Ph.D. Theses

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Modeling of drug delivery by modifying tumor microenvironment
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2023., 2023) Yılmaz, Defne.; Ünlü, Mehmet Burçin.
The tumor microenvironment is a critical aspect in the formation and progression of tumors. Its complex nature is commonly distinguished by abnormal vascular structure and tumor-induced states, including hypoxia, elevated interstitial pressure, and acidity. The tumor microenvironment has a significant impact on the effectiveness of anticancer treatments, so it is crucial to adopt strategies in cancer research that target tumor microenvironment features. In this regard, to study the therapeutic strategies targeting the tumor microenvironment, a mathematical modeling approach is utilized here to incorporate vascular tumor growth and response to treatments. The spatiotemporal model captures the tumor cell growth, vasculature, and oxygenation, along with drug treatments. The effects of drugs targeting hypoxia, an important feature of the tumor microenvironment, are investigated when given in combination with chemotherapy drugs. Different scheduling and sequencing are applied, and the tumor response to these various cases is examined. In addition, to examine the response of the tumor to chemotherapy treatment, a deep learning approach is employed, and a response prediction model is presented. The model includes tumor growth and drug response behaviors. The goal of the presented model is to represent the tumor response behavior according to the ongoing treatment schedule. The model predicts future tumor microenvironment maps using drug scalars and tumor microenvironment channels as inputs, which might aid in determining the optimum schedule and dosages of combination therapy for patients.
On BPS dyons in N = 2 supersymmetric theories
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2023., 2023) Şanlı, Canberk.; Bleeken, Dieter Van den.
N = 4 superconformal mechanics of the (3, 4, 1) multiplet describes the scaling limit of the effective Coulomb quiver mechanics. There is an equivalent hyperK¨ahler with torsion (HKT) gauged sigma model (4, 4, 0) reformulation which becomes equivalent to the original (3, 4, 1) formulation (only) on the quotient spaceM/G whereMis the HKT target space of the gauged model and G is the group of (gauged) isometries of M. This opens a genuinely new class of superconformal mechanical models not explored before. We systematically study N = 0, 1, 2 supersymmetric analogs as well. These gauged sigma models are only scale invariant before gauging, and become (super) conformally invariant after gauging. We obtain these results by explicitly checking the superconformal invariance of the Lagrangian which gives the conformal geometry constraints and by checking the superconformal algebra satisfied by the Noether charges which provide additional constraints. For the N = 2, 4 gauged sigma models, we obtain a differential geometric description for the prequantum Hilbert space, and discuss the applications of our work to the pure AdS2/CFT1 duality. There is a well-known self-dual formulation of point dyons in classical electromagnetism. Starting from Dirac’s original theory of monopoles, we obtain a particularly useful self-dual reformulation, and employ it to describe the coupling of BPS point dyons to N = 2, d = 4 low energy effective actions which in particular recovers Denef’s core-halo description for BPS bound states but also allows for certain generalizations when the bound state radius becomes small as one approaches to the Higgs phase. We explain how these two descriptions for N = 2 BPS (bound) states, as well as various other faces of them, obtain a unified view in the context of quiver (quantum) mechanics. NOTE Keywords : Lie symmetries, Conformal symmetry, Black holes, Quantum field theory, Effective field theories, Conformal invariant field theories, Supersymmetry, Effective string theory, Quiver theory, Superconformal algebras.
Design and simulation of a UHF band radio frequency quadrupole (RFQ) proton accelerator
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2023., 2023) Çelebi, Emre.; Özcan, Veysi Erkcan.; Ünel, N. Gökhan.
This thesis introduces DemirciPro, a computer program created to design and simulate low-energy beamlines, along with the UHF band radio frequency quadrupole (RFQ) designed as part of the Proton Testbeam At Kandilli (PTAK) project. The RFQ has undergone trial production and measurements, and this thesis documents its results. DemirciPro is a computer program that is developed within KahveLAB. DemirciPro strives to design low- energy beamlines with a single robust interface. It simulates all the critical elements of a low-current beamline except for the ion source. Moreover, DemirciPro has the potential to govern other design and simulation codes using a single interface, thereby enabling the simulation of the whole beamline using industrystandard programs. In this thesis, we provide a brief overview of the Proton Testbeam At Kandilli (PTAK) project, including the current test results of its components. We then delve into the details of RFQ, including its original RFQ vane-tip design and the novel design approach followed. We also present the findings of the completed trial production over the past years. Lastly, we compare the vane-tip measurements of test production with the design and discuss the effects of production errors on the simulation results.
Investigating magneic properties of dot
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2023., 2023) Öztürk, Zeynep Reyhan.; Kurnaz, M. Levent.; Yıldız, Fikret.
Magnetic insulators with perpendicular magnetic anisotropy enable low-power spin-orbit torque switching, high-frequency oscillation, and fast domain-wall motion [1]. Ferromagnetic insulators with PMA are key for studying skyrmions, magnetooptics, and memory/logic applications [2–4]. Yttrium iron garnet (YIG), with its perpendicular magnetization, revolutionizes current spintronics and magnonics. This work focuses on growing thin/ultrathin YIG films on Silicon substrates using Pulsed Laser Deposition (PLD) and explores the strain’s impact on magnetic anisotropy. We achieved perpendicular magnetic anisotropy without additional layers or doping materials. Additionally, we studied the magnetic properties of Permalloy (NiFe) thin films with dot/antidot arrays that have the ability to modify the magnetic behavior of magnetic thin films such as the magnetization reversal mechanism, magnetic domains, and control the magnetic anisotropy. The FMR technique determined the magnetization dynamics of the dot/antidot patterned NiFe thin films. The magnetization reversal of various shape, size, and thickness of dot/antidot array were investigated by a micromagnetic simulation, Mumax3 which solves the Landau-Lifshitz-Gilbert (LLG) equation. The remnant magnetization and coercive fields of the structures were extracted from the hysteresis loops. We discovered the excitation of spin-wave modes related to the shape effect from the FMR spectrum and found out different domain wall structures corresponding to different dot/antidot shapes from the simulations.
Thermodynamical properties of bosons and fermions for lattice motivated dispersion relationships
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2023., 2023) Güner, Metin Mehmet.; Akant, Levent.; Arık, Metin.
In this thesis, we have studied the coupled simple harmonic oscillator physics of one-dimensional strings, for acoustical phonons, and n-dimensional physics for the optical case (for arbitrary n). For the acoustical modes, we have calculated only the Bosonic Simple Harmonic Oscillator (BSHO), for the optical mode, we have included both BSHO and Fermionic Simple Harmonic Oscillator (FSHO). For the optical modes, for both BSHO’s and FSHO’s, we have found that the problem is reduced to the physics of a single oscillator with a partial vibration frequency. For the FSHO’s in the optical cases, we have encountered the presence of negative temperatures for all n. In both cases, we have discovered the presence of correct asymptotical behavior as T → 0 and T → ∞. In the acoustical case, we have used Mellin Transform and a summation formula due to Euler to calculate the partition function and the thermodynamical properties thereof. Again we have obtained the correct asymptotical behavior as T → 0 and T →∞. We note that whenever possible, we have included more than one method of calculation and obtained similar results.
The role of trigger waves in cancer angiogenesis
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Debir, Birses.; Ünlü, Mehmet Burçin.
This study includes mathematical modeling of biological observations and numer ical solutions of these models. Our study focused on different phenomenons, including; vessel formation, intracellular calcium ion concentration, traveling wave solutions, and cytosol elasticity. Understanding signaling in diseases is essential for a proper response. For this reason, understanding the secondary messenger signaling that mechanisms of ten prefer to use and their interaction with the mechanisms enables the system’s re sponse to be better understood. This study examined the interaction of angiogenesis, a mechanism that contributes to tumor growth, and cytosolic calcium ion, an intracel lular secondary messenger. Therefore, we simulated a mathematical model involving essential angiogenesis and calcium homeostasis elements using previously used models. In our simulations, we developed two and multiple cell scenarios and examined the results of our system in distributions of different angiogenic stimulus uptake. Since angiogenesis requires the cell to move in a specific direction, we simulated the cytosolic gelation- solution mechanism, inspired by another model used in the literature. We examined the condition under which the wave direction persists in the traveling wave configuration.
Analytic solutions of scalar field cosmology with minimal and nonminimal coupling and deformed discrete and finite quantum systems
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) İldeş, Medine.; Turgut, Osman Teoman.; Arık, Metin.
In this thesis first, we study analytic solutions of cosmology. We investigate the most general cosmological model with real scalar field which is minimally coupled to gravity and Brans- Dicke cosmology. Field equations consist of three differential equations. We switch independent variable from time to scale factor by change of variable ˙a/a = H(a). Thus a new set of differential equations are analytically solvable with known methods. a(t) can be explicitly found as long as methods of integration techniques are available. We investigate the dynamics of the universe at early times as well as at late times in light of these formulas. We find mathematical machinery which turns on and turns off early accelerated expansion. On the other hand late time accelerated expansion is explained by cosmic domain walls. φ 4 potential is studied in Brans-Dicke Cosmology. In this thesis we also study discrete and finite quantum systems. We define a deformed kinetic energy operator for a discrete position space with a finite number of points. The structure may be either periodic or nonperiodic with well-defined end points. It is shown that for the nonperiodic case the translation operator becomes nonunitary due to the end points. This uniquely defines an algebra which has the desired unique representation. Energy eigenvalues and energy wave functions for both cases are found. In addition, we uncover the mathematical structure of the Schwinger algebra and introduce almost unitary Schwinger operators which are derived by considering translation operators on a finite lattice.
Perturbative and non-perturbative physics from singularities
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022), 2022) Pazarbaşı, Cihan.; Bleeken, Dieter Van den.
A function that representing a physical quantity has singularities which con tain perturbative or non-perturbative information about the physical system under investigation. Moreover, the theory of resurgence tells us that these perturbative and non-perturbative parts are intimately connected and it is possible to use one of them to obtain the other one. In this thesis, we combine these two ideas with a focus on the functions formulated in integral representations. Specifically, first consider ing two different examples on the semi-classical expansion in quantum mechanics and the pair production problem in electromagnetic backgrounds, we will concentrate on the quantum action which we express in the Schwinger’s integral representation. We will show that the perturbative and non-perturbative information about the physical system is hidden in singularities of the propagator TrU(t). The way we obtain the non-perturbative one is very similar to the Borel method which is used to handle the divergent perturbation series. Contrary to the Borel method, by probing the singular ities of TrU(t) directly and using the iε prescription, we will be able to prevent the Borel ambiguity problem in the physical cases that it leads to the violation of the uni tarity. Later, we will turn our attention to the renormalon problem in non-relativistic quantum mechanics. After presenting the existence of the renormalon divergence in a scattering problem with a background potential consisting of 2D δ-potential perturbed with a tilted 1D δ-potential, we will argue that the Borel ambiguity in the summation of the divergent series can be prevented again by a careful application of the iε pre scription and the resulting non- perturbative contribution due to the renormalon obeys the causality condition.
Numerical modeling, design, and time-frequncy metrology applications of low-noise optical frequency combs
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Şenel, Çağrı.; Ünlü, Mehmet Burçin.
Optical frequency combs are very versatile tools, which revolutionized many ar eas, and enabled novel applications, especially in time-frequency metrology. In this thesis, I tried to convey a complete picture of the road that goes from conception through design to applications of optical frequency combs. To this end, implementation details of a free, publicly- available numerical pulse propagation simulation software, which can be used for, but not limited to, simulating and designing complete fem tosecond laser and amplifier systems, supercontinuum generation, and Raman lasers, is presented. An algorithmic design methodology, which was used for designing all of the femtosecond lasers and fiber amplifiers presented in this thesis, is introduced, which might pave the way to discovery of new laser pulse evolution regimes. Sev eral optical frequency combs were developed and utilized in time- frequency metrology applications. A fully-stabilized, robust, and transportable low-noise Yb-fiber optical frequency comb was developed and used for absolute frequency measurements of stabi lized lasers. High-harmonic mode-locking and repetition rate multiplication methods were used to generate ultra-low phase noise RF-MW signals with frequencies ranging from 78.125 MHz to 10 GHz by utilizing optical frequency combs. A femtosecond fiber laser that can be automatically mode-locked was also developed during these studies. Finally, an atomic frequency standard based on a passively mode-locked femtosecond laser was developed, for the first time to the best of my knowledge. Resulting 87Rb atomic frequency standard performed at a similar level to the commercial Rb atomic frequency standards, with much better phase noise at high frequency offsets.
Probing mechanical and chemical properties of biological materials by multiple modalities
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Pesen, Medine Tuna.; Ünlü, Mehmet Burçin.
Mechanical and chemical alterations are the signature of most diseases, such as endometriosis, sickle cell, malaria, and cancer. Detecting such alterations has vital importance for early diagnosis and efficient treatments. In the first project, we aimed to establish a non-invasive diagnostic technique for endometriosis disease by detect ing chemical changes in blood serum using Raman spectroscopy and machine learning algorithms. We measured 49 endometriosis patients’ and 45 healthy women’s blood sera for this project. A diagnostic tool was built-up by applying machine learning algorithms (kNN and SVM) to the Raman spectral data. The results of this study indicated diagnosis sensitivity and specificity values of 80.5% and 89.7%, respectively. In the following study, we investigated the effect of repetitive bi-axial stretch-relaxation cycles on the individual erythrocyte cells with optical tweezers to assess their mechan ical response. According to the findings, the cells became stiffer with each stretch and became completely undeformable after several stretch-relaxation cycles. Also, we showed that stiffness changes with each stretching cycle as a sign of weak power law. Therefore, we showed that cell rheology is scale-free at a single and whole-cell level for the first time. In the last project, we studied the camel erythrocytes compared to the human erythrocytes in their mechanochemical features since camel erythrocytes have the most distinctive cell features among the vertebrates. Using optical tweezers, we quantified that camel erythrocytes are almost ten times more rigid than human ery throcytes. The chemical analysis by the Raman spectroscopy revealed the difference in the lipid:protein ratio of these two cell types.
A ramond-nevu schwarz string one end fixed
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2005., 2005.) Arapoğlu, A. Savaş.; Turgut, Osman Teoman.; Saçlıoğlu, Cihan,
We study an open string with one end free and the other fixed on a D0-brane as a qualitative guide to the spectrum of hadrons containing one very heavy quark. We first consider the bosonic degrees of freedom, then introduce the fermionic degrees offreedom through the world sheet supersymmetry. The mixed boundary conditions break half of the world sheet supersymmetry and allow only odd-moded a and even-moded d oscillators in the Ramond sector, while the Neveu-Schwarz oscillators b's become odd-integer moded. Boson-fermion masses can still be matched if space-time is 9 dimensional; thus SO(8) triality still plays a role in the spectrum, although full space-time supersymmetry does not survive. We quantize the system in a temporal-like gauge where X^0 ~ t. Although the gauge choice eliminates negative-norm states at the outset, there are still even-moded Virasoro and even(odd) moded super-Virasoro constraints to be imposed in the NS(R) sectors. The Casimir energy is now positive in both sectors; there are no tachyons. States for a' M^2 b 13/4 are explicitly constructed and found to be organized into SO(8) irreps by (super)constraints, which include a novel ``" operator in the NS and I in the R-sectors. GSO projections are not allowed. The pre-constraint states above the ground state have matching multiplicities, indicating spacetime supersymmetry is broken by the (super)constraints. A distinctive physical feature of the system is a slope twice that of the open RNS string. When both ends are fixed, all leading and subleading trajectories are eliminated, resulting in a spectrum qualitatively similar to the J/ and particles.
Quantum group structures associated with invariances of some physical algebras
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2005., 2005.) Kayserilioğlu, Ufuk.; Arık, Metin.
In this study, the anticommuting spin algebra is introduced and it is shownto be invariant under the action of the quantum group SOq=-1(3). Furthermore, itsrepresentations and Hopf algebra structure are studied and found to be closely resemble the similar results for the angular momentum algebra. The invariance propertiesof the bosonic and fermionic oscillator algebras under inhomogeneous transformationsare also studied. The bosonic inhomogeneous symplectic group, BISp(2d,R) , andthe fermionic inhomogeneous orthogonal group, FIO(2d,R) , are defined as the inhomogeneous invariance quantum groups of these algebras. The sub(quantum)groupsand contractions of these quantum groups are studied as a source for new quantumgroups. Finally, the fermionic inhomogeneous orthogonal quantum group is defined forodd number of dimensions and its sub(quantum)groups and contractions are studied.
Dense molecular gas tracers in water megamaser galaxies
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2021., 2021.) Farhan, Ahlam.; Ercan, Nihal.
Since their discovery in 1979, water megamasers (H2OMMs) have been of special interest for both astrophysicists and cosmologists. The unique environment they are emitted from is interesting, being located inside accretion disks surrounding SMBHs. H2OMMs have helped astrophysicists accurately calculate SMBH masses and determine the kinematics of gas falling into the central SMBHs. More important, H2OMMs gave the golden chance to find the distance to a special kind of H2OMM galaxies, i.e., disk masers, accurately and directly, without any need to use standard candles or the cosmic microwave background (CMB) radiation. Unfortunately, detecting H2OMMs is very difficult, only 180 galaxies have been found out of more than 6000 galaxies searched for H2OMMs. The low detection rate is motivating many researchers to find a method that would help to increase the detecting rate. As H2OMMs are believed to be created inside dense gas clouds at galaxy centers, a correlation between dense gas and maser emission was suggested. Until now, this correlation has been barely studied. Only one work has been done so far in 2014 with a small sample number. In this work, we tried to study this possible correlation. A sample of HCN(J=1-0) and HCO+(J=1-0) emission lines was successfully collected and used as dense gas tracers via normalizing them by CO(J=1-0) emission lines, to get dense gas fractions. Subsequently, a sample of 30 H2OMM galaxies along with the dense gas fractions has been statistically analysed. Both of the dense gas fractions show a correlation with water maser emission, with HCO+ being stronger than HCN. Moreover, after removing (Ultra)Luminous IR Galaxies (U)LIRG sources, the HCO+ correlation becomes very tight and HCN correlation becomes stronger. Our results agree with previous works on dense gas as a favorable parameter to be used in H2OMM surveys. According to our analyses, HCO+ is preferable to HCN as a dense gas tracer in MM sources.
Multiparameter generalization of deformed particle algebras
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2004., 2004.) Arıkan, Ali Serdar.; Arık, Metin.
In this study, we generalize deformed boson and fermion algebras by considering as many deformation parameters as possible. The restriction on the number of deformation parameters emerges from the relations leaving the algebra consistent. In the bosonic part, we consider the Newton oscillator which is a one parameter generalization of the harmonic oscillator as a starting point. Then, we achieve the multiparameter generalization with d(d+1) parameters for d dimensional oscillator system. We also present the Fock representation of this oscillator. In the fermionic part, we first study the quantum group covariant two-parameter deformed fermion algebra. Then we show that if the quantum group symmetry is not preserved, then the number of deformation parameters in d dimension can be increased to 2(d-1). We study the limiting case where all deformation parameters go to zero such that the deformed fermionic oscillator reduces to the orthofermion algebra. Finally, we investigate the symmetry properties of this limiting case.
Normal forms, nonlocal chaotic behaviour in sportt and NMR systems
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2002., 2002.) Perdahçı, Nazım Ziya.; Oğuz, Ömer.; Hacinliyan, Avadis.
Poincaré's theory of normal forms is applied to a number of simple chaotic Sprott flows that have resonant eigenvalues. It is shown that the normal form expansion can give significant information not limited to the local properties of chaotic attractors, but also, on nonlocal properties such as positive and zero Liapunov exponents for systems that have the Hopf bifurcation property. Existence of a zero Liapunov exponent is indicated if the system has hyperbolic fixed points. The method is not directly applicable where an eigenvalue of the linearized part vanishes, because of the complexity of the normal form. Rational transformations that change the eigenvalue spectrum of the linearized parts are employed on the Sprott C and E systems to obtain simpler systems. A proposel on the possible use of fractal analysis methods on functional MRI data and preliminary results on possible source of chaotic behavior inherent in nuclear spin systems are presented.
Adiabatic solutions in general relativity and boundary symmetries
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2020., 2020.) Kutluk, Emine Şeyma.; Bleeken, Dieter Van den.
We investigate adiabatic solutions to general relativity for a spacetime with spatial slices with boundary, by Manton approximation. This approximation tells us for a theory with a Lagrangian in the natural form, a motion that is described as a slow motion on the space of vacua-static solutions that minimize the energy- is a good approximate solution. To apply this to the case of general relativity we rst bring it to the natural form by splitting space and time and choosing Gaussian normal coordinates, where a spacetime is described by the metric on its spatial slices. Then following Manton we propose slow solutions such that each slice is a slowly changing di eomorphism of a reference slice, and thus each solution is described by a vector eld on the spatial slice. These solutions will have the property that the action will become a functional of the vector elds on the boundaries of the spatial slices. Moreover using the Hodge- Morrey-Friedrichs decomposition we will show that the constraints of general relativity will identify a unique solution for a given boundary value. Then we comment on the structure of the space of vacua which we show to be a (pseudo)-Riemannian homogeneous space. We illustrate our procedure for a speci c reference slice we choose: the 3d Euclidean round ball.
Some new cosmological applications of the energy-momentum squared gravity
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2021., 2021.) Uzun, Nebiye Merve.; Turgut, Osman Teoman.; Akarsu, Özgür.
We investigate two specific forms of a new-type of modified gravity called Energy Momentum Squared Gravity (EMSG) constructed by the addition of the term f(TµνT µν) to the Einstein-Hilbert action with a cosmological constant (Λ). First, we propose a new model of EMSG, called Energy-Momentum Log Gravity (EMLG) described by f(TµνT µν) = α ln(λ TµνT µν). This choice is made as a specific way of including new terms in the right-hand side of the Einstein field equations, resulting in constant effective inertial mass density and leading to an explicit exact solution of the dust energy density in terms of redshift. We look for viable cosmologies, in particular, an extension of standard ΛCDM model. The EMLG provides an effective dynamical dark energy passing below zero at large redshifts, accommodating a mech anism for screening Λ, in line with suggestions for alleviating some of the tensions between observational data sets prevailing within the ΛCDM model. We present a theoretical investigation and then constrain the free parameters of the model using the latest observational data, and discuss the results. Second, we simultaneously replace the spatially flat Robertson-Walker metric with its simplest anisotropic extension, and couple the CDM to the gravity via the EMSG of the form f(TµνT µν) ∝ TµνT µν. These two modifications can mutually cancel out, namely, the shear scalar can be screened completely, and reproduce mathematically the same Friedmann equation of the ΛCDM model. This evades the BBN limits on the anisotropy, and thereby provides an op portunity to manipulate the CMB quadrupole temperature fluctuation at the desired amount. We also discuss the consequences of the model on the very early times and far future of the Universe.
Measurement of heat, strain, and refractive index with multicore optical fibers
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2020., 2020.) Güvenç Kılıç, Sema.; İnci, Naci.
In this thesis, different types of multicore optical fibers are designed and employed in the optical sensing experimental setups to investigate their detection abilities of temperature, heat transfer rate, strain, and refractive index. In the first section of this comprehensive investigation, a four-core optical fiber is introduced as a calorimetric gauge for the measurements of one-dimensional heat transfer rates. Transient heat pulses from an Nd:YAG laser are sent onto Aluminum coated surface, provided that the laser pulse is focused onto one of the optical fiber cores only. As a result of this, a phase shift in the interference pattern occurs and the corresponding optical signal is recorded by a CMOS camera. In the second section, the thermal expansion property of a stainless-steel cylinder is used to generate a strain on the optical fiber cores. Multicore fiber is coiled around a solid stainless-steel cylinder to obtain a circular optical fiber loop and this cylinder is placed onto the heater. Heating the stainless-steel cylinder results in thermal expansion in radial direction and a shear strain in the optical fiber cores. Thereby, a phase shift in the transmission spectrum is observed and monitored by the camera. As a third study, a four-core fiber is proposed and demonstrated for temperature sensing and heat transfer rate measurements. Laser heat pulses from are sent onto this Aluminum coated surface, which results in a variation in the interference pattern as a result of change in the refractive index and physical lengths of the cores. In addition to this experiment, hot air vortices were sent onto Aluminum coated surface of the fiber and variations in interference pattern are recorded. In the last section, a seven-core optical fiber with two fiber Bragg gratings is employed to detect the refractive index changes of surrounding environment. A small portion of optical fiber is etched in order to further expose the outer six cores to the surrounding environment. As the outer cores are affected by both temperature and refractive index change, the center core is only affected by the temperature.
Light-matter interaction in photonic cavities
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2020., 2020.) Gökbulut, Belkıs.; İnci, Naci.
In this thesis, light‒matter interaction in several novel photonic platforms are investigated by time-resolved fluorescence lifetime measurements along with their theoretical and numerical analyses. Firstly, a simple leaky-mode wedge-type photonic nanocavity is proposed for the inhibition of spontaneous light emission of the CdTe/CdS quantum dots (QDs) by enforcing the suppression of optical modes under the weak coupling regime. Thus, for the first time, the inhibited spontaneous emission rate is achieved using a low Q-factor photonic structure. Secondly, a fluorescence lifetime imaging technique is employed to investigate the spontaneous transition rate of the BODIPY dye molecules, which are doped in a single PEG nanofiber. The spontaneous emission rate of the confined dye molecules is observed to inhibit and enhance periodically upon swelling of the nanofiber through its exposure to relative humidity, suggesting an efficient optical switch mechanism. In the third photonic platform, the critical role of the confined hybrid mode in a CdTe QDs-doped single nanofiber, which is partially decorated by gold nanoparticles, on the modification of the spontaneous emission dynamics of the fluorescent emitters in a low Q-factor photonic-plasmonic nanocavity is explored for the first time. This is followed by chemically growing hollow cylindrical nanocavities on the surface of a single polymer microfiber, which is uniformly doped with perovskite nanowires; the assembly is employed for the localization of light to meaningfully alter the emission rate of the fluorescent nanowires; due to interaction of the electromagnetic field of the whispering gallery modes, supported by the microfiber, with the boundaries of the cylindrical hollow nanocavities. Finally, a quasi-optical cavity, originating from the transverse Anderson localization of the light waves in an entirely three-dimensional disordered medium, formed by polymer medium with randomly positioned air bubbles, is realized as an alternative to conventional photonic cavities for the intense light localization to strongly enhance the spontaneous emission rate of the fluorescent emitters.
Frequency-resolved photoconductivity studies in amorphous chalcogenide films
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2000., 2000.) Akat, Ercüment.; Aktaş, Gülen, 1951- .
Frequency-resolved photocurrent measurements (FRPC) were carried out on amorphous chalcogenide films such as AsSeSb AsSe and AsSeTe, by a method called frequency-resolved spectroscopy (FRS) as a function of temperature (between 230 K and 395 K), of excitation intensity (between 40 and 600 ) and in the frequency range of 10 Hz to 1.1x104 Hz, yielding the lifetime distribution directly. The results indicate that the recombination is through distant pairs, since the lifetime does depend on the excitation intensity, through ; as at 295 K and at 395 K, rather than by geminate (twin) pairs. It can also be concluded that there is a continuous distribution of states in the mobility gap implied by the value of the exponent 0.72 (in ), which is between 0.5 and 1.0, the figures that according to Rose (1978), correspond respectively to monomolecular and bimolecular recombination. The temperature exponent in the relationship has turned out to be between 7 and 10 for a frequency of some (=200 Hz); and 11 and 13 for (=3322 Hz); the value of the exponent showing a diminishing behaviour as the generation rate (intensity of excitation) is elevated. Finally, the exponent in the relationship is found to depend on both temperature and the generation rate in a manner such that the photocurrent will be more sensitive (inversely) to the frequency of excitation as either one of the above variables or both are lowered.

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