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Publication Metadata only A deformation-based approach to tuning of magnetic micromechanical resonators(2018) Yalçınkaya, Arda D.; Department of Mechanical Engineering; N/A; Department of Mechanical Engineering; Biçer, Mahmut; Esfahani, Mohammad Nasr; Alaca, Burhanettin Erdem; Researcher; PhD Student; Faculty Member; Department of Mechanical Engineering; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); College of Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; N/A; 115108Resonance frequency tuning in magnetic micromechanical resonators remains a primary field of study for frequency reference applications. The use of magnetic micromechanical resonators for innovative timing, oscillator and sensing applications necessitates a platform for the precise control of the resonance frequency. The present work addresses a deformation based technique for tuning the resonance frequency of nickel micromechanical resonators. Frequency response is measured through magnetic actuation and optical readout. The tuning approach is based on a combination of flexural deformation and uniaxial strain. The bending deformation is achieved by using a DC current through the microbeam. This magnetomotive mechanism reduces the resonance frequency by about 13% for a maximum DC current of 80 mA. A substrate bending method is used for applying uniaxial strain to increase the resonance frequency by about 8%. A bidirectional frequency modulation is thus demonstrated by utilizing both deformation techniques. The interpretation of results is carried out by finite element analysis and electromechanical analogy in an equivalent circuit. Using deformation techniques, this study provides a rigorous approach to control the resonance frequency of magnetic micromechanical resonators.Publication Open Access A physical channel model and analysis for nanoscale molecular communications with Förster resonance energy transfer (FRET)(Institute of Electrical and Electronics Engineers (IEEE), 2012) Kuşcu, Murat; Akan, Özgür Barış; Faculty Member; College of EngineeringIn this study, a novel and physically realizable nanoscale communication paradigm is introduced based on a well-known phenomenon, Forster resonance energy transfer (FRET), for the first time in the literature. FRET is a nonradiative energy transfer process between fluorescent molecules based on the dipole-dipole interactions of molecules. Energy is transferred rapidly from a donor to an acceptor molecule in a close proximity such as 0 to 10 nm without radiation of a photon. Low dependence on the environmental factors, controllability of its parameters, and relatively wide transfer range make FRET a promising candidate to be used for a high-rate nanoscale communication channel. In this paper, the simplest form of the FRET-based molecular communication channel comprising a single transmitter-receiver nanomachine pair and an extended version of this channel with a relay nanomachine for long-range applications are modeled considering nanomachines as nanoscale electromechanical devices with some sensing, computing, and actuating capabilities. Furthermore, using the information theoretical approach, the capacities of these communication channels are investigated and the dependence of the capacity on some environmental and intrinsic parameters is analyzed. It is shown that the capacity can be increased by appropriately selecting the donor-acceptor pair, the medium, the intermolecular distance, and the orientation of the molecules.Publication Metadata only Analysis of the knight shift data on Li and Zn substituted YBa2Cu3O6+x(Elsevier, 2001) Department of Physics; Bulut, Nejat; Faculty Member; Department of Physics; College of Sciences; N/AThe Knight shift data on Li and Zn substituted YBa2Cu3O6+x are analyzed using an itinerant model with short-range antiferromagnetic correlations. The model parameters, which are determined by fitting the experimental data on the transverse nuclear relaxation rate T-2(-1) of pure YBa2Cu3O6+x, are used to calculate the Knight shifts for various nuclei around a nonmagnetic impurity located in the CuO2 planes. The calculations are carried out for Li and Zn impurities substituted into optimally doped and underdoped YBa2Cu3O6+x. The results are compared with the Li-7 and Y-89 Knight shift measurements on these materials. (C) 2001 Elsevier Science B.V. All rights reserved.Publication Metadata only Bose-einstein condensate in a harmonic trap with an eccentric dimple potential(Institute of Physics (IOP) Publishing, 2008) Uncu, H.; Tarhan, D.; Demiralp, E.; Department of Physics; Müstecaplıoğlu, Özgür Esat; Faculty Member; Department of Physics; College of Sciences; 1674We investigate Bose-Einstein condensation of noninteracting gases in a harmonic trap with an offcenter dimple potential. We specifically consider the case of a tight and deep dimple potential, which is modeled by a point interaction. This point interaction is represented by a Dirac delta function. The atomic density, chemical potential, critical temperature and condensate fraction, and the role of the relative depth and the position of the dimple potential are analyzed by performing numerical calculations.Publication Metadata only Dynamic modeling of soft magnetic film actuated scanners(IEEE-Inst Electrical Electronics Engineers Inc, 2009) N/A; N/A; Department of Electrical and Electronics Engineering; Işıkman, Serhan Ömer; Ürey, Hakan; Master Student; Faculty Member; Department of Electrical and Electronics Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 8579Dynamic behavior of magnetic thin film actuators is investigated in detail and applied to various laser scanning applications. Magnetic hysteresis effects are incorporated into the model developed in the prior work, which assumes linear magnetization as a function of magnetic field and is based on the distributed point-by-point calculation of the magnetostatic moments and forces across the film surface. A simple functional form is used to model the major B-H loop of ferromagnetic films. The model is validated with permalloy (Ni-Fe) plated polymer actuators. The actuators are excited using an external electro-coil and the structures deflect due to magnetic anisotropy torque. The ac deflection of the actuators is modeled by calculating the point-by-point moments on the magnetic film and the solution can handle nonuniform external field and unsaturated magnetic film cases. A 25 degrees optical scan angle is demonstrated for laser scanning display and imaging applications with a nonoptimum coil. Scaling the model to MEMS devices is also discussed.Publication Metadata only Dynamics of spacing adjustment and recovery mechanisms of ABAC-type growth pattern in ternary eutectic systems(Elsevier, 2017) N/A; N/A; Department of Mechanical Engineering; Mohagheghi, Samira; Şerefoğlu, Melis; PhD Student; Researcher; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; College of Engineering; 329277; 44888In directionally solidified 2D samples at ternary eutectic compositions, the stable three-phase pattern is established to be lamellar structure with ABAC stacking, where A, B, and C are crystalline phases. Beyond the stability limits of the ABAC pattern, the system uses various spacing adjustment mechanisms to revert to the stable regime. In this study, the dynamics of spacing adjustment and recovery mechanisms of isotropic ABAC patterns were investigated using three-phase In-Bi-Sn alloy. Unidirectional solidification experiments were performed on 23.0 and 62.7 mu m-thick samples, where solidification front was monitored in real-time from both sides of the sample using a particular microscopy system. At these thicknesses, the pattern was found to be 2D during steady-state growth, i.e. both top and bottom microstructures were the same. However, during spacing adjustment and recovery mechanisms, 3D features were observed. Dynamics of two major instabilities, lamellae branching and elimination, were quantified. After these instabilities, two key ABAC pattern recovery mechanisms, namely, phase invasion and phase exchange processes, were identified and analyzed. After elimination, ABAC pattern is recovered by either continuous eliminations of all phases or by phase exchange. After branching, the recovery mechanisms are established to be phase invasion and phase exchange.Publication Metadata only Effects of dilute nonmagnetic impurities on the Q = (π,π) spin-fluctuation spectrum in YBa2Cu3O7(Elsevier, 2001) Department of Physics; Bulut, Nejat; Faculty Member; Department of Physics; College of Sciences; N/AThe effects of nonmagnetic impurities on the Q = (pi, pi) spin-fluctuation spectral weight Im chi (Q, omega) are studied within the framework of the two-dimensional Hubbard model using the random phase approximation. In the first part of the paper, the effects of the nonmagnetic impurities on the magnetic susceptibility of the noninteracting (U = 0) system, chi (0)(q, omega), are calculated with the self-energy and the vertex corrections using various forms of the effective electron-impurity interaction. Here, the range and the strength of the impurity potential are varied as well as the concentration of the impurities. It Is shown that the main effect of dilute impurities on X,(Q, w) is to cause a weak smearing. In the second part, Im chi (Q, omega) is calculated for the interacting system. Here, the calculations are concentrated on the processes which involve the impurity scattering of the spin fluctuations with finite momentum transfers. Results are given for various values of the model parameters, and comparisons are made with the neutron scattering data on Zn substituted YBa2Cu3O7. (C) 2001 Elsevier Science B.V. All rights reserved.Publication Metadata only FR4 laser scanner with dynamic focus(IEEE-Inst Electrical Electronics Engineers Inc, 2009) Sprague, Randy B.; N/A; Department of Electrical and Electronics Engineering; Işıkman, Serhan Ömer; Ürey, Hakan; Master Student; Faculty Member; Department of Electrical and Electronics Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 8579An electromagnetically actuated optical scanner made using standard printed circuit board technology with integrated dynamic focusing feature is presented. Dynamic focus is achieved with an independently controlled plunger machined on the flame retardant-4 (FR4) platform. Integration of a laser diode and lens, torsional scanner, and the plunger for dynamic focus adjustment on FIN platform greatly improves the form factor of the device for imaging applications. A peak-to-peak mechanical scan angle of 50 degrees is achieved. The dynamic focus control allows for shifting the beam waist location from 80 mm up to 650 mm.Publication Metadata only Low-cost low-threshold diode end-pumped Tm:YAG laser at 2.016 um(Springer, 2012) Kurt, Adnan; N/A; N/A; Department of Physics; Beyatlı, Ersen; Naghizadeh, Solmaz; Sennaroğlu, Alphan; PhD Student; PhD Student; Faculty Member; Department of Physics; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Sciences; 229317; N/A; 23851We report a low-threshold continuous-wave Tm:YAG laser that can be excited near 785 nm with low-cost, single-mode AlGaAs laser diodes. Low-threshold operation was achieved using a tightly focused, astigmatically compensated x-cavity containing a 2-mm-thick Tm:YAG crystal with 5 % Tm3+ concentration. Two linearly polarized single-mode diodes operating at 785.8 nm were polarization coupled to end pump the resonator. With a 6 % output coupler, as high as 32 mW of output power could be obtained at 2016 nm with 184 mW of incident pump power. The output could be further tuned in the 1935-2035 nm range. Slope efficiency measurements indicated that cross-relaxation was very effective at this doping level. With a 2 % output coupler, lasing could be obtained with as low as 32.3 mW of pump power. In the limit of vanishing output coupling, the incident threshold pump power could be reduced to as low as 25 mW. To our knowledge, this is among the lowest lasing thresholds reported to date for continuous-wave, room-temperature thulium lasers.Publication Metadata only Modeling and characterization of comb-actuated resonant microscanners(Iop Publishing Ltd, 2006) N/A; Department of Electrical and Electronics Engineering; Ataman, Çağlar; Ürey, Hakan; PhD Student; Faculty Member; Department of Electrical and Electronics Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 8579The dynamics of the out-of-plane comb-drive actuator used in a torsional resonant mode microscanner is discussed. The microscanner is fabricated using the standard SOI technology by Fraunhofer, IPMS and utilized in various display, barcode scanning, spectroscopy and other imaging applications. The device is a parametrically excited system and exhibits hysteretic frequency response, nonlinear transient response, subharmonic oscillations, multiple parametric resonances, and alternating-oscillation-frequency behavior. Analytical and numerical models are developed to predict the parametric system dynamics. The analytical model is based on the solution of the linear Mathieu equation and valid for small angular displacements. The numerical model is valid for both small and large deflection angles. The analytical and numerical models are validated with the experimental results under various ambient pressures and excitation schemes and successfully predict the dynamics of the parametric nature of the microscanner. As many as four parametric resonances are observed at 30 mTorr. The models developed in this paper can be used to optimize the structure and the actuator.