Publications with Fulltext
Permanent URI for this collectionhttps://hdl.handle.net/20.500.14288/6
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Publication Open Access Topological superfluid phases of an atomic Fermi gas with in- and out-of-plane Zeeman fields and equal Rashba-Dresselhaus spin-orbit coupling(American Physical Society (APS), 2013) Subaşı, Ahmet Levent; Department of Physics; Işkın, Menderes; Faculty Member; Department of Physics; College of Sciences; 29659We analyze the effects of in-and out-of-plane Zeeman fields on the BCS-Bose-Einstein condensation (BEC) evolution of a Fermi gas with equal Rashba-Dresselhaus (ERD) spin-orbit coupling (SOC). We show that the ground state of the system involves gapless superfluid phases that can be distinguished with respect to the topology of the momentum-space regions with zero excitation energy. For the BCS-like uniform superfluid phases with zero center-of-mass momentum, the zeros may correspond to one or two doubly degenerate spheres, two or four spheres, two or four concave spheroids, or one or two doubly degenerate circles, depending on the combination of Zeeman fields and SOC. Such changes in the topology signal a quantum phase transition between distinct superfluid phases and leave their signatures on some thermodynamic quantities. We also analyze the possibility of Fulde-Ferrell-Larkin-Ovchinnikov (FFLO)-like nonuniform superfluid phases with finite center-of-mass momentum and obtain an even richer phase diagram.Publication Open Access Dynamical properties of a coupled nonlinear dielectric waveguide-surface-plasmon system as another type of Josephson junction(Society of Photo-optical Instrumentation Engineers (SPIE), 2011) Department of Physics; Özok, Yasa Ekşioğlu; Müstecaplıoğlu, Özgür Esat; Güven, Kaan; Faculty Member; Faculty Member; Department of Physics; College of Sciences; N/A; 1674; 52290We demonstrate that a weakly-coupled nonlinear dielectric waveguide surface-plasmon (DWSP-JJ) system can be formulated in analogy to bosonic Josephson junction of atomic condensates at very low temperatures, yet it exhibits different dynamical features. Such a system can be realized along a metal - dielectric interface where the dielectric medium hosts a nonlinear waveguide (e. g. fiber) for soliton propagation. The inherently dynamic coupling parameter generates novel features in the phase space.Publication Open Access Discrete-time quantum walk with nitrogen-vacancy centers in diamond coupled to a superconducting flux qubit(American Physical Society (APS), 2013) Xue, Peng; Shikano, Yutaka; Sanders, Barry C.; Department of Physics; Hardal, Ali Ümit Cemal; Müstecaplıoğlu, Özgür Esat; Faculty Member; Department of Physics; Graduate School of Sciences and Engineering; College of Sciences; N/A; 1674We propose a quantum-electrodynamics scheme for implementing the discrete-time, coined quantum walk with the walker corresponding to the phase degree of freedom for a quasimagnon field realized in an ensemble of nitrogen-vacancy centers in diamond. The coin is realized as a superconducting flux qubit. Our scheme improves on an existing proposal for implementing quantum walks in cavity quantum electrodynamics by removing the cumbersome requirement of varying drive-pulse durations according to mean quasiparticle number. Our improvement is relevant to all indirect-coin-flip cavity quantum-electrodynamics realizations of quantum walks. Our numerical analysis shows that this scheme can realize a discrete quantum walk under realistic conditions.Publication Open Access Fundamental transfer matrix and dynamical formulation of stationary scattering in two and three dimensions(American Physical Society (APS), 2021) Loran, Farhang; Department of Mathematics; Department of Physics; Mostafazadeh, Ali; Faculty Member; Department of Mathematics; Department of Physics; College of Sciences; 4231We offer a consistent dynamical formulation of stationary scattering in two and three dimensions (2D and 3D) that is based on a suitable multidimensional generalization of the transfer matrix. This is a linear operator acting in an infinite-dimensional function space which we can represent as a 2 x 2 matrix with operator entries. This operator encodes the information about the scattering properties of the potential and enjoys an analog of the composition property of its one-dimensional ancestor. Our results improve an earlier attempt in this direction [Phys. Rev. A 93, 042707 (2016)] by elucidating the role of the evanescent waves. We show that a proper formulation of this approach requires the introduction of a pair of intertwined transfer matrices, each related to the time-evolution operator for an effective nonunitary quantum system. We study the application of our findings in the treatment of the scattering problem for delta-function potentials in 2D and 3D and clarify its implicit regularization property which circumvents the singular terms appearing in the standard treatments of these potentials. We also discuss the utility of our approach in characterizing invisible (scattering-free) potentials and potentials for which the first Born approximation provides the exact expression for the scattering amplitude.Publication Open Access Acoustic superradiance from an optical-superradiance-induced vortex in a Bose-Einstein condensate(American Physical Society (APS), 2014) Ghazanfari, Nader; Department of Physics; Müstecaplıoğlu, Özgür Esat; Faculty Member; Department of Physics; College of Sciences; 1674We consider the scattering of an acoustic wave from a vortex induced by optical superradiance. The vortex is created by pumping a large amount of angular momentum with a Laguerre-Gaussian light beam in an atomic Bose-Einstein condensate. We derive the mean-field dynamical equations of the light-superfluid system, and obtain the equations governing the elementary excitation of the system, which result in a massless Klein-Gordon equation with source terms. This equation describes the propagation of the sound wave in an effective space-time. Employing a simplifying draining bathtub model for the vortex, we investigate the scattering of the acoustic wave in the vortex phase and obtain a condition for the acoustic superradiance. We conclude that Laguerre-Gaussian-beam-induced sudden transition from homogeneous to vortex state in the superfluid leads to a prominent observation of the acoustic superradiance.Publication Open Access Dynamic accommodation measurement using Purkinje reflections and ML algorithms(Society of Photo-optical Instrumentation Engineers (SPIE), 2023) Department of Electrical and Electronics Engineering; N/A; Aygün, Uğur; Şahin, Afsun; Ürey, Hakan; Faculty Member; Faculty Member; Department of Electrical and Electronics Engineering; Koç University Research Center for Translational Medicine (KUTTAM) / Koç Üniversitesi Translasyonel Tıp Araştırma Merkezi (KUTTAM); College of Engineering; School of Medicine; N/A; N/A; N/A; 171267; 8579We developed a prototype device for dynamic gaze and accommodation measurements based on 4 Purkinje reflections (PR) suitable for use in AR and ophthalmology applications. PR1&2 and PR3&4 are used for accurate gaze and accommodation measurements, respectively. Our eye-model was developed in Zemax and matches the experiments well. Our model predicts the accommodation from 25cm to infinity (<4 diopters) with better than 0,25D accuracy. We performed repeatability tests and obtained accurate gaze and accommodation estimations using 15 subjects. We are generating a large synthetic data set using physically accurate models and machine learning algorithms.Publication Open Access Coagulation measurement from whole blood using vibrating optical fiber in a disposable cartridge(Society of Photo-optical Instrumentation Engineers (SPIE), 2017) Çivitci, Fehmi; Barış, İbrahim; Yaralıoğlu, Göksenin; Department of Electrical and Electronics Engineering; Yaras, Yusuf Samet; Gündüz, Ali Bars; Sağlam, Gökhan; Ölçer, Selim; Ürey, Hakan; Other; Faculty Member; Department of Electrical and Electronics Engineering; College of Engineering; N/A; N/A; N/A; N/A; 8579In clinics, blood coagulation time measurements are performed using mechanical measurements with blood plasma. Such measurements are challenging to do in a lab-on-a-chip (LoC) system using a small volume of whole blood. Existing LoC systems use indirect measurement principles employing optical or electrochemical methods. We developed an LoC system using mechanical measurements with a small volume of whole blood without requiring sample preparation. The measurement is performed in a microfluidic channel where two fibers are placed inline with a small gap in between. The first fiber operates near its mechanical resonance using remote magnetic actuation and immersed in the sample. The second fiber is a pick-up fiber acting as an optical sensor. The microfluidic channel is engineered innovatively such that the blood does not block the gap between the vibrating fiber and the pick-up fiber, resulting in high signal-to-noise ratio optical output. The control plasma test results matched well with the plasma manufacturer's datasheet. Activated-partial-thromboplastin-time tests were successfully performed also with human whole blood samples, and the method is proven to be effective. Simplicity of the cartridge design and cost of readily available materials enable a low-cost point-of-care device for blood coagulation measurements.Publication Open Access Atom-dimer and dimer-dimer scatterings in a spin-orbit-coupled Fermi gas(American Physical Society (APS), 2021) Department of Physics; Işkın, Menderes; Faculty Member; Department of Physics; College of Sciences; 29659Using the diagrammatic approach, here we study how spin-orbit coupling (SOC) affects the fermion-dimer and dimer-dimer scattering lengths in the Born approximation, and we benchmark their accuracy with the higher-order approximations. We consider both isotropic and Rashba couplings in three dimensions and show that the Born approximation gives accurate results in the 1/(mαas)≪-1 limit, where m is the mass of the fermions, α is the strength of the SOC, and as is the s-wave scattering length between fermions. This is because the higher-loop contributions form a perturbative series in the 1/(mαas)<0 region that is controlled by the smallness of the residue Z of the dimer propagator. In sharp contrast, since Z grows with the square root of the binding energy of the dimer in the 1/(mαas)>0 region, all of the higher-loop contributions are of similar order.Publication Open Access Advanced materials and device architectures for magnetooptical spatial light modulators(Wiley-VCH, 2019) N/A; Department of Electrical and Electronics Engineering; Kharratian, Soheila; Onbaşlı, Mehmet Cengiz; Ürey, Hakan; Faculty Member; Faculty Member; Department of Electrical and Electronics Engineering; Graduate School of Sciences and Engineering; N/A; 258783; 8579Faraday and Kerr rotations are magnetooptical (MO) effects used for rotating the polarization of light in transmission and reflection from a magnetized medium, respectively. MO effects combined with intrinsically fast magnetization reversal, which can go down to a few tens of femtoseconds or less, can be applied in magnetooptical spatial light modulators (MOSLMs) promising for nonvolatile, ultrafast, and high-resolution spatial modulation of light. With the recent progress in low-power switching of magnetic and MO materials, MOSLMs may lead to major breakthroughs and benefit beyond state-of-the-art holography, data storage, optical communications, heads-up displays, virtual and augmented reality devices, and solid-state light detection and ranging (LIDAR). In this study, the recent developments in the growth, processing, and engineering of advanced materials with high MO figures of merit for practical MOSLM devices are reviewed. The challenges with MOSLM functionalities including the intrinsic weakness of MO effect and large power requirement for switching are assessed. The suggested solutions are evaluated, different driving systems are investigated, and resulting device architectures are benchmarked. Finally, the research opportunities on MOSLMs for achieving integrated, high-contrast, and low-power devices are presented.Publication Open Access Transfer matrices as nonunitary S matrices, multimode unidirectional invisibility, and perturbative inverse scattering(American Physical Society (APS), 2014) Department of Mathematics; Mostafazadeh, Ali; Faculty Member; Department of Mathematics; College of Sciences; 4231We show that in one dimension the transfer matrix M of any scattering potential v coincides with the S matrix of an associated time-dependent non-Hermitian 2 x 2 matrix Hamiltonian H(tau). If v is real valued, H(tau) is pseudo-Hermitian and its exceptional points correspond to the classical turning points of v. Applying time-dependent perturbation theory to H(tau) we obtain a perturbative series expansion for M and use it to study the phenomenon of unidirectional invisibility. In particular, we establish the possibility of having multimode unidirectional invisibility with wavelength-dependent direction of invisibility and construct various physically realizable optical potentials possessing this property. We also offer a simple demonstration of the fact that the off-diagonal entries of the first-order Born approximation for M determine the form of the potential. This gives rise to a perturbative inverse scattering scheme that is particularly suitable for optical design. As a simple application of this scheme, we construct an infinite-range unidirectionally invisible potential.