Research Outputs
Permanent URI for this communityhttps://hdl.handle.net/20.500.14288/2
Browse
16 results
Search Results
Publication Metadata only Analysis of an optical biosensor based on elastic light scattering from diamond-, glass-, and sapphire microspheres(Wiley-V C H Verlag Gmbh, 2012) Murib, Mohammed S.; Tran, Anh Quang; De Ceuninck, Ward; Schöning, Michael J.; Nesladek, Milos; Wagner, Patrick; Department of Physics; Serpengüzel, Ali; Faculty Member; Department of Physics; College of Sciences; 27855Deoxyribonucleic acid (DNA) and protein recognition are now standard tools in biology. In addition, the special optical properties of microsphere resonators expressed by the high quality factor (Q-factor) of whispering gallery modes (WGMs) or morphology dependent resonances (MDRs) have attracted the attention of the biophotonic community. Microsphere-based biosensors are considered as powerful candidates to achieve label-free recognition of single molecules due to the high sensitivity of their WGMs. When the microsphere surface is modified with biomolecules, the effective refractive index and the effective size of the microsphere change resulting in a resonant wavelength shift. The transverse electric (TE) and the transverse magnetic (TM) elastic light scattering intensity of electromagnetic waves at 600 and 1400?nm are numerically calculated for DNA and unspecific binding of proteins to the microsphere surface. The effect of changing the optical properties was studied for diamond (refractive index 2.34), glass (refractive index 1.50), and sapphire (refractive index 1.75) microspheres with a 50 mu m radius. The mode spacing, the linewidth of WGMs, and the shift of resonant wavelength due to the change in radius and refractive index, were analyzed by numerical simulations. Preliminary results of unspecific binding of biomolecules are presented. The calculated shift in WGMs can be used for biomolecules detection.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 Comparison of the NMR and neutron scattering experiments on cuprates with nonmagnetic impurities(World Scientific Publ Co Pte Ltd, 2000) Department of Physics; Department of Physics; Bulut, Nejat; Yaz, Ümit; Faculty Member; Undergraduate Student; Department of Physics; College of Sciences; College of Sciences; 4963; N/AThe NMR and neutron scattering experiments on YBa2Cu3O7 with U and Zn impurities are compared using a simple framework exhibiting short-range antiferomagnetic correlations. The model parameters are determined by fitting the NMR rate T-2(-1) of the pure system, and the effective impurity potential is approximated by a static extended form. The results of the calculations are compared with the experimental data, and their implications are discussed.Publication Metadata only Coupled cavity DQW semiconductor lasers(Elsevier Science Sa, 2000) Sağol, B.E.; Avrutin, Eugene A.; De la Rue, R.M.; Laybourn, P.J.R.; Stanley, C.R.; Department of Physics; Serpengüzel, Ali; Faculty Member; Department of Physics; College of Sciences; 27855Coupled cavity effects has been observed in the electroluminescence spectra of monolithic GaAs/GaAlAs double quantum well graded index separate confinement heterostructure semiconductor diode lasers. A time domain analysis has been performed in order to simulate the experimentally observed results. The theoretically calculated spectra are in good agreement with the experimentally observed spectra.Publication Metadata only D=3 anisotropic and d=2 tj models: phase diagrams, thermodynamic properties, and chemical potential shift(Springer, 2006) Hinczewski, M.; Department of Physics; Berker, Ahmet Nihat; Faculty Member; Department of Physics; College of Sciences; 179795The anisotropic d=3 tJ model is studied by renormalization-group theory, yielding the evolution of the system as interplane coupling is varied from the isotropic three-dimensional to quasi-two-dimensional regimes. Finite-temperature phase diagrams, chemical potential shifts, and in-plane and interplane kinetic energies and antiferromagnetic correlations are calculated for the entire range of electron densities. We find that the novel tau phase, seen in earlier studies of the isotropic d=3 tJ model, persists even for strong anisotropy. While the tau phase appears at low temperatures at 30-35% hole doping away from [n(i)]=1, at smaller hole dopings we see a complex lamellar structure of antiferromagnetic and disordered regions, with a suppressed chemical potential shift, a possible marker of incommensurate ordering in the form of microscopic stripes. An investigation of the renormalization-group flows for the isotropic two-dimensional tJ model also shows a clear pre-signature of the tau phase, which in fact appears with finite transition temperatures upon addition of the smallest interplane coupling.Publication Metadata only Dx2-y2 superconductivity and the Hubbard model(Taylor & Francis, 2002) Department of Physics; Bulut, Nejat; Faculty Member; Department of Physics; College of Sciences; 4963The numerical studies of d(x2-y2)-wave pairing in the two-dimensional (2D) and the 2-leg Hubbard models are reviewed. For this purpose, the results obtained from the determinantal Quantum Monte Carlo and the Density-Matrix Renormalization-Group calculations are presented. These are calculations which were motivated by the discovery of the high-T-c cuprates. In this review, the emphasis is placed on the microscopic many-body processes which are responsible for the d(x2-y2)-wave pairing correlations observed in the 2D and the 2-leg Hubbard models. In order to gain insight into these processes, the results on the effective pairing interaction as well as the magnetic, density and the single-particle excitations will be reviewed. In addition, comparisons will be made with the other numerical approaches to the Hubbard model and the numerical results on the t-J model. The results reviewed here indicate that an effective pairing interaction which is repulsive at (pi,pi) momentum transfer, and enhanced single-particle spectral weight near the (pi,0) and (0,pi) points of the Brillouin zone, create optimum conditions for d(x2)-(y2)-wave pairing. These are two effects which act to enhance the d(x2-y2)-wave pairing correlations in the Hubbard model. Finding additional ways is an active research problem.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 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 Open Access Optimization of room-temperature continuous-wave cr (4+)-Doped solid-state lasers: experiment and modeling(Institute of Electrical and Electronics Engineers (IEEE), 2001) Department of Physics; Sennaroğlu, Alphan; Faculty Member; Department of Physics; College of Sciences; 23851A novel study to determine the optimum crystal and resonator parameters for Cr4+ doped lasers subjected to lifetime thermal loading was permormed. The comparison of the results of lasing threshold, pump absorption saturation, and power efficiency measurements was done with the predictions of a theoretical model to determine the laser cross sections. Using the best-fit cross-section values, numerical optimization studies were carried out to determine the optimum crystal absorption, resonator reflectivity, and crystal length which maximized the output power.Publication Metadata only Optofluidic lasers with aqueous quantum dots(Amer Chemical Soc, 2015) Chen, Qiushu; Fan, Xudong; Department of Physics; Kiraz, Alper; Faculty Member; Department of Physics; College of Sciences; 22542We achieved two types of laser emissions from aqueous quantum dots (Qps) using the same high-Q:factor optofluidic ring resonator (OFRR) platform. In the first type, 2 mu M QDs were in bulk buffer solution that filled the entire OFRR cavity volume. The lasing threshold was 0.1 mu J/mm(2), over 3 orders of magnitude lower than the state-of-the-art. In the second type of laser, the QDs were immobilized as a single layer on the interface between the OFRR inner wall and buffer solution with a surface density as low as 3 X 10(9)-10(10) cm(-2). The lasing threshold of 60 mu J/mm(2) was achieved. In both bulk solution and single-layer lasing cases, the laser emission persisted even under 5-10 min of uninterrupted pulsed optical excitation that was well above the corresponding lasing threshold, indicative of high photostability of the QD laser. This was in sharp contrast to organic-dye-based lasers, which underwent quick photobleaching during the laser operation under similar pumping conditions. Theoretical analysis is also carried out to elucidate the advantages of QD-based optofluidic lasers over those based on dyes. Our work opens the door to a plethora of applications where optofluidic QD lasers can replace dye-based optofluidic lasers in biosensing and on-chip miniaturized laser development.