Browsing by Subject "Dispersion"

Now showing 1 - 12 of 12

Open Access
An analytical approach to the design of multiple mode rectangular cavity waveguide filters
(Institute of Electrical and Electronics Engineers Inc., 2017) Kelleci, C.; Atalar, Abdullah
The multiple mode rectangular cavity structure with square corner cuts is revisited. An attempt to predict the physical dimensions of the cavity for dual mode second-order and triple mode third-order filters is made. Analytic expressions are formed to be used in the design process. The classical triple mode cavity filter structure is altered to give a finite frequency transmission zero either in the lower or upper sideband of the center frequency. The concept is illustrated with example designs. A novel additive manufacturing technique is used to fabricate a selected filter structure. The experimental results are in agreement with the expectations.
Open Access
Application of a mode-locked fiber laser for highly time resolved broadband absorption spectroscopy and laser-assisted breakdown on micro-plasmas
(IOP Publishing, 2012-05-29) Niermann, B.; Budunoglu, I. L.; Gurel, K.; Boke, M.; Ilday, F. O.; Winter, J.
Absorption spectroscopy is known to be a powerful tool to gain spatially and temporally resolved information on excited and reactive species in a plasma discharge. Furthermore, the interaction of the discharge with short intense laser pulses can trigger the ignition and the transition into other transient states of the plasma. In this context laser-assisted 'pump-probe' experiments involving simultaneously generated supercontinuum radiation yield highly temporally resolved and spatially well-defined information on the transient phenomena. In this paper we demonstrate the possibility for 'pump-probe' experiments by initiating breakdown on a picosecond time scale ('pump') with a high-power beam and measuring the broadband absorption with the simultaneously provided supercontinuum ('probe'). Since both pulses are generated from the same mode-locked master oscillator, they have a strong level of synchronization.
Open Access
Crossed chiral band approximation for wide-band self-collimation of light
(Institute of Physics Publishing, 2020) Gümüş, M. A.; Akçaalan, Önder; Kurt, H.
We propose a perspective to the evaluation of the wide bandwidth phenomenon for low-symmetric photonic structures, by introducing the band tailoring and chiral band approximation on the self-collimation effect. In the case of the crossing of the bands, we claim the excitation of the lower mode can provide the utilization of the entire bandwidth by suppressing the intersection regions where the frequencies tend to mix. Thereby, we design broadband self-collimation capable, defect-free photonic structures and examine their performances. A fractional bandwidth of 0.35 (a/λ = 0.429 − 0.607) and 0.37 (a/λ = 0.481 − 0.701) are achieved for radii of r = 0.25a and r = 0.23a, respectively. We explore the full-range collimation by using transmission and E-field intensity analyses in addition to band diagrams and group velocity dispersions. Moreover, we indicate all-angle collimation validity even for highly tilted sources up to an angle of 80◦ .
Open Access
Dispersion irrelevant wideband asymmetric transmission in dielectric photonic crystal gratings
(Optical Society of America, 2012-11-22) Serebryannikov, A. E.; Colak, E.; Cakmak, A. O.; Özbay, Ekmel
Wideband suppression of zero order and relevant strongly asymmetric transmission can be obtained in photonic crystal gratings that are made of linear isotropic materials and show the broken structural (axial) symmetry, even if zero diffraction order may be coupled to a Floquet-Bloch (FB) wave at the incidence and exit interfaces. The studied mechanism requires that the peculiar diffractions at the corrugated exit interface inspire strong energy transfer to higher orders, including those not coupled to an FB wave. At the opposite direction of incidence, transmission due to zero and some higher orders that may be coupled at the corrugated input interface can vanish. This leads to the alternative scenario of wideband unidirectional transmission, which itself does not need but can coexist with the other scenario based on the merging of asymmetric diffraction and dispersion of the FB mode.
Open Access
Dispersion of multi-walled carbon nanotubes in an aqueous medium by water-dispersible conjugated polymer nanoparticles
(2010) Baykal, B.; Ibrahimova, V.; Er, G.; Bengü, E.; Tuncel, D.
Vertically aligned multi-walled carbon nanotubes (MWCNTs) synthesized by the alcohol catalytic CVD (ACCVD) technique are dispersed in water with the aid of water-dispersible conjugated polymer nanoparticles (CPNs). The interactions between CPNs and CNTs are studied with spectroscopy (UV-Vis, fluorescence and Raman) and electron microscopy techniques are used to confirm attachment of CPNs to the CNT sidewalls.
Open Access
Dynamic correlation effects on the plasmon dispersion in a two-dimensional electron gas
(The American Physical Society, 2003) Yurtsever, A.; Moldoveanu, V.; Tanatar, Bilal
The charge-density oscillations (plasmons) of a low-density two-dimensional uniform electron gas are studied within the framework of finite temperature and frequency dependent (dynamic) version of Singwi, Tosi, Land, and Sjölander theory and compared with the recent experimental results. The use of the Hartree-Fock approximation for the static structure factor leads to a finite temperature dynamical counterpart of the static Hubbard approximation. We observe important differences between dynamic and static local-field factors as well as between the corresponding plasmon dispersion laws. Our calculated plasmon energies that include dynamic correlations are in very-good agreement with the recent experimental results.
Open Access
Dynamic correlations in double-layer electron systems
(The American Physical Society, 2001) Tanatar, Bilal; Davoudi, B.
We study the effects of dynamic correlations on the ground-state properties of a double-layer two-dimensional electron gas within the quantum Singwi-Tosi-Land-Sjölander theory (STLS). The intralayer and interlayer static structure factors, the pair-correlation functions, and the wave vector and frequency-dependent local-field factors have been calculated for a range of electron densities and layer separations. We find that the local-field factors have an oscillatory frequency dependence and the magnitude of interlayer local-field factors is about an order of magnitude smaller than that of the intralayer. Our results are compared with the random-phase approximation and the static STLS approximation to assess the importance of dynamical correlations. We also calculate the dispersion relations for the optical and acoustic plasmons and the damping of these modes to compare them with other mean-field theories, and we comment on the relevance of our results to the recent experiments.
Open Access
Improving the accuracy of a time lens
(Optical Society of America, 1997-11) Yurchenko, V. B.
A method for improving the accuracy of temporal imaging with an imperfect time lens is proposed. Signal distortion arising from complicated dispersion of the delay lines can be reduced considerably by appropriate choice of the phase-modulation function including the second harmonic of the basic modulation frequency and a specific phase shift of the modulation with respect to the main signal. The method is of particular interest for picosecond and femtosecond optical pulse generation.
Open Access
Numerical study on the dispersion and deposition of particles in evaporating sessile droplets
(2022-09) Erdem, Ali Kerem
Evaporating sessile droplets including dispersed particles are utilized in the coating, printing, and biomedical applications. Modeling this problem is a challenging process, therefore diﬀerent assumptions are used in the literature. It is important to have a model which covers both pinned and moving contact line regimes for the droplet, thus whole evaporation process and deposition proﬁle can be understood. Therefore, in this work, a numerical and mathematical model is derived to simulate two-dimensional symmetric thin evaporating sessile droplets whose contact line is ﬁrstly pinned and then moving. This model is derived by combining diﬀerent models in literature with the help of lubrication theory and rapid vertical diﬀusion assumption. This model includes a temporal change in the droplet’s surface height, contact line dynamics, particle dispersion, and deposition. The ﬁnite diﬀerence method is used in the numerical solution. Cases including pinned and moving contact lines in the literature are solved separately by diﬀerent numerical algorithms developed in this work and these algorithms were combined. This new algorithm ﬁrst solves a mathematical model in the pinned contact line regime. When the contact angle goes below the deﬁned limit, the second part of the algorithm solves the mathematical model in the moving contact line regime until 95 percent of the total particle mass is deposited. A parametric study has been done with the developed algorithm. A set of parameters is deﬁned and chosen parameters are changed to see their eﬀects. It is observed that increasing the Marangoni number and Capillary number, increased particle accumulation near the center. Decreasing evaporation number and increasing Damkohler number result in more uniform particle deposition.
Open Access
Spatio-temporal evolution of ultrashort pulses in graded-index multimode fiber at normal dispersion
(IEEE, 2017) Teğin, Uğur; Ortaç, Bülend
Summary form only given. A novel nonlinear phenomenon at normal dispersion regime inside of graded-index multimode fiber (GRIN MMF), geometric parametric instability (GPI), can be observed while propagating beam experiences spatio-temporal evolution. Longhi theoretically predicted this GPI and associated with periodic refocusing inside GRIN MMF [1]. Later his work is verified by Krupa et al. [2] and Lopez-Galmiche et al. [3] and discrete GPI sidebands are reported with more than 120 THz detuning from to pump frequency. These studies focused on quasi-continues pulse evolution in GRIN MMF and the spatio-temporal evolution of femtosecond pulses at normal dispersion regime remains unknown. Here, we present the first experimental observation of GPI sidebands with ultrashort pulses in GRIN MMF. To gain detailed understanding, we perform numerical simulations and theoretical calculations and obtained results are well-aligned with experimental observations.
Open Access
Varentropy decreases under the polar transform
(Institute of Electrical and Electronics Engineers Inc., 2016) Arıkan, E.
We consider the evolution of variance of entropy (varentropy) in the course of a polar transform operation on binary data elements (BDEs). A BDE is a pair (X,Y) consisting of a binary random variable X and an arbitrary side information random variable Y. The varentropy of (X,Y) is defined as the variance of the random variable-log pX|Y(X|Y). A polar transform of order two is a certain mapping that takes two independent BDEs and produces two new BDEs that are correlated with each other. It is shown that the sum of the varentropies at the output of the polar transform is less than or equal to the sum of the varentropies at the input, with equality if and only if at least one of the inputs has zero varentropy. This result is extended to polar transforms of higher orders and it is shown that the varentropy asymptotically decreases to zero when the BDEs at the input are independent and identically distributed.
Open Access
Volume based texture mapping
(1995) Salk, Gürkan
'I'he most realistic and attractive computer generated images are usually those that contain a large arnount of visual complexity and detail. Texturing is a widely used way of adding visual complexity and detail to computer generated iniciges. Traditionally surface texturing was only used to simulate surface detail. In this thesis we generate textures dehiuid throughout a region of three-dimensional space and map those textures together with their geometric definition onto complex objects. The textured object is rendered volume based with a. backward mapping algorithm (ray tracing). Hence the texture aJfects the definition and the realism of the object. In rendering tlie scene, natural phenomena such cis dispersion and absorption of light is also incorporated.