sekani students, a mild or temporary hearing loss is not significantly associated with an external LOC orientation. It seems that school related variables and demographic variables commonly associated with LOC orientation in the samples described in the literature are also present in the sample studied in this project.

This thesis reports results from the investigation of optically-induced carrier dynamics in graphite and graphitic carbon nanostructures. In this first set of experiments, the dynamics of photo-excited carriers in exfoliated graphene and thin graphitic films are studied by optical pump-probe spectroscopy. Samples ranging in thickness from 1 to 260 carbon layers are deposited onto an oxidized silicon substrate. Time-resolved reflectivity and transmissivity are measured at 1300 nm, following excitation by 150 fs, 800 nm pump pulses at room temperature. Two time scales are identified over which the extracted transient dielectric function returns to its quiescent value. A fast decay time of ~200 fs in graphene is associated with hot phonon emission and increases to ~300 fs for thicknesses greater than only a few carbon layers. The slow decay time, associated with hot phonon interaction and/or carrier recombination, increases more gradually, from ~2.5 to 5 ps over ~30 layers. A simple model suggests the thickness dependence of the slow decay time is likely a result of thermal diffusion into the substrate. In the second set of experiments, coherently-controlled two-colour injection photocurrents are generated via quantum interference of single- and two-photon absorption in bulk graphite and a variety of single-walled carbon nanotube samples, such as a CVD-grown aligned forest of nanotubes (tube diameter dt = 2.5 ± 1.5 nm), and both arc discharge (dt = 1.44 ± 0.15 nm) and HiPco (dt = 0.96 ± 0.14 nm) nanotube films separated by electronic type (metallic vs. semiconducting). At pump wavelengths of 1500 and 750 nm, the emitted terahertz radiation is used to estimate a peak current density of ~12 kA/cm² in graphite and a peak current of ~8 nA per nanotube. From the dependence of the injected current on pump polarization, the relative values of the current injection tensor elements are measured, and information is gained on the alignment and birefringence of the nanotube samples. The dependence of the injected current on pump wavelength implies that the currents are likely based on band-band electronic transitions and not on excitonic effects, which govern most linear optical processes.

This thesis reports results from the investigation of optically-induced carrier dynamics in graphite and graphitic carbon nanostructures. In this first set of experiments, the dynamics of photo-excited carriers in exfoliated graphene and thin graphitic films are studied by optical pump-probe spectroscopy. Samples ranging in thickness from 1 to 260 carbon layers are deposited onto an oxidized silicon substrate. Time-resolved reflectivity and transmissivity are measured at 1300 nm, following excitation by 150 fs, 800 nm pump pulses at room temperature. Two time scales are identified over which the extracted transient dielectric function returns to its quiescent value. A fast decay time of ~200 fs in graphene is associated with hot phonon emission and increases to ~300 fs for thicknesses greater than only a few carbon layers. The slow decay time, associated with hot phonon interaction and/or carrier recombination, increases more gradually, from ~2.5 to 5 ps over ~30 layers. A simple model suggests the thickness dependence of the slow decay time is likely a result of thermal diffusion into the substrate. In the second set of experiments, coherently-controlled two-colour injection photocurrents are generated via quantum interference of single- and two-photon absorption in bulk graphite and a variety of single-walled carbon nanotube samples, such as a CVD-grown aligned forest of nanotubes (tube diameter dt = 2.5 ± 1.5 nm), and both arc discharge (dt = 1.44 ± 0.15 nm) and HiPco (dt = 0.96 ± 0.14 nm) nanotube films separated by electronic type (metallic vs. semiconducting). At pump wavelengths of 1500 and 750 nm, the emitted terahertz radiation is used to estimate a peak current density of ~12 kA/cm² in graphite and a peak current of ~8 nA per nanotube. From the dependence of the injected current on pump polarization, the relative values of the current injection tensor elements are measured, and information is gained on the alignment and birefringence of the nanotube samples. The dependence of the injected current on pump wavelength implies that the currents are likely based on band-band electronic transitions and not on excitonic effects, which govern most linear optical processes.

Aukšto lygio žmogiškas kapitalas gamina didelės pridėtinės vertės turtą – pasaulyje paklausias prekes. Visuomenės gerovę pirmiausia lemia investicijos į žmones ir jų žinias. Žmonių įgujamu sugebėjimai – jų išsimokslinimas, patirtis, įgūdžiai, sveikata – yra svarbiausi siekiant ekonominės pažangos. Bet kurios visuomenės pagrindinis turtas yra žmonės, o ekonominio-socialinio progreso galutinis kriterijus yra žmogus ir jo poreikių platesnis patenkinimas. Žmogiškasis kapitalas yra vertingiausias šiuolaikinės visuomenės resursas, svarbesnis už gamtos turtus ar žmonių sukauptą materialinį turtą. Šiuolaikinėje visuomenėje, padidėjus žinių vaidmeniui, automatiškai sutelkiamas asmenų ir visos šalies konkurencingumas tam, kad būtų stiprinami svarbūs žmogiškojo kapitalo raiškos komponentai, konkrečiai, aukštasis išsimokslinimas. Darbo tikslas: įvertinti aukštojo išsilavinimo, kaip investavimo į žmones reikšmę bei nagrinėti kaip žmogiškojo kapitalo raiška sąveikauja su karjera ir jos realizavimu Lietuvoje. Empirinio tyrimo tikslas: įvertinti studijuojančių žmonių nuostatas į aukštąjį mokslą kaip investiciją į žmones, bei nustatyti tų nuostatų pokyčius profesinės karjeros planavimo aspektu. Tyrimo dalykas: respondentų nuomonės ir nuostatos žmogiškojo kapitalo raiškai bei profesiniam realizavimui įvertinimas. Tyrimo objektas: Studijuojančio jaunimo žmogiškojo kapitalo profesinio realizavimo problemos ir panaudojimo galimybės Lietuvoje. ... [toliau žr. visą tekstą] / The main property of any society is human beings while the final criterion of economical social progress is an individual and wider satisfaction of his needs. Human capital is the most valuable recourse of modern society, even more important than natural resources or material capital pilled up by human beings. In modern society when the role of knowledge is increased, the competition of people and all country is concentrated in order to strengthen the components of human capital expression, especially higher education. The aim of the thesis: to evaluate the importance of high education as the investment in human capital and to research how this aspect influence the carrer and its realisation in Lithuania The aim of the empirical research: to evaluate the point of view of studying persons towards high education as investment in people and how this investment could influence their professional carrer in Lithuania. The subject of the research: the evaluations of the opinions of interviewed students. The object of the research: the problems of professional realisation and employment possibilities of studying persons in Lithuania. The main tasks of the thesis: 1. to ground the meaning of human capital. 2. to ground theoreticaly the importance of higher education in the formation and expression of human capital. 3. according the analysis of statistic data to evaluate the expression of social aspects in human capital, specificaly the role of high education in... [to full text]