Browsing by Subject "Oscillations"
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Item Open Access Aharanov-Bohm effects induced by light in a fiber(A I P Publishing LLC, 1996-08-02) Kulik, I. O.; Shumovsky, A.A weakly coupled normal-metal ring surrounding an optical fiber is considered under the condition that the frequency of light in fiber is larger than the conduction bandwidth of the metal. It is shown that in the presence of static magnetic field parallel to the fiber axis, the resistance of the ring is a nonmonotone function of the optical intensity and an oscillating function of the static magnetic flux with period equal to flux quantum hc/e. The temperature dependence of oscillations requires that inelastic mean free path of electrons is larger than the ring size, and does not relate to the energy level spacing to temperature ratio.Item Open Access AlGaN/GaN HEMT-based fully monolithic X-band low noise amplifier(Wiley, 2005-04) Schwindt, R.; Kumar, V.; Aktas, O.; Lee, J. W.; Adesida, I.A fully monolithic AlGaN/GaN HEMT-based low noise amplifier is reported. The circuit demonstrated a noise figure of 3.5 dB, gain of -7.5 dB, input return loss of -7.5 dB, and output return loss of -15 dB at 8.5 GHz. The dc characteristics of individual 0.25-μm × 150-μm transistors were: maximum current density of 1.0 A/mm, maximum transconductance of 170 mS/mm and a threshold voltage of -6.8 V. The devices have a typical short circuit current gain cutoff frequency of 24.5 GHz and a maximum oscillating frequency of 48 GHz. The devices demonstrated a minimum noise figure of 1.6 dB with an associated gain of 10.6 dB at 10 GHz.Item Open Access Bi-angular lens for material characterization(IEEE, 1994) Yaralıoğlu, Göksen Göksenin; Atalar, Abdullah; Köymen, HayrettinIn this paper a new lens design is proposed for characterization of layered materials. Lamb wave lens employs Lamb waves for this purpose since these waves propagate along interfaces. However, below cut-off angle, the critical angles of Lamb wave modes are low and the generated V(z) curves have small number of oscillations, which in turn causes measurement difficulties and accuracy degradation. Bi-angular lens described in this paper, generates an extra obliquely incident wave, instead of normally incident beam, in order to provide the reference specular reflection. Simulation results as well as experimental results are presented and it is shown that a high sensitivity can be obtained by using this new lens.Item Open Access Experimental study of linear closed-loop control of subsonic cavity flow(2006) Yan P.; Debiasi, M.; Yuan X.; Little J.; Özbay, Hitay; Samimy, M.A study is presented of the modeling and implementation of different concepts for linear feedback control of a single-mode resonance shallow cavity flow. When a physics-based linear model is used for cavity pressure oscillations-, an H∞ controller was designed and tested experimentally. It significantly reduced the main Rossiter mode for which it was designed, while leading to strong oscillations at other Rossiter modes. Other linear control methods such as Smith predictor controller and proportional integral derivative (PID) controller exhibited similar results. The ineffectiveness of using fixed linear models in the design of controllers for the cavity flows is discussed. A modification of the PID design produced a parallel-proportional with time-delay controller that remedied this problem by placing zeros at the frequencies corresponding to other resonance states. Interestingly, it was observed that introducing the same zero to the H∞ controller can also successfully avoid the strong oscillations at other Rossiter modes otherwise observed in the single-mode-based design. The parallel-proportional with time-delay controller was compared to a very effective open-loop method for reducing cavity resonance and exhibited superior robustness with respect to departure of the Mach number from the design conditions. An interpretation is presented for the physical mechanisms by which the open-loop forcing and the parallel-proportional with time-delay controllers reduce the cavity flow noise. The results support the idea that both controls induce in the system a rapid switching between modes competing for the available energy that can be extracted from the mean flow.Item Open Access The Fractional Fourier transform and harmonic oscillation(Springer, 2002) Kutay, M. A.; Özaktaş, Haldun M.The ath-order fractional Fourier transform is a generalization of the ordinary Fourier transform such that the zeroth-order fractional Fourier transform operation is equal to the identity operation and the first-order fractional Fourier transform is equal to the ordinary Fourier transform. This paper discusses the relationship of the fractional Fourier transform to harmonic oscillation; both correspond to rotation in phase space. Various important properties of the transform are discussed along with examples of common transforms. Some of the applications of the transform are briefly reviewed.Item Open Access On a problem of H.Shapiro(Elsevier, 2004-02) Ostrovskii, I.; Ulanovskii, A.Let μ be a real measure on the line such that its Poisson integral M(z) converges and satisfies M(x+ iy) ≤ Ae-cyα, y → + ∞, for some constants A, c > 0 and 0 < α ≤ 1. We show that for 1/2 < α ≤ 1 the measure μ must have many sign changes on both positive and negative rays. For 0 < α ≤ 1/2 this is true for at least one of the rays, and not always true for both rays. Asymptotical bounds for the number of sign changes are given which are sharp in some sense. © 2003 Elsevier Inc. All rights reserved.Item Open Access Plane-wave dynamics of optical parametric oscillation with simultaneous sum-frequency generation(Institute of Electrical and Electronics Engineers, 1999-06) Dikmelik, Y.; Akgün, G.; Aytür, O.This paper presents a theoretical analysis of sum-frequency generating optical parametric oscillators where a single nonlinear crystal is used for both parametric generation and sum-frequency generation. In these devices, the parametric and sum-frequency generation processes are both phase matched for the same direction of propagation inside the crystal. Different polarization geometries for which this simultaneous phase-matching condition can potentially be satisfied are identified and categorized, for both birefringent and quasi-phase-matching methods. Plane-wave coupled-mode equations are presented for each of these categories. Solutions of these coupled mode equations and calculation of the single-pass saturated signal gain are outlined. Intracavity signal photon flux density calculations based on these solutions lead to stable steady-state upconversion, multistability, and chaos. The dependence of the photon conversion efficiency on various design parameters are investigated.Item Open Access Plasma modes in layered superconductors(Elsevier B.V., 2005) Askerzade, I. N.; Tanatar, BilalAn expression for the plasmon spectrum in the layered superconductors with arbitrary thickness of planes, which varies within a wide range is obtained. The obtained result can be attractive for the explanation of experimental data on plasmon modes in cuprates and other recently discovered superconductors.Item Open Access Plasmons and the drag effect in a strong magnetic field(Elsevier B.V., 2002) Manolescu, A.; Tanatar, BilalWe study the effect of magnetoplasmons on the drag resistance in a strong magnetic field, at finite temperatures. The typical magnetic field is about 1 T, and the temperature is up to 10 K. The Landau levels are broadened by disorder, but well separated in energy. We discuss intra-Landau level magnetoplasmons, with low frequencies, below ωc, and inter-Landau level magnetoplasmons (also called Bernstein modes), with high frequencies, close to multiples of ωc. We compare the temperature dependence of the minima and maxima of the Shubnikov-de Haas oscillations of the transresistance.Item Open Access Simulation of millimeter-wave gunn oscillations in gallium nitride(2004) Sevik, C.; Bulutay, C.High field transport in wide bandgap semiconductors like GaN is of great technological importance. The negative differential mobility regime at high fields, under suitable conditions, can lead to millimeterwave Gunn oscillations. Using extensive simulation based an ensemble Monte Carlo technique, the prospects of GaN Gunn diodes are theoretically investigated. The possibility of operating these Gunn diodes at their higher harmonic modes are explored. The main finding of this research is that the carrier dynamics in GaN can be tailored by an optimum choice of doping profile, temperature and bias conditions so that the efficiency of higher harmonic Gunn oscillations can be boosted. Finally, the physical origin of these Gunn oscillations is sought exploring whether it is the intervalley scattering mechanism, the Γ valley nonparabolicity, or the effective mass discrepancy between the Γ and the lowest satellite valleys as the responsible mechanism.Item Open Access Simultaneous non-contact atomic force microscopy (nc-AFM)/STM imaging and force spectroscopy of Si(1 0 0)(2 × 1) with small oscillation amplitudes(Elsevier Science B.V., 2002) Özer, H. Ö.; Atabak, M.; Ellialtoğlu, R. M.; Oral, A.Si(1 0 0)(2 × 1) surface is imaged using a new non-contact atomic force microscopy (nc-AFM)/STM with sub-Ångström oscillation amplitudes using stiff tungsten levers. Simultaneous force gradient and STM images of individual dimers and atomic scale defects are obtained. We measured force-distance (f-d) curves with different tips. Some of the tips show long force interactions, whereas some others resolve short-range interatomic force interactions. We observed that the tips showing short-range force interaction give atomic resolution in force gradient scans. This result suggests that short-range force interactions are responsible for atomic resolution in nc-AFM.Item Open Access Vortex lattice of a Bose-Einstein condensate in a rotating anisotropic trap(American Physical Society, 2004) Oktel, M. Ö.The behavior of Bose-Einstein condensates (BEC) with a vortex lattice in a rotating anisotropic trap was studied. The single-particle wave functions for a rotating anisotropy trap were found by exactly diagonalizing the Hamiltonian. The wave functions in the lowest Landau level (LLL) were shown to be written as simple analytic functions in the small anisotropy limit. The results show that the regularity of the vortex configurations observed in an axisymmetric trap is disturbed by the anisotropy of the confinement.