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Permanent URI for this collectionhttps://hdl.handle.net/20.500.14288/3
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Publication Metadata only Time-dependent diffeomorphisms as quantum canonical transformations and the time-dependent harmonic oscillator(Iop Publishing Ltd, 1998) N/A; Department of Mathematics; Mostafazadeh, Ali; Faculty Member; Department of Mathematics; College of Sciences; 4231Quantum canonical transformations corresponding to time-dependent diffeomorphisms of the configuration space are studied. A special class of these transformations which correspond to time-dependent dilatations is used to identify a previously unknown class of exactly solvable time-dependent harmonic oscillators. The Caldirola-Kanai oscillator belongs to this class. For a general time-dependent harmonic oscillator, it is shown that choosing the dilatation parameter to satisfy the classical equation of motion, one obtains the solution of the Schrodinger equation. A simple generalization of this result leads to the reduction of the Schrodinger equation to a second-order ordinary differential equation whose special case is the auxiliary equation of the Lewis-Riesenfeld invariant theory. The time-evolution operator is expressed in terms of a positive red solution of this equation in a closed form, and the time-dependent position and momentum operators are calculated.Publication Metadata only Chaos in rotating triatomic clusters(Editions Physique, 1997) Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129Lyapunov exponent distributions of rotating triatomic Lennard-Jones clusters are calculated to analyze the effects of the vibrational and rotational motion on the extent of chaotic behavior. Initial momentum components are assigned to atoms either to rotate the clusters around symmetry axes or to generate random angular momenta. In both cases, it is seen that the initial kinetic energy assigned to vibrational modes is the dominant factor which determines the degree of chaos.Publication Metadata only Comparison between difference-frequency generation and parametric fluorescence in quasi-phase-matched lithium niobate stripe waveguides(IEEE, 1996) Baldi, P.; El Hadi, K.; De Micheli, M. P.; Ostrowsky, D.B.; Department of Physics; Sundheimer, Michael; Faculty Member; Department of Physics; College of Sciences; N/ATuning curves and gain are two relevant parameters for integrated optical parametric oscillators. We show in this paper that they can be determined with a good precision without building a high-finesse cavity by measurement of the optical parametric fluorescence and difference-frequency generation. In the first part of this paper, we compare theoretically the guided optical parametric fluorescence and the guided difference-Frequency generation in the quasi-phase matching configuration. In the second part, we compare experimental results on optical parametric fluorescence in the 1.2-2.2-μm region and optical difference-frequency generation from a 1.55-μm laser diode using a pump wavelength between 775-795 nm in quasi-phase-matched lithium niobate stripe waveguides. This comparison shows that the gain measured by both methods is identical, but, while parametric fluorescence allows us to obtain the quasi-phase-matching curve, the difference-frequency generation gives a simpler and more accurate measurement of the gain. The combination of these two techniques provides therefore a powerful tool for evaluating the different fabrication processes of the nonlinear waveguides, without actually fabricating a parametric oscillator.Publication Metadata only Experimental and numerical investigation of thermal effects in end-pumped Cr/sup 4+/: forsterite lasers near room temperature(IEEE-Inst Electrical Electronics Engineers Inc, 1998) Department of Physics; Department of Physics; Sennaroğlu, Alphan; Pekerten, Barış; Faculty Member; Undergraduated Student; Department of Physics; College of Sciences; College of Sciences; 23851; N/AThe results of a study which employs both experimental and theoretical methods to investigate the role of thermal effects in room-temperature Cr4+:forsterile lasers are presented. A novel model was developed to calculate the incident threshold pump power required to attain oscillation by taking into account absorption saturation and pump-induced thermal loading in the gain medium, Experimentally, the incident threshold pump power was measured as a function of the crystal boundary temperature for three Cr4+:forsterite laser crystals with different small-signal differential absorption coefficients alpha(p0) and/or cross-sectional areas. Excellent agreement was obtained between theory and experiment for values of the stimulated emission cross section comparable to those from previously reported data. The model was then used to numerically determine the optimum value of alpha(p0) which minimizes the incident threshold pump power in room-temperature Cr4+:forsterite lasers, At a crystal boundary temperature of 15 degrees C, the optimum value of alpha(p0) was determined to be 0.64 cm(-1) for a 2-cm-long Cr4+:forsterite crystal, corresponding to an unsaturated absorption of 72%. The use of crystals with an optimum absorption coefficient should lead to the realization of highly efficient CW Cr4+:forsterite lasers at room temperature.Publication Metadata only Efficient continuous-wave operation of a radiatively cooled CR4+: forsterite laser at room temperature(Optical Society of America, 1997) Department of Physics; Department of Physics; Sennaroğlu, Alphan; Kurt, Adnan; Faculty Member; Teaching Faculty; Department of Physics; College of Sciences; College of Sciences; 23851; 194455Results of a detailed experimental investigation aimed at reducing the thermal loading problem in a cw Cr(4+):forsterite laser at elevated temperatures are presented. From a Cr(4+):forsterite crystal with a differential absorption coefficient of 0.57 cm(-1), as much as 900 mW of cw output power has been obtained at 1.26 mum and at a crystal boundary temperature of 15 degrees C with an absorbed pump power of only 4.5 W at 1.06 mum. No chopping of the pump beam was necessary. An efficient radiative cooling technique was further employed to cool the laser and no subsequent power fading was observed. To the author's knowledge, the measured absorbed power slope efficiency of 29.5% represents the highest cw power performance reported to date from a Cr(4+):forsterite laser pumped by a Nd:YAG laser around room temperature. The role of the low differential absorption coefficient in the reduction of thermal loading is further elucidated by presenting comparative cw power performance data with a second Cr(4+):forsterite crystal having a differential absorption coefficient of 1.78 cm(-1) in the temperature range between 12 and 35 degrees C. Finally, some interesting multipulse effects of the laser observed in the millisecond regime during quasi-cw operation at 50% duty cycle are described.Publication Metadata only A theoretical study of structural defects in conjugated polymers(Elsevier Science Sa, 1999) Yurtsever, Mine; Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129Accurate ab-initio calculations are performed for pyrrole and thiophene oligomers bonded through alpha and beta carbons. The thermodynamical stabilitiy of all possible binding types including the branched forms of tetramers and pentamers are compared. Employing the probabilities obtained from these calculations, a Monte Carlo type growth scheme is applied to predict branching as functions of the chain length and temperature. A high degree of branching for polypyrrole is reported whereas the linear chains dominate the structure of polythiophene.Publication Metadata only The sensitivity of conformational free energies of the alanine dipeptide to atomic site charges(Wiley, 1997) Maye, Peter V.; Mezei, Mihaly; Department of Physics; Reşat, Haluk; Faculty Member; Department of Physics; College of Sciences; N/ADifferent atomic point charge sets are obtained for the are and C7,eq conformations of the alanine dipeptide by fitting the charges of each conformation to the respective ab initio electrostatic potential surfaces both individually and simultaneously, in both the united atom and the all-atom representations. Using these charge sets, the sensitivity of the relative conformational aqueous free energies to the atomic site charges is investigated. For this particular system, we find that the solute-water contributions to the conformational free energy differences have a rather weak dependence on site charges; the calculated intramolecular contributions, however, show a rather strong dependence on the atomic site charges. It is suggested that the calculated results for the alanine dipeptide using a single, simultaneously fit set of charges for both conformations are in better agreement with experiments than the calculations carried out with charges determined individually for each conformation. © 1997 John Wiley & Sons, Inc.Publication Metadata only Finite-element method for quantum scattering(Kluwer Academic Publ, 1993) Department of Mathematics; Aşkar, Attila; Faculty Member; Department of Mathematics; College of Sciences; 178822The finite element method is introduced and applied to quantum mechanical scattering problems. In this procedure the space is discretized on a grid with the unknown quantities being the wavefunction values. Local polynomials approximate the wave function and no global basis set expansion is required. The scattering solution is constructed by a suitable combination of independent standing wave solutions. These latter solutions are generated numerically by using real, not complex, arithmetic. A one-dimensional barrier crossing is studied as a first example to illustrate finite element discretization and the construction of the scattered wave forms in an uncomplicated situation. A two variable generalization is given next. The method is then sucessfully applied to a model collinear problem which is analytically soluble and to the collinear H + H2 system. Next, a three variable formulation of the co-planar A + BC system is discussed with specific reference to co-planar H + H2 . Some comments on the generalization of the technique complete the discussion.Publication Metadata only Experimental and numerical investigation of thermal effects in end-pumped Cr/sup 4+/: forsterite lasers near room temperature(Optica Publishing Group, 1997) Department of Physics; Department of Physics; Sennaroğlu, Alphan; Yılmaz, Mehmet Burak; Faculty Member; Undergraduate Student; Department of Physics; College of Sciences; College of Sciences; 23851; N/ATheory of cw thermal loading in saturable absorbers with temperature-dependent lifetime is presented. Excellent agreement with experimental data is obtained with use of chromium-doped YAG saturable absorbers.Publication Metadata only Phase-space reconstruction in Hamiltonian systems through multiple time series(Elsevier Science Bv, 1997) Department of Chemistry; Atay, Fatihcan; Yurtsever, İsmail Ersin; Faculty Member; Faculty Member; Department of Chemistry; College of Sciences; 253074A method which combines the various time series originating from a single source in order to reconstruct the phase-space dynamics is applied to a chaotic Hamiltonian system. It is shown that for a large energy range the variation of the maximum Lyapunov exponents can be reproduced more accurately than the results obtained from a single time series, for both clean and noisy signals. Especially in cases where the maximum exponent is close to zero, using multiple series can give better results, providing a more reliable way of detecting chaos.
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