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Publication Open Access Bistable behavior of a two-mode Bose-Einstein condensate in an optical cavity(Maik Nauka/Interperiodica Publishing, 2013) Safaei, S.; Tanatar, B.; Department of Physics; Müstecaplıoğlu, Özgür Esat; Faculty Member; Department of Physics; College of Sciences; 1674We consider a two-component Bose-Einstein condensate in a one-dimensional optical cavity. Specifically, the condensate atoms are taken to be in two degenerate modes due to their internal hyperfine spin degrees of freedom and they are coupled to the cavity field and an external transverse laser field in a Raman scheme. A parallel laser also excites the cavity mode. When the pump laser is far detuned from its resonance atomic transition frequency, an effective nonlinear optical model of the cavity-condensate system is developed under the discrete mode approximation (DMA), while matter-field coupling has been considered beyond the rotating wave approximation. By analytical and numerical solutions of the nonlinear dynamical equations, we examine the mean cavity field and population difference (magnetization) of the condensate modes. The stationary solutions of both the mean cavity field and normalized magnetization demonstrate bistable behavior under certain conditions for the laser pump intensity and matter-field coupling strength.Publication Open Access Collective excitations of a laser driven atomic condensate in an optical cavity(Maik Nauka/Interperiodica Publishing, 2013) Öztop, B.; Türeci, H. E.; Department of Physics; Müstecaplıoğlu, Özgür Esat; Faculty Member; Department of Physics; College of Sciences; 1674We theoretically examine collective excitations of an optically driven atomic Bose-Einstein condensate, coupled to a high-finesse optical cavity. This open system has been recently used for the experimental demonstration of the Dicke superradiance of cavity photons, which is simultaneously and mutually triggered by spontaneous breaking of translational symmetry of the condensate into a crystalline order. We first develop a Hartree-Fock mean field dynamical model of the physical system. Using this model, we compute the dynamics of the cavity photons, the condensate density profile and the Dicke phase transition diagram. Both the imaginary-time and real-time evolution methods are used in the calculations. Collective excitations are determined by the solving Bogoliubov-de Gennes equations. The spectrum, softening of the modes and energetic hierarchy of excitations are determined.
Publication Metadata only İnelastic neutron scattering peak in Zn substituted YBa2Cu3O7(Elsevier Science Bv, 2000) 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 are studied in the normal state using the framework of the Hubbard model. It is shown that the impurity scattering of the spin fluctuations with momentum transfers near 2k(F) could lead to a peak in Im chi (Q, omega) at omega = 2\mu\, where mu is the chemical potential. the results on the single-layer and the bilayer CuO2 models are compared with the neutron scattering data on Zn substituted YBa2Cu3O7.Publication Metadata only Low-threshold, 12MHz, multipass-cavity femtosecond Cr 4+: forsterite laser(IOP Publishing Ltd, 2009) Fujimoto, J. G.; N/A; Department of Physics; Çankaya, Hüseyin; Sennaroğlu, Alphan; Researcher; Faculty Member; Department of Physics; Graduate School of Sciences and Engineering; College of Sciences; N/A; 23851We report on the development of an extended-cavity, low-threshold femtosecond Cr4+:forsterite laser operating near 1270 nm. The resonator was end-pumped by a continuous-wave Yb-fiber laser at 1060 nm. The 195-MHz short cavity could be operated with absorbed threshold pump powers as low as about 300 mW. To scale up the pulse energy, the cavity length was increased by using a q-preserving multipass cavity (MPC) consisting of a flat and a curved (R = 4 m), notched high-reflector. Kerr-lens mode locking was used to generate femtosecond pulses and double-chirped mirrors were included in the cavity for dispersion compensation. With an absorbed pump power of 1250 mW, the mode-locked laser produced 4 nJ of pulse energy at an average output power of only 47 mW. The corresponding repetition rate of the extended cavity was 11.7 MHz. The pulse-width was 90 fs and the spectral bandwidth was 23 nm, corresponding to a time-bandwidth product of 0.37.Publication Metadata only Nonlinear dynamics of a spinor bose-einstein condensate in a double-well potential(Institute of Physics (IOP) Publishing, 2006) Zhang, M.; You, L.; Department of Physics; Müstecaplıoğlu, Özgür Esat; Faculty Member; Department of Physics; College of Sciences; 1674We examine the nonlinear dynamical behavior of a spinor Bose-Einstein condensate in a double-well potential. Considering a condensate with large number of atoms, such that it can be described using the mean field theory, we separate the spinor dynamics from the spatial dynamics under the single-mode approximation. We limit ourselves to certain initial conditions under which the spatial mode is frozen so that we can focus on the spinor dynamics only. Identifying collective spin variables of our system, we derive the corresponding nonlinear equations of motion for them. Employing standard stability analysis, we find and characterize fixed points of the system. For a wide range of physical parameters such as tunneling strength and nonlinear interactions, as well as for various initial preparations of the system, we identify qualitatively different dynamical regimes possible in the system. In particular, complete and incomplete oscillations of spin variables between quantum wells are found. We also show that by bringing some fixed points close to each other in the phase space of the system, it is possible to induce amplitude modulation to those otherwise regular tunneling oscillations.Publication Metadata only Quantum correlations of spin-1 atoms in an optical lattice(Institute of Physics (IOP) Publishing, 2009) Oztop, B.; Oktel, M. O.; Department of Physics; Müstecaplıoğlu, Özgür Esat; Faculty Member; Department of Physics; College of Sciences; 1674In this work, we investigate the system of cold spin-1 atoms in a one dimensional optical lattice in relation with squeezing and entanglement. By using the corresponding Bose-Hubbard Hamiltonian, both superfluid and Mott-insulator phases are studied by using numerical methods in the mean-field approximation. To observe the presence of entanglement, we used a squeezing measure as a criterion for quantum correlations. We further investigate the two interaction regimes, namely ferromagnetic and antiferromagnetic in the case of zero and nonzero but very small angle between the counterpropagating laser beams that form the optical lattice. States in the superfluid phase are calculated analytically by using the perturbation theory.Publication Metadata only Resistive hydrogen sensors based on nanostructured metals and metal alloys(Amer Scientific Publishers, 2013) N/A; Department of Electrical and Electronics Engineering; Kılınç, Necmettin; Researcher; Department of Electrical and Electronics Engineering; College of Engineering; 59959Hydrogen (H-2), as a renewable energy source, has numerous applications such as chemical production, fuel cell technology, rocket engines, fuel for cars etc. The detection of H-2 is so important in safety issue due to the flammable and explosive properties of H-2 gas, in a H-2 source for leak detection and in H-2 production process because of real-time quantitative analysis of production. This paper reviews resistive type H-2 sensor based on palladium (Pd), platinum (Pt) and their alloy nano-structures in the forms of thin films, nanoporous films, nanowires, nanoparticles, nanotubes, etc. The sensing mechanism of the nanostructured Pd and Pt resistive sensor is discussed in separated section. Nanostructured Pd sensors show a decrease or an increase in their resistance towards H-2 gas depending on continuity of the nanostructure and will be examined in two parts: discontinuous (nano-gap based) and continuous Pd and Pd alloy nanostructure sensors. on the contrary to Pd nanostructure sensor, nanostructured Pt sensors require oxygen (O-2) to operate. There are limited numbers of publications about nanostructured Pt and Pt alloy sensors, so further investigation are needed to well understand sensing mechanism of the Pt sensors.Publication Metadata only Strain modulated band gaps of semiconducting zigzag single walled carbon nanotubes(Natl Inst Optoelectronics, 2015) Eyecioğlu, Önder.; Mısırlıoğlu, Banu Süngü; Department of Physics; Dereli, Gülay; Other; Department of Physics; College of Sciences; N/AStrain can alter the electronic properties of materials. At the nanoscale, small displacements of atoms could have large effects. In this study, we have examined how elastic strain can modify the energy band gaps of semiconducting zigzag Single Walled Carbon Nanotubes (SWCNTs). The electronic structure of SWCNTs have been computed for each deformed configurations by means of real space, Order(N) Tight Binding Molecular Dynamic (O(N) TBMD) simulations. During the applications of uniaxial strain, carbon atoms are moved slightly from their equilibrium positions, but their atomic bonds are not broken. Three different kinds of semiconducting zigzag SWCNTs are chosen. (12,0) SWCNT, although a semiconducting SWCNT, is quasi-metallic in its pristine state. Application of stretching and compression opens its band gap. Thus under strain (12,0) SWCNT shows metallic-semiconducting transitions. (13,0) and (14,0) zigzag SWCNTs are semiconductors having energy band gap values of 0.44eV and 0.55eV in their pristine state. The energy band gap of (13,0) SWCNT decreases with increasing absolute value of compression. On the other hand, the energy band gap of (14,0) SWCNT decreases with increasing value of tension. So in both cases, the energy band gap closes and semiconducting metallic transitions are observed. Flexibilities of the stretched hexagonal network of SWCNTs are displayed in terms of carbon-carbon bond-lengths, bond-angles and radial distribution functions. Correlations between the strain induced structural changes and the electronic properties of SWCNTs are discussed.Publication Metadata only Vortex lattice of a Bose-Einstein condensate as a photonic band gap material(Institute of Physics (IOP) Publishing, 2009) Tasgin, M. E.; Oktel, M. O.; Department of Physics; Müstecaplıoğlu, Özgür Esat; Faculty Member; Department of Physics; College of Sciences; 1674Photonic crystal behavior of a rotating Bose-Einstein condensate with a triangular vortex lattice is reviewed and a scheme for getting much wider band gaps is proposed. It is shown that photonic band gaps can be widened an order of magnitude more by using a Raman scheme of index enhancement, in comparison to previously considered upper level microwave scheme.