Master Degree / Yüksek Lisans Tezleri
Permanent URI for this collectionhttps://hdl.handle.net/11147/3008
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Master Thesis Emission Characteristics of Two and Three Level Systems(Izmir Institute of Technology, 2022) Yılmaz, Teyfik; Çakır, Özgür; Çakır, ÖzgürIn this thesis, we mainly focus on the two subjects. Firstly, we investigate the spontaneous emission from a V-type three-level atom. We mainly study the influence of quantum interference between the decay processes from the two upper levels to a lower level to which the upper levels are coupled by the same vacuum modes. The effects of quantum interference on the spontaneous emission spectrum are studied. These effects are shown to induce spectral narrowing and a dark line in the spectrum. The influence of the interference on the upper level populations is also examined. It is seen that the upper level populations are not simple exponential decays. In the second part of this study, the fluorescence spectrum of a driven two-level atom is evaluated. Both the resonance and the off-resonance cases, and the weak and the strong coupling regimes are investigated.Master Thesis Electron Optics in Graphene(01. Izmir Institute of Technology, 2022) Coşgel, Gürcan; Çakır, ÖzgürNegative refraction, also known as Veselago lensing, was first predicted by Victor Veselago in 1968 (Veselago (1968)). Its unique effect has a great potential for both scientific and technological applications such as superlenses. Unlike the conventional positive refractive index, focusing effect can be observed by negative refraction. In this thesis, the focusing effect was investigated theoretically through on n-p junction in graphene. The opposite chirality of electrons and holes enable the negative refraction where electrons( holes) have their momentum parallel(anti-parallel) to the group velocity. The case when potential barrier is directed perpendicular to KK direction, where K and K are the Dirac points were considered. The Green’s functions were calculated analytically and derived the susceptibility using the Green’s functions for various positions of the sources and the receiver at various Fermi energies. The spatial Green’s functions were calculated analytically and derived the static susceptibility (response function).Master Thesis Interactions Between Metal Surfaces and Sulfur-Containing Amino Acids(01. Izmir Institute of Technology, 2022) Çevlikli, Mustafa; Ataman, EvrenWith Covid-19 pandemic, the scientific studies over viruses gained a big acceleration. Some of these studies show that SARS-CoV-2 can infect through direct ways from a patient. Besides that, surfaces that are contaminated from adsorption of the virus can indirectly infect a person. Because of this, the studies on the interaction between viruses and environment is an important field on virus studies. Spike proteins which project out from protective structure called lipid bilayer and they are outmost elements of SARS-CoV-2 viruses. Spike proteins play a vital role on functions like viral entry to the host cell and attachment to the surfaces. Purpose of this study is to contribute to the existing knowledge about surface interaction of these proteins which have an important role on virus-surface interactions. Because of the complicated physical and chemical form of proteins, in this study, the interaction of amino acids, which are building block of proteins, with metal surfaces are investigated. 0.01M, 0.02M and 0.05M L-cysteine and L-methionine aqueous solution was dropped to the metals frequently used in daily life, chrome (Cr) and iron (Fe) polycrystalline substrates and left to dry out at room temperature and under atmospheric conditions. Since the water solubility of L-cystine is low, only a saturated solution of Lcystine was prepared and same process was applied. After, dried out samples were analyzed with x-ray photoelectron spectroscopy (XPS). Functional sulfur groups on L-cysteine/Cr, L-methionine/Cr and L-Methionine/Fe system stayed intact. On the other hand, sulfur atoms on L-Cysteine/Fe system oxidized and formed --SO x species. While all other systems behaved oppositely, the amount of protonated functional amino species ( ) on L-methionine/Fe system decreased relative to functional amino group () with increase in coverage. Due to surface amino acid interaction of L-methionine in different substrates, binding energy of sulfur on iron was measured 1 eV lower than on chromium. In L-cystine/Cr system, while disulfide bonds stayed intact, and functional groups were observed on the surface together.Master Thesis B92 Based Quantum Key Distribution With Faint Pulsed Laser(01. Izmir Institute of Technology, 2021) Mutlu, Görkem; Ateş, Serkan; Çakır, ÖzgürIn quantum key distribution (QKD), photons are used to share the key between the transmitter and receiver, and in principle, single photon sources should be used to create a secure communication channel. Nowadays, attenuated laser sources are used in many studies. While it is practical to use attenuated laser pulses for QKD system, it poses many safety issues due to the possibility of multiple photons in the laser pulses. In addition, the key rate is waived to increase the level of security. However, the use of single photon sources is not as easy and practical as using attenuated laser sources. Today, studies of single photon sources to be used for QKD continue. In order for these single photon sources to be used actively, a photon source that operates at room temperature, operates in a wide band-gap range for different areas of use (underwater, optical fiber-based and free space) and can be excited at high speed is required. Since hBN defect centers are a material that can produce single photons at room temperature and have a wide band gap, it seems very ideal for these studies. In this thesis, studies have been carried out on the realization of the protocol, which is a part of QKD, with solid-state materials that produce single photons. In the studies, a key was produced with a faint pulsed laser. Also, data is encrypted using the key of the transmitter. Then the data is successfully decrypted with the key measured by the receiver.Master Thesis Effect of Random Structural Variations on the Optical Properties of Honeycomb Photonic Crystals(01. Izmir Institute of Technology, 2021) Tunçtürk, Yiğit; Sözüer, Hüseyin SamiPeriodic dielectric structures called photonic crystals(PhCs) are being used in various sensors and devices. Since PhCs are designed to operate within certain frequency ranges, accuracy in structure becomes important. During the manufacturing process, random errors in geometry can be encountered. Two types of errors come to the forefront, surface roughness and positional randomness. Once the periodic structure becomes imperfect due to random errors, calculations for the desired frequency ranges must be performed using supercells. In this thesis, effect of surface roughness and positional ran- domness on photonic density of states are investigated for both TM and TE modes in two-dimensional honeycomb photonic crystals.Master Thesis Electronic Properties of Artificial Graphene Nanostructure(01. Izmir Institute of Technology, 2021) Okcu, Emre; Güçlü, Alev DevrimArtificial graphene is an artificial honeycomb structure which mimics the interesting properties of graphene. Such as Dirac cone in energy dispersion, zero band gap etc. Wide range of production type makes artificial graphene valuable material. It can be engineered by lasers, molecules and semiconductors. Semiconductor based artificial graphene can be produced by dot lattice with honeycomb patterned attractive potential or by antidot lattice with triangular patterned repulsive potential. In the following calculations, semiconductor (GaAs) based artificial graphene was used to compute electronic properties. Like in graphene, artificial graphene has Dirac cones in energy dispersion. However, graphene has 1.42 angstrom carbon to carbon atom distance. This distance can not be changed but artificial graphene offers us tunability. Different parameters yield tons of band structure. It offers not only Dirac cone but also gaped bands in energy dispersion. This graphene-like feature and tunability make artificial graphene an important and researchable subject. Besides, we added another tunable parameter stiffness to control the shape of potential. Stiffness became another important parameter in our calculations. We observed that stiffness dramatically changes the band structure of the material. As a first step, artificial graphene band structures are calculated from the single-electron approximation. Some parameters are compared with other works and the same results are found. Dirac cones are achieved in band structures. Hopping and Hubbard U values are computed. Those parameters are essential for computing finite structures. Mean-field Hubbard can be solved, and wave functions can be used as input for input required methods such as quantum Monte Carlo. As a second step, we used the density functional theory method to investigate electron-electron interactions. Local density approximation was chosen to solve the Kohn-Sham equation. Hopping parameters obtained from DFT are much realistic than the single-electron approximation. Stiffness plays a big role in DFT energy dispersioMaster Thesis Coulomb Impurities in Graphene Quantum Dots in a Magnetic Field(01. Izmir Institute of Technology, 2021) Eren, İsmail; Güçlü, Alev DevrimIn this thesis, we investigate the atomic collapse of Graphene Quantum Dots (GQDs) in a magnetic field with the tight-binding (TB) model and mean-field Hubbard (MFH) approximation. We placed a charged impurity at the center of GQDs, and we systematically investigated the atomic collapse effect in the magnetic field by adjusting the charge of the impurity, size of the quantum dots, and magnitude of the magnetic field. It is shown that the electronic state with the lowest energy of Graphene resembled the same effect of the lowest bound state (TLBS) of atomic collapse. We confirmed the earlier findings, and we showed that the required critical charges of TLBS of the GQDs to collapse below the Fermi level are almost equal. Additionally, we investigate the formation of resonance states of GQDs, and among these resonance states, we study the evolution of the first-formed resonance state (R1). Applying a perpendicular magnetic field to GQD, decreased the critical charge of each structure, and we found that the decrease is dependent on the dot size. Moreover, we also found that TLBS of GQDs of varying sizes are crossed each other at a particular impurity charge and energy. We used the relation between the magnetic field and magnetic length (lB), and we compared B with the radius of the GQD (RGQD) in varying sizes. We found that TLBS of a GQD still converges to a particular crossing point (in terms of impurity charge and energy) as in no magnetic field when lB > RGQD. However, TLBS of a GQD diverges from the crossing point when lB < RGQD. It is studied that the continuum form of the R1 state became a chain of separated Landau levels in a magnetic field. Here we show that Landau level formation is more noticeable, and the inter-level separation of the Landau levels becomes more prominent when the lB < RGQD. Lastly, we investigated the atomic collapse of the Hofstadter's butterflies in GQDs. We found that increasing the impurity charge collapsed the energy levels. Also, increasing the impurity charge decreased (increased) the local density of states of the impurity center at the top (bottom) part of the spectrum of the Hofstadter's butterflies.Master Thesis Optical Spectroscopy of Single Defects in Hexagonal Boron Nitride(01. Izmir Institute of Technology, 2021) Birinci, Ayşenur; Ateş, SerkanSingle photon sources are main component for several applications in quantum information technologies. Hexagonal boron nitride (hBN) is a suitable material to create heterostructures with two dimensional materials. It is a popular two dimensional material and single photon source due to having stable and bright emission in visible range. In this thesis, optical properties of single defects in bulk hBN were investigated. Defects have been selected using the micro-Photoluminescence setup, and it was observed that the defects have high degree of polarization and show typical optical saturation behavior. Time-resolved photoluminescence measurements were done by time-correlated single photon counting. Single photon nature of generated light from individual defects were demonstrated using Hanbury-Brown and Twiss interferometer.Master Thesis The Impact of Adsorbates on the Optoelectronic Properties of Graphene/Silicon Based Schottky Barrier Photodiodes(01. Izmir Institute of Technology, 2020) Şahan, Nusret; Çelebi, CemThe aim of this study is to investigate the effect of atmospheric adsorbates on the electronic and optoelectronic properties of graphene/n-type Silicon (Gr/n-Si) based Schottky barrier photodiodes. Wavelength resolved photocurrent spectroscopy and transient photocurrent spectroscopy measurements conducted under high-vacuum conditions revealed that the adsorbates cause hole doping in graphene and hence increase the zero-bias Schottky barrier height of the Gr/n-Si heterojunction from 0.71 to 0.78 eV. Adsorbate induced increment in the barrier height promotes the separation of photo-excited charge carriers at the depletion region of the heterojunction and leads to an improvement in the maximum spectral response (e.g., from 0.39 to 0.46 A W^-1) and response speed of the Gr/n-Si photodiode in the near-infrared region. The experimentally obtained results are expected to give an insight into the adsorbate induced variations in the rectification and photo-response characters of the heterojunctions of graphene and other 2D materials with different semiconductors.Master Thesis Travel Time in Quantum Theory and Ionization Times of Noble Gases(01. Izmir Institute of Technology, 2020) Paçal, Serkan; Güçlü, Alev DevrimTime in Quantum mechanics, stands as an unresolved problem from the first time the theory was established to the present day. The present thesis consists of three main studies, in the first part, some time formulas that have been proposed in the past are included. In the second part, time is formulated by using David Bohm's "the guiding equation". In the third part, time formula has been applied to atomic potentials (He, Ar and Kr noble gases) for which time measurements done. It has been shown that the ionization time of the noble gases we have calculated gives results very compatible with the experiments.