Browsing by Subject "Optics"
Now showing 1 - 20 of 27
Results Per Page
Sort Options
Item Open Access 83 W, 1 ns, 3.1 MHz all-fiber laser for micromachining(IEEE, 2011) Özgören, Kıvanç; Öktem, Bülent; Yılmaz, Sinem; İlday, F. Ömer; Pasin, E.; Eken, K.Fiber lasers are commonly used for various material processing applications. The advantages (such as simplicity of the system, high material removal rate) and disadvantages (larger heat-affected zone, reduced precision) of nanosecond pulses over sub-picosecond pulses are well known. © 2011 IEEE.Item Open Access About the Wigner distribution of a graded index medium and the fractional fourier transform operation(SPIE, 1993-08) Özaktaş, Haldun M.; Mendlovic, D.; Lohmann, A. W.Upon propagation through quadratic graded index media, the Wigner distribution of the wavefunction of light rotates uniformly. As a consequence, a definition of fractional Fourier transforms based on rotating the functions Wigner distribution, and another based on propagation through graded index media, are equivalent.Item Open Access Beyond Nyquist sampling: a cost-based approach(Optical Society of America, 2013) Özçelikkale, A.; Özaktaş, Haldun M.A sampling-based framework for finding the optimal representation of a finite energy optical field using a finite number of bits is presented. For a given bit budget, we determine the optimum number and spacing of the samples in order to represent the field with as low error as possible. We present the associated performance bounds as trade-off curves between the error and the cost budget. In contrast to common practice, which often treats sampling and quantization separately, we explicitly focus on the interplay between limited spatial resolution and limited amplitude accuracy, such as whether it is better to take more samples with lower amplitude accuracy or fewer samples with higher accuracy. We illustrate that in certain cases sampling at rates different from the Nyquist rate is more efficient.Item Open Access Directional selectivity through the subwavelength slit in metallic gratings(IEEE, 2011) Çakmakyapan, Semih; Çaglayan, Hümeyra; Serebryannikov, Andriy; Özbay, EkmelAn approach for obtaining strong directional selectivity through a single subwavelength slit in non-symmetric metallic gratings is shown theoretically and experimentally. Directionality effect originates from the different resonance frequencies of two interfaces. © 2011 OSA.Item Open Access Effect of spatial distribution of partial information on the accurate recovery of optical wave fields(Optical Society of America, 2017) Oktem, F. S.; Özaktaş, Haldun M.We consider the problem of recovering a signal from partial and redundant information distributed over two fractional Fourier domains. This corresponds to recovering a wave field from two planes perpendicular to the direction of propagation in a quadratic-phase multilens system. The distribution of the known information over the two planes has a significant effect on our ability to accurately recover the field. We observe that distributing the known samples more equally between the two planes, or increasing the distance between the planes in free space, generally makes the recovery more difficult. Spreading the known information uniformly over the planes, or acquiring additional samples to compensate for the redundant information, helps to improve the accuracy of the recovery. These results shed light onto redundancy and information relations among the given data for a broad class of systems of practical interest, and provide a deeper insight into the underlying mathematical problem.Item Open Access Electron momentum and energy relaxation rates in GaN and AlN in the high-field transport regime(The American Physical Society, 2003) Bulutay, C.; Ridley, B. K.; Zakhleniuk, N. A.Momentum and energy relaxation characteristics of electrons in the conduction band of GaN and AlN are investigated using two different theoretical approaches corresponding to two high electric-field regimes, one up to 1-2 MV/ cm values for incoherent dynamics, and the other at even higher fields for coherent dynamics where semiballistic and ballistic processes become important. For the former, ensemble Monte Carlo technique is utilized to evaluate these rates as a function of electron energy up to an electric-field value of 1 MV/cm (2 MV/cm) for GaN (AlN). Momentum and energy relaxation rates within this incoherent transport regime in the presence of all standard scattering mechanisms are computed as well as the average drift velocity as a function of the applied field. Major scattering mechanisms are identified as polar optical phonon (POP) scattering and the optical deformation potential (ODP) scattering. Roughly, up to fields where the steady-state electron velocity attains its peak value, the POP mechanism dominates, whereas at higher fields ODP mechanism takes over. Next, aiming to characterize coherent dynamics, the total out-scattering rate from a quantum state (chosen along a high-symmetry direction) due to these two scattering mechanisms are then computed using a first-principles full-band approach. In the case of POP scattering, momentum relaxation rate differs from the total out-scattering rate from that state; close to the conduction-band minimum, momentum relaxation rate is significantly lower than the scattering rate because of forward-scattering character of the intravalley POP emission., However, close to the zone boundary the difference between these two rates diminishes due to isotropic nature of intervalley scatterings. Finally, a simple estimate for the velocity-field behavior in the coherent transport regime is attempted, displaying a negative differential mobility due to the negative band effective mass along the electric-field direction.Item Open Access Fast and accurate algorithms for quadratic phase integrals in optics and signal processing(SPIE, 2011) Koç, A.; Özaktaş, Haldun M.; Hesselink L.The class of two-dimensional non-separable linear canonical transforms is the most general family of linear canonical transforms, which are important in both signal/image processing and optics. Application areas include noise filtering, image encryption, design and analysis of ABCD systems, etc. To facilitate these applications, one need to obtain a digital computation method and a fast algorithm to calculate the input-output relationships of these transforms. We derive an algorithm of NlogN time, N being the space-bandwidth product. The algorithm controls the space-bandwidth products, to achieve information theoretically sufficient, but not redundant, sampling required for the reconstruction of the underlying continuous functions. © 2011 SPIE.Item Open Access Feature issue of digital holography and 3D imaging (DH) introduction(Optical Society of America (OSA), 2014-07) Hayasaki, Y.; Zhou, C.; Popescu, G.; Onural, LeventThe OSA Topical Meeting "Digital Holography and 3D Imaging (DH)," was held in Seattle, Washington, July 13-17, 2014. Feature issues based on the DH meeting series have been released by Applied Optics (AO) since 2007. This year Optics Express (OE) and AO jointly decided to have one such feature issue in each journal. The DH meeting will continue in the future, as expected, and the next meeting is scheduled to be held on 24-28 May 2015, in Shanghai Institute of Optics and Fine Mechanics, Shanghai, China. © 2014 Optical Society of AmericaItem Open Access Förster-type nonradiative energy transfer directed from colloidal quantum dots to epitaxial quantum wells for light harvesting applications(Optical Society of America, 2011) Nizamoğlu, Sedat; Sarı, Emre; Baek J.-H.; Lee I.-H.; Demir, Hilmi VolkanWe report on Frster-type nonradiative energy transfer directed from CdSe/ZnS core/shell quantum dots to InGaN/GaN quantum wells with 69.6% efficiency at 1.527 ns-1 rate at room temperature for potential light harvesting and solar cells applications. © 2011 OSA.Item Open Access Fractional Fourier transforms and their optical implementation: I(Optical Society of America, 1993) Mendlovic, D.; Özaktaş, Haldun M.Fourier transforms of fractional order a are defined in a manner such that the common Fourier transform is a special case with order a = 1. An optical interpretation is provided in terms of quadratic graded index media and discussed from both wave and ray viewpoints. Several mathematical properties are derived.Item Open Access Grating coupler integrated photodiodes for plasmon resonance based sensing in fluidic systems(IEEE, 2011) Türker, Burak; Güner, Hasan; Ayaş, Sencer; Ekiz, Okan O.; Acar, Handan; Güler, Mustafa O.; Dâna, AykutluWe demonstrate an integrated sensor combining a grating-coupled plasmon resonance surface with a planar photodiode. Plasmon enhanced transmission is employed as a sensitive refractive index (RI) sensing mechanism and monitored via the integrated photodiode. © 2011 OSA.Item Open Access How can we use nanophotonics to help in combating climate change?(IEEE, 2008-08) Demir, Hilmi VolkanThe paper is about emerging technologies enabled by nanophotonics to combat climate change. We developed and demonstrated 1.) nanocrystal hybridized white light sources and those also embedded with plasmonic metal nanoparticles for the reduction of global energy consumption, 2.) photovoltaic nanocrystal scintillators hybridized on solar cells for the generation of efficient green energy sources, and 3.) photocatalytic nanoparticle and nanocomposite systems for massive environmental decontamination, all to contribute to the reduction of greenhouse gas amount.Item Open Access Image representation and compression with the fractional Fourier transform(Elsevier, 2001-04-01) Yetik, İ. Ş.; Kutay, M. A.; Özaktaş, Haldun M.We discuss the application of fractional Fourier transform-based filtering configurations to image representation and compression. An image can be approximately represented (and stored or transmitted) as the coefficients of the minimum mean square filtering configuration approximating the image matrix. An order of magnitude compression is possible with moderate errors with the raw method. While inferior to commonly available compression algorithms, the results presented correspond to the basic method without any refinement or combination with other techniques, suggesting that the approach may hold promise for future development. Regardless of its practical usefulness, the fact that the information inherent in an image can be decomposed or factored into fractional Fourier domains is of considerable conceptual significance. The information contained in the image is distributed to the different domains in an unequal way, making some domains more dispensible than others in representing the image. (C) 2001 Published by Elsevier Science B.V.Item Open Access Infrared digital holography applications for virtual museums and diagnostics of cultural heritage(SPIE, 2011) Paturzo, M.; Pelagotti, A.; Geltrude, A.; Locatelli, M.; Poggi P.; Meucci, R.; Ferraro P.; Stoykova, E.; Yaraş F.; Yöntem, A. Özgür; Kang H.; Onural, LeventInfrared digital holograms of different statuettes are acquired. For each object, a sequence of holograms is recorded rotating the statuette with an angular step of few degrees. The holograms of the moving objects are used to compose dynamic 3D scenes that, then, are optically reconstructed by means of spatial light modulators (SLMs) using an illumination wavelength of 532 nm. This kind of reconstruction allows to obtain a 3D imaging of the statuettes that could be exploited for virtual museums. © 2011 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).Item Open Access Linear algebraic theory of partial coherence: discrete fields and measures of partial coherence(SPIE, 2003-08) Özaktaş, Haldun M.; Yüksel, S.; Kutay, M. A.We present a linear algebraic theory of partial coherence which allows precise mathematical definitions of concepts such as coherence and incoherence. This not only provides new perspectives and insights, but also allows us to employ the tools of linear algebra in applications. We define a scalar measure of the degree of partial coherence of an optical field which is zero for complete incoherence and unity for full coherence.Item Open Access Memory-efficient multilevel physical optics algorithm for fast computation of scattering from three-dimensional complex targets(IEEE, 2007) Manyas, Alp; Gürel, LeventMultilevel physical optics (MLPO) algorithm provides a speed-up for computing the physical-optics integral over complex bodies for a range of aspect angles and frequencies. On the other hand, when computation of the RCS pattern as a function of θ, φ, and frequency is desired, the O N3 memory complexity of the algorithm may prevent the solution of electrically large problems. In this paper, we propose an improved version of the MLPO algorithm, for which the memory complexity is reduced to O N2 log N . The algorithm is based on the aggregation of only some portion of the scattering patterns at each aggregation step. This way, memory growth in each step is prevented, and a significant amount of saving is achieved.Item Open Access Negative phase advance in polarization independent, multi-layer negative-index metamaterials(Optical Society of American (OSA), 2008) Aydın, Koray; Zhaofeng, Li; Şahin, Levent; Özbay, EkmelWe demonstrate a polarization independent negative-index metamaterial (NIM) at microwave frequencies. Transmission measurements and simulations predict a left-handed transmission band with negative permittivity and negative permeability. A negative-index is verified by using the retrieval procedure. Effective parameters of single-layer and twolayer NIMs are shown to be different. Negative phase advance is verified within the negative-index regime by measuring the phase shift between different sized negative-index metamaterials. Backward wave propagation is observed in the numerical simulations at frequencies where the phase advance is negative. ©2008 Optical Society of America.Item Open Access Negative refraction and subwavelength focusing using left-handed composite metamaterials(SPIE, 2008-01) Özbay, Ekmel; Aydın, KorayWe review experimental studies performed on left-handed metamaterials (LHM) at microwave frequencies. The metamaterial structure is composed of periodic arrays of split-ring resonators and wire meshes and exhibits a left-handed propagation band at frequencies of negative permittivity and negative permeability. Negative refraction is verified using prism shaped LHM and also by beam-shifting method. Subwavelength focusing of a point source is achieved with a resolution of 0.13λ, through a flat LHM superlens.Item Open Access On the elemination of infinities in the PO component of equivalent edge currents(Elsevier, 1993) Oguzer, T.; Altıntaş, A.; Büyükdura, O. M.Equivalent edge currents for the Physical Optics (PO) contribution to the edge diffraction are derived by choosing a proper coordinate system for a half plane. It is seen that these expressions for the PO component are free from singularities except at shadow and reflection boundaries on the Keller Cone. To find the total diffracted field, the expressions for the fringe contribution given earlier by Michaeli should be used along with the PO contribution obtained here. The procedure is applied to the problems of backscattering from square and triangular plates. Higher order diffractions are also accounted for by combining the equivalent currents with the Uniform Theory of Diffraction (UTD). The results obtained are compared with previous solutions and measured data.Item Open Access Optimal Filtering with Linear Canonical Transformations(Elsevier BV * North-Holland, 1997-02-01) Barshan, B.; Kutay, M. A.; Özaktaş, Haldun M.Optimal filtering with linear canonical transformations allows smaller mean-square errors in restoring signals degraded by linear time- or space-variant distortions and non-stationary noise. This reduction in error comes at no additional computational cost. This is made possible by the additional flexibility that comes with the three free parameters of linear canonical transformations, as opposed to the fractional Fourier transform which has only one free parameter, and the ordinary Fourier transform which has none. Application of the method to severely degraded images is shown to be significantly superior to filtering in fractional Fourier domains in certain cases.