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Ultra-düşük gürültülü fiber lazer sistemleri ve uygulamaları
Fiber laser systems are intensely studied for and already utilized in a wide range of scientific, biomedical and industrial applications. Scientifically, fiber lasers are widely used for spectroscopy, laser-matter interactions, nonlinear and quantum optics experiments, among others. The industrial applications range from the well-established, such as laser-material processing, laser marking, and various forms of optical sensing to niche or upcoming applications such as high- speed circuit testing, inspection of packaged foods, additive manufacturing. In all applications outside the research laboratory, long-term stability of the lasers operation is of paramount importance. Fiber lasers are clearly advantageous in this respect, as the optical fibers provide isolated paths for light propagation, minimizing the impact of environmental effects, and generally render the laser system nearly or completely free from mechanical misalignment. In addition to long-term stability of the laser operation, short-term (typically less than 1 second) stability, or fluctuations of the laser output is of crucial importance as in many situations, it effectively determines the signal-to-noise ratio, sets the resolution or otherwise limits the quality of the measurement. Fluctuations or noise impact both the intensity and phase of the laser output. As part of this thesis, first, the intensity noise of mode-locked fiber lasers is characterized systematically for the major mode-locking regimes over a wide range of parameters. It is found that equally low-noise performance can be obtained in all regimes. Losses in the cavity influence noise strongly without a clear trace in the pulse characteristics. Noise level is found to be virtually independent of pulse energy below a threshold for the onset of nonlinearly induced instabilities. Instabilities that occur at high pulse energies are characterized. It is found that continuous-wave peak formation and multiple pulsing influence noise performance moderately. However, at high pulse energies, an abrupt increase of the intensity noise is encountered, corresponding to up to 2 orders of magnitude increase in noise. These results effectively constitute guidelines for minimization of the laser noise in mode-locked fiber lasers. For the high-power laser systems that utilize external amplification in fiber amplifiers, the added noise due to amplification is usually predominantly determined by the pump source, assuming that the amplifier design is correctly made and amplified spontaneous emission (ASE) is minimized. Many high-power amplifiers utilized multi-mode pump diodes, which have much higher noise levels. A high-power fiber laser system where the amplifiers are seeded by low intensity noise pulses is analyzed in detail. When operating at its maximum power level (10 W), the amplified output exhibits an integrated (from 3 Hz to 250 kHz) intensity noise of 0.2%, whereas the seed signals intensity noise is less than 0.03%. The origins of the added noise is analyzed systematically using modulation transfer functions to ascertain contributions of the pump source. The transfer of the noise in the seed signal is also analyzed, as well as contributions of ASE, which can be significant. Prediction of intensity noise by modulation transfer functions supplies a lower limit for the intensity noise of fiber lasers and amplifiers. The second part of the thesis applies the know-how on low-noise fiber lasers that was developed in the first part to a scientific problem. As part of a col- laboration with researchers from Ruhr-University at Bochum, Germany, we have developed a custom, low-noise laser system for spectroscopy of micro-plasma dis- charges. Absorption spectroscopy is a commonly used technique to determine the presence of a particular substance or to quantify the amount of substance present in the plasma discharge. However, the absorbance is usually small, at the level of one part in a thousand or less. Therefore, low-noise laser signals are required to detect such low differences. We developed a low-noise fiber laser system for the absorption spectroscopy studies of reactive species in a micro-plasma discharge. The laser setup also produces high-energy picosecond pulses, which are powerful enough to trigger the plasma ignition and transition into other transient states of plasma. Since both pulses are generated from the same mode-locked oscilla- tor, they have excellent mutual synchronization. We demonstrate the possibility for pump-probe experiments by initiating breakdown on a picosecond time scale (pump) with a high-power beam and measuring the broadband absorption with the simultaneously provided supercontinuum (probe). The third part of this thesis the laser-noise know-how to address a technolog- ical problem, namely the development custom, low-noise fiber lasers for LADAR applications. Two different fiber laser systems are constructed as transmitter sources of direct detection and coherent detection LADAR systems and tested for realistic scenarios. Both LADAR systems succeeded to detect 1 cm-diameter wire from a distance of 1 km in a measurement time shorter than 100 s, which is comparable to the best performing commercial LADAR systems.
Artırılmış optik etkinlik için fotokatalitik nanokompozitler
To combat environmental pollution, photocatalytic decomposition provides degradation of organic and inorganic contaminants near the surface of the photocatalyst nanoparticles by converting optical energy of the absorbed light into chemical energy for the redox reactions. However, photocatalytic activities of such semiconductor metal-oxide nanoparticles are limited with their bandgap energy that allows for optical absorption typically in the ultraviolet spectral range. Yet another limitation is that the photocatalytic activity of these semiconductor nanoparticles is substantially reduced when they are immobilized in solid thin films, resulting from their effectively decreased active surface area. But such immobilized nanoparticles are much more desired in industrial applications, e.g., for mass environmental decontamination and outdoors/indoors self-cleaning on large surfaces. To address these issues, in this thesis, we investigated and demonstrated the spectral behavior and time evolution of optical activity curves of immobilized TiO2 and ZnO nanoparticles. We studied the nanoparticle size effect for the optical activity and demonstrated significant increase in the resulting photocatalysis with decreasing the size of such immobilized nanoparticles for the first time. We obtained optimal excitation conditions for TiO2 and ZnO nanocomposite films separately. We achieved maximum optical recovery levels of 93% for TiO2 nanoparticles and 55% for ZnO nanoparticles at the excitation wavelengths of 310 nm and 290 nm, respectively, after optical irradiation with an excitation density of 7.3 J/cm2, where we observed no optical recovery for their respective negative control groups (with no nanoparticles). In these comparative spectral studies, we showed strong correlation between the differential optical recovery and the photocatalytic activity. For further substantial enhancement in the near ultraviolet and visible spectral ranges, we also proposed and demonstrated the use of a unique combination of TiO2-ZnO nanoparticles integrated together into the same resin. In this novel approach, we observed higher levels of photocatalytic activity under optical irradiation at and above 380 nm compared to the cases of only TiO2 or only ZnO nanocomposite films with the same total metal-oxide nanoparticle density. At 400 nm in the visible, we accomplished an optical recovery level of ~30% with the combination of TiO2-ZnO nanoparticles together while this level was only ~14% for the TiO2 nanoparticles alone and ~3% for the ZnO nanoparticles alone under identical conditions. Even at 440 nm, we obtained ~20% optical recovery using the TiO2-ZnO photocatalytic synergy, despite the optical activity of the single type of nanoparticles alone close to the zero base-line of their control group. These proof-of-concept experimental demonstrations indicate that such TiO2-ZnO combined nanocomposite films hold great promise for efficient environmental decontamination in daylight. Keywords: Nanoparticles, nanocomposite films, TiO2, ZnO; optical recovery, ultraviolet, visible; photocatalytic synergy; photocatalysis, environmental decontamination.
Silisyum ve germanyum nanoörgülerde tasıyıcı dinamiği
This is a computational work on the Si and Ge nanocrystals (NCs) embedded in wide band gap host matrices. As the initial task, xtensive ab initio work on the structural and electronic properties of various NC host matrices, namely, SiO2, GeO2, Si3N4, and Al2O3 are preformed. The structural parameters, elastic constants, static and optical dielectric constants are obtained in close agreement with the available results. Furthermore, recently reported high density cubic phase of SiO2 together with GeO2 and SnO2 are studied and their stable highdielectric constant alloys are identified.Based on the ab initio study of host matrices two related high field phenomena vital especially for the electroluminescence in Si and Ge NCs, are examined. These are the hot carrier transport through the SiO2 matrix and the subsequent quantum-confined impact ionization (QCII) process which is responsible for the creation of electron-hole pairs within the NCs. First, the utility and the validity of the ab initio density of states results are demonstrated by studying the high field carrier transport in bulk SiO2 up to fields of 12 MV/cm using the ensemble Monte Carlo technique. Next, a theoretical modeling of the impact ionization of NCs due to hot carriers of the bulk SiO2 matrix is undertaken. An original expression governing the QCII probability as a function of the energy of the hot carriers is derived.Next, using an atomistic pseudopotential approach the electronic structures for embedded Si and Ge NCs in wide band-gap matrices containing several thousand atoms are employed. Effective band-gap values as a function of NC diameter reproduce very well the available experimental and theoretical data. To further check the validity of the electronic structure on radiative processes, direct photonemission rates are computed. The results for Si and Ge NCs as a function of diameter are in excellent agreement with the available ab initio calculations for small NCs.In the final part, non-radiative channels, the Auger recombination (AR) and carrier multiplication (CM) in Si and Ge NCs are investigated again based on the atomistic pseudopotential Hamiltonian. The excited electron and excited hole type AR and CM and biexciton type AR lifetimes are calculated for different sized and shaped NCs embedded in SiO2 and Al2O3. Asphericity is also observed to increase the AR and CM rates. An almost monotonous size-scaling and satisfactory agreement with experiment for AR lifetime is obtained considering a realistic interface region between the NC core and the host matrix. It is further shown that the size-scaling of AR can simply be described by slightly decreasing the established bulk Auger constant for Si to 1.0×10?30cm6s?1. The same value for germanium is extracted as 1.5×10?30cm6s?1 which is very close to the established bulk value. It is further shown that both Si and Ge NCs are ideal for photovoltaic efficiency improvement via CM due to the fact that under an optical excitation exceeding twice the band gap energy, the electrons gain lion?s share from the total excess energy and can cause a CM. Finally, the electron-initiated CM is predicted to be enhanced by couple orders of magnitude with a 1 eV of excess energy beyond the CM threshold leading to subpicosecond CM lifetimes.Bu çalışma, Si ve Ge nanoörgülerin (Nö?lerin) sayısal hesaplamaları hakkındadır. Başlangıç olarak SiO2, GeO2, Si3N4, Ge3N4 ve Al3O3 gibi Nö matrislerinin elektronik ve yapısal özellikleri temel prensipler yöntemiyle incelenmiştir. Bunun sonucunda, yapısal özellikler, elastik sabitler, statik ve optik dielektrik sabitler için mevcut çalışmalar ile oldukça uyumlu değerler elde edilmiştir. Ayrıca, SiO2?nun henüz yayımlanmış yüksek yoğunluklu kübik fazı GeO2 ve SnO2 da ele alınarak calışılmış ve bu malzemelerin yüksek dielektrik sabitli durağan alaşım formları elde edilmiştir.
Grafen tabanlı yapılarda elektronik-manyetik özellikler ve fononik enerji yitimi
With the synthesis of a single atomic plane of graphite, namely graphene honeycomb structure, active research has been focused on the massless Dirac fermion behavior and related artifacts of the electronic bands crossing the linearly at the Fermi level. This thesis presents a theoretical study on the electronic and magnetic properties of graphene based structures, and phononic energy dissipation. First, functionalization of these structures by 3d-transition metal (TM) atoms is investigated. The binding energies, electronic and magnetic properties have been investigated for the cases where TM-atoms adsorbed to a single side and double sides of graphene. It is found that 3d-TM atoms can be adsorbed on graphene with binding energies ranging between 0.10 to 1.95eV depending on their species and coverage density. Upon TM-atom adsorption graphene becomes a magnetic metal. TM-atoms can also be adsorbed to graphene nanoribbons with armchair edge shapes (AGNRs). Binding of TM-atoms to the edge hexagons of AGNR yield the minimum energy state for all TM-atom species examined in this work and in all ribbon widths under consideration. Depending on the ribbon width and adsorbed TM-atom species, AGNR, a non-magnetic semiconductor, can either be a metal or a semiconductor with ferromagnetic or anti-ferromagnetic spin alignment. Interestingly, Fe or Ti adsorption makes certain AGNRs half-metallic with a 100% spin polarization at the Fermi level. These results indicate that the properties of graphene and graphene nanoribbons can be strongly modified through the adsorption of 3d TM atoms. Second, repeated heterostructures of zigzag graphene nanoribbons of different widths are shown to form multiple quantum well structures. Edge states of specific spin directions can be confined in these wells. The electronic and magnetic state of the ribbon can be modulated in real space. In specific geometries, the absence of reflection symmetry causes the magnetic ground state of whole heterostructure to change from antiferromagnetic to ferrimagnetic. These quantum structures of different geometries provide novel features for spintronic applications. Third, as apossible device application, a resonant tunnelling double barrier structure formed from a finite segment of armchair graphene nanoribbon with varying widths has been proposed based on first-principles transport calculations. Highest occupied and lowest unoccupied states are confined in the wider region, whereas the narrow regions act as tunnelling barriers. These confined states are identified through the energy level diagram and isosurface charge density plots which give rise to sharp peaks originating from resonant tunnelling effect. Finally, we studied dynamics of dissipation of local vibrations to the surrounding substrate. A model system consisting of an excited nano-particle which is weakly coupled with a substrate is considered. Using three different methods, the dynamics of energy dissipation for different types of coupling between the nano-particle and the substrate is studied, where different types of dimensionality and phonon densities of states were also considered for the substrate. Results of this theoretical analysis are verified by a realistic study. To this end the phonon modes and interaction parameters involved in the energy dissipation from an excited benzene molecule to the graphene are calculated performing first-principles calculations.
İki sıcaklıklı Ising modelin kesin bir sınır değerindeki çözümü
We analyze the order-disorder transition for a two dimensional Ising model.We consider a ferromagnetic exchange interaction between the nearest neighbor Ising spins.The spin exchanges are introduced in two different temperatures, at infinite and finite temperatures.The model is first proposed by Praestgaard, Schmittmann, and Zia (Eur. Phys.J. B 18, 675 (2000)).In this thesis, we look at a limit of the system where the spin exchange at infinitetemperature proceeds at a very fast rate in one of the lattice direction (the "y-direction").In the other direction (the "x-direction"), the spin exchange at a finite temperature is drivenby one of several possible exchange dynamics such as Metropolis, Glauber, and exponential rates.We investigate an exact nonequilibrium stationary state solution of the model far from equilibrium.We apply basic stochastic formalisms such as the Master equation and the Fokker-Planck equation.Our main interest is to analyze the possibility of various types of phase transitions.Using the magnetization as a phase order parameter, we observe two kinds ofphase transitions: transverse segregation and longitudinal segregation with respect tothe direction x. We find analytically the transition temperature and the nonequilibriumstationary state for small magnetizations at an exact limit. We show that depending on thetype of microscopic interaction (such as Metropolis, Glauber, exponential spin exchange rates)the transition temperature and the phase boundary vary. For some exchange rates, we observe no transverse segregation.
Çok frekanslı akıgeçiş manyetik kuvvet mikroskopisi
In the recent years, progress in atomic force microscopy (AFM) led to the multifrequencyimaging paradigm in which the cantilever-tip ensemble is simultaneouslyexcited by several driving forces of different frequencies. By using multifrequencyexcitation, various interaction forces of different physical origin suchas electronic interactions or chemical interactions can be simultaneously mappedalong with topography. However, a multifrequency magnetic imaging techniquehas not been demonstrated yet. The difficulty in imaging magnetic forces usinga multifrequency technique partly arises from difficulties in modulation of themagnetic tip-sample interaction. In the traditional unmodulated scheme, measurementof magnetic forces and elimination of coupling with other forces is obtainedin a double pass measurement technique where topography and magneticinteractions are rapidly measured in successive scans with different tip-sampleseparations. This measurement scheme may suffer from thermal drifts or topographicalartifacts. In this work, we consider a multifrequency magnetic imagingmethod which uses first resonant flexural mode for topography signal acquisitionand second resonant flexural mode for measuring the magnetic interactionsimultaneously. As in a fluxgate magnetometer, modulation of magnetic momentof nickel particles attached on the apex of AFM tip can be used to modulatethe magnetic forces which are dependent on external DC fields through the nonlinearmagnetic response of the nickel particles. Coupling strength can be variedby changing coil current or setpoint parameters of Magnetic Force Microscopy(MFM) system. Special MFM tips were fabricated by using Focused Ion Beam(FIB) and magnetically characterized for the purpose of multifrequency imaging.In this work, the use of such a nano-flux-gate system for simultaneous topographicand magnetic imaging is experimentally demonstrated. The excitation and detectionscheme can be also used for high sensitivity cantilever magnetometry.
Grafin nano-şeritlerin elektronik yapısı
Graphite is a known material to human kind for centuries as the lead of a pencil. Graphene as a two dimensional material, is the single layer of graphite. Many theoretical works have been done about it so far, however, it newer took attention as it takes nowadays. In 2004, Novoselov et al. was able to produce graphene in 2D. Now that, making experiments on graphene is possible scientists have to renew their theoretical knowledge about systems in two dimension because graphene, due to its electronic structure, is able to prove the ideas in quantum relativistic phenomena. Indeed, recent theoretical studies were able to show that, electrons and holes behave as if they are massless fermions moving at a speed about 1 million m/s (c/300, c being speed of light) due to the linear electronic band dispersion near K points in the brillouin zone which was observed experimentally as well.Having zero band gap, graphene cannot be used directly in applications as a semiconductor. Graphene Nano-Ribbons (GNRs) are finite sized graphenes. They can have band gaps differing from graphene, so they are one of the new candidates for band gap engineering applications such as field effect transistors. This work presents theoretical calculation of the band structures of Graphene Nano-Ribbons in both one (infinite in one dimension) and zero dimensions (finite in both dimensions) with the help of tight binding method. The calculations were made for Zigzag, Armchair and Chiral Graphene Nano-Ribbons (ZGNR,AGNR,CGNR) in both 1D and 0D. Graphene nano-ribbons with zero band gap (ZGNR and AGNR) are observed in the calculations as well as the ribbons with finite band gaps (AGNR and CGNR) which increase with the decrease in the size of the ribbon making them much more suitable and strong candidate to replace silicon as a semiconductor.
Optoelektronik aygıtlar için germanyum bileşikleri
Silicon has been the backbone of the mainstream electronics of the last fifty years. It is however, used in conjunction with other matierals, mainly with its oxides and nitrides. Germanium, on the other hand, is also a group IV element and has been used in the early stages of transistor and detector development. In addition to Si/Ge heterojunctions, bandgap engineering through SiGe alloys has also been used in photodetectors. Recent progress in light emitting devices utilizing Si nanocrystals suggest the use of Ge1-xNx layers as barriers due to its suitable band offsets [1]. Experiments have shown that Ge1-xNx is also a promising material for applications in photodiodes, amplifiers, optic fibers, protective coatings, etc [2]. Both Si and Ge are, however indirect bandgap semiconductors, lacking efficient light emission. On the other hand, strong light emission observed in Si nanocrystals has made the study of semiconductor nanocrystals an expanding field of interest due to potential applications in novel optoelectronic devices [1]. These nanocrystals exhibit strong luminescence and nonlinear optical properties that usually do not ppear in the bulk materials [3-4]. SiGe nanocrystals attract attention due to the possibility of a tunable band gap with composition.In this study, formation of Ge1-xNx thin films and SiGe nanocrystals by plasma enhanced chemical vapor deposition (PECVD) reactor has been studied. We present the growth conditions and experimental characterization of the resulting thin films and nanocrystals. We used ellipsometry, Raman Spectrometry, Fourier Infrared Spectrometry (FTIR) and X-ray photoelectron Spectroscopy (XPS). For SiGe nanocrystals, 4 peaks in the Raman Spectra were observed around 295 cm-1, 400 cm-1, 485 cm-1 and 521 cm-1. These peaks are assigned to the Ge-Ge, Si-Ge, local Si-Si and crystalline Si-Si vibrational modes, respectively [5]. For the Ge1-xNx thin films FTIR spectrum showed the existence of the Ge-N bonds and its band offsets determined by XPS confirm its suitability for optoelectronic devices.
Silisyum nano örgülerde gerilme
In this Thesis we present our achievements towards an understanding of atomistic strain mechanisms and interface chemistry in siliconnanocrystals.The structural control of silicon nanocrystals embedded in amorphous oxide is currently an importanttechnological problem. First, our initial attempt is described to simulate the structural behaviorof silicon nanocrystals embedded in amorphous oxide matrix based on simple valence force fieldsas described by Keating-type potentials. Next, the interface chemistry of silicon nanocrystals (NCs) embeddedin amorphous oxide matrix is studied through molecular dynamics simulations with the chemical environment being governed by the reactive force field model. Our results indicate that the Si NC-oxide interface is more involved than the previously proposed schemes which were based on solely simple bridge or double bonds. We identify different types of three-coordinated oxygen complexes, previously not noted. The abundance and the charge distribution of each oxygen complex is determined as a function of the NC size as well as the transitions among them.Strain has a crucial effect on the optical and electronic properties of nanostructures. We calculatethe atomistic strain distribution in silicon NCs up to a diameter of 3.2 nm embedded inan amorphous silicon dioxide matrix. A seemingly conflicting picture arises when the strain fieldis expressed in terms of bond lengths versus volumetric strain. The strain profile in either caseshows uniform behavior in the core, however it becomes nonuniform within 2-3 \AA~distance tothe NC surface: tensile for bond lengths whereas compressive for volumetric strain. Wereconcile their coexistence by an atomistic strain analysis.Vibrational density of states (VDOS) affects the optical properties of Si-NCs. VDOS obtained by calculating velocity autocorrelation function (VACF) using velocities of the atoms is extracted from the molecular dynamics simulations. The information on bonding topology enables classification of atoms in the system with respect to their neighbor atoms. With help of this information we separate contributions of different type of atoms to the VDOS. Calculating VACF of different type of atoms such as surface atoms and core atoms of nanocrystal, to the system facilitates understanding of the effects of strain fields and interface chemistry to the VDOS.
Kırınım ağı tabanlı plazmonik kovuklar
Surface plasmon polaritons are dipole carrying electromagnetic excitations occur-ing at metal-dielectric interfaces. Metallic periodic structures exhibit modi¯edtransmission and re°ection spectra owing to the interaction of propagating SPPswith the periodicity. These periodic surfaces are used to demonstrate localiza-tion of propagating SPPs. Thin metallic ¯lms surrounded by Bragg re°ectors,selective loading of biharmonic metallic surfaces and Moire patterns are used todemonstrate plasmonic cavity formation. The quality factor, Q, a characteristicvalue that indicates rate of energy loss relative to the stored energy in the cavityis a crucial parameter for classifying these cavities. It was proposed that theQ factor should strongly depend on the surface geometry. However, there wasnot a sytematic study on the Q factor of these cavity structures. In this work,we report on a comparative study of grating based plasmonic band gap cavities.Numerically, we calculate the quality factors of the cavities based on three typesof grating surfaces; uniform, biharmonic and Moirµe surfaces. Experimentally,we demonstrate the existence of plasmonic cavities based on uniform gratings.E®ective index perturbation and cavity geometries are obtained by additionaldielectric loading. Furthermore, we fabricate 2D plasmonic structures, observeplasmonic band gaps in the symetry axis and propose cavity geometries for thisstructure.