Browsing by Subject "Dispersion (waves)"

Now showing 1 - 13 of 13

Open Access
Conversion from constitutive parameters to dispersive transmission line parameters for multi-band metamaterials
(Taylor and Francis Ltd., 2016) Ozturk, Y.; Yilmaz, A. E.; Özbay, Ekmel
In this study, we explain an approach including conversion from constitutive parameters to dispersive transmission line parameters using the double-band DNG (double-negative) properties of the circular type fishnet metamaterials. After designing the metamaterial structure, the numerical calculations and the composite right/left-handed (CRLH) modeling of circular-type metamaterials are realized in free space. Detailed dispersion characteristics give us the opportunity to explain the true behavior of the inclusions during the analysis stage. By combining the results coming from the standard retrieval procedure with the conventional CRLH theory, we calculate the actual values of the transmission line parameters for all frequency regimes. The constitutive parameters of an equivalent CRLH transmission line are derived and shown to be negative values. It is shown that the constitutive parameters present the same behavior for all negative refractive index regimes. The double-negative properties and the phase advance/lag behavior of metamaterials are observed based on the dispersive transmission line parameters.
Open Access
Diffraction inspired unidirectional and bidirectional beam splitting in defect-containing photonic structures without interface corrugations
(American Institute of Physics Inc., 2016) Colak, E.; Serebryannikov, A. E.; Usik, P. V.; Özbay, Ekmel
It is shown that strong diffractions and related dual-beam splitting can be obtained at transmission through the nonsymmetric structures that represent two slabs of photonic crystal (PhC) separated by a single coupled-cavity type defect layer, while there are no grating-like corrugations at the interfaces. The basic operation regimes include unidirectional and bidirectional splitting that occur due to the dominant contribution of the first positive and first negative diffraction orders to the transmission, which is typically connected with different manifestations of the asymmetric transmission phenomenon. Being the main component of the resulting transmission mechanism, diffractions appear owing to the effect exerted by the defect layer that works like an embedded diffractive element. Two mechanisms can co-exist in one structure, which differ, among others, in that whether dispersion allows coupling of zero order to a wave propagating in the regular, i.e., defect-free PhC segments or not. The possibility of strong diffractions and efficient splitting related to it strongly depend on the dispersion properties of the Floquet-Bloch modes of the PhC. Existence of one of the studied transmission scenarios is not affected by location of the defect layer.
Open Access
Femtosecond pulse generation from a Ti3+: Sapphire laser near 800 nm with voltage reconfigurable graphene saturable absorbers
(OSA - The Optical Society, 2017) Baylam, Işınsu; Özharar, Sarper; Kakenov, Nurbek; Kocabaş, Coşkun; Sennaroglu, Alphan
We experimentally show that a voltage-controlled graphene-gold supercapacitor saturable absorber (VCG-gold-SA) can be operated as a fast saturable absorber with adjustable linear absorption at wavelengths as low as 795 nm. This was made possible by the use of a novel supercapacitor architecture, consisting of a high-dielectric electrolyte sandwiched between a graphene and a gold electrode. The high-dielectric electrolyte allowed continuous, reversible adjustment of the Fermi level and, hence, the optical loss of the VCG-gold-SA up to the visible wavelengths at low bias voltages of the order of a few volts (0-2 V). The fast saturable absorber action of the VCG-gold-SA and the bias-dependent reduction of its loss were successfully demonstrated inside a femtosecond Ti3+:sapphire laser operating near 800 nm. Dispersion compensation was employed by using dispersion control mirrors and a prism pair. At a bias voltage of 1.2 V, the laser operated with improved power performance in comparison with that at zero bias, and the VCG-gold-SA initiated the generation of nearly transform-limited pulses as short as 48 fs at a pulse repetition rate of 131.7 MHz near 830 nm. To the best of our knowledge, this represents the shortest wavelength where a VCG-gold-SA has been employed as a mode locker with adjustable loss. © 2017 Optical Society of America.
Open Access
Filtering characteristics of hybrid integrated polymer and compound semiconductor waveguides
(IEEE, 2002) Ozturk, C.; Huntington, A.; Aydınlı, Atilla; Byun, Y.T.; Dagli, N.
This paper reports a study on a compact filter fabricated using hybrid integration of compound semiconductors and polymers. A GaAs epilayer is glued onto a polymer channel waveguide forming a highly asymmetrical directional coupler. This approach results in a narrow band filter due to very different dispersion characteristics of the compound semiconductor and the polymer materials. Furthermore, fiber coupling loss has been significantly reduced, since the input and output coupling is done through the polymer waveguide. Filtering characteristics can be engineered by changing the thickness and the length of the semiconductor epilayer. This can be done precisely using etch stop layers and noncritical lithography. The spectral response of such a filter can also be tuned electronically either using the electro-optic properties of the compound semiconductor or the thermo-optic properties of the polymer.
Open Access
Hybrid plasmon-phonon polariton bands in graphene-hexagonal boron nitride metamaterials [Invited]
(Optical Society of America, 2017) Hajian, H.; Ghobadi, A.; Dereshgi, S. A.; Butun, B.; Özbay, Ekmel
We theoretically investigate mid-infrared electromagnetic wave propagation in multilayered graphene-hexagonal boron nitride (hBN) metamaterials. Hexagonal boron nitride is a natural hyperbolic material that supports highly dispersive phonon polariton modes in two Reststrahlen bands with different types of hyperbolicity. Due to the hybridization of surface plasmon polaritons of graphene and hyperbolic phonon polaritons of hBN, each isolated unit cell of the graphene-hBN metamaterial supports hybrid plasmon-phonon polaritons (HPPs). Through the investigation of band structure of the metamaterial we find that, due to the coupling between the HPPs supported by each unit cell, the graphene-hBN metamaterial can support HPP bands. The dispersion of these bands can be noticeably modified for different thicknesses of hBN layers, leading to the appearance of bands with considerably flat dispersions. Moreover, analysis of light transmission through the metamaterial reveals that this system is capable of supporting high-k propagating HPPs. This characteristic makes graphene-hBN metamaterials very promising candidates for the modification of the spontaneous emission of a quantum emitter, hyperlensing, negative refraction, and waveguiding. © 2017 Optical Society of America.
Open Access
Lattice dynamics and elastic properties of lanthanum monopnictides
(2008) Gökoǧlu G.; Erkişi, A.
In this study, first principles calculation results of the second order elastic constants and lattice dynamics of two lanthanum monopnictides, LaN and LaBi, which crystallize in rock-salt structure (B1 phase), are presented. Calculations were based on plane wave basis sets and pseudopotential methods in the framework of Density Functional Theory (DFT) with generalized gradient approximation. Elastic constants are calculated by tetragonal and orthorhombic distortions on cubic structure. Phonon dispersion spectra was constructed in the linear response approach of the Density Functional Perturbation Theory (DFPT). The complete phonon softening with negative frequencies and large elastic anisotropy were observed for LaN single crystal as a sign of the structural instability. The phonon dispersion curve for LaBi is typical for lanthanum monopnictides and does not show any anomalous physical property. The calculated structural quantities for both LaN and LaBi systems agree well with the available experimental and theoretical data. © 2008 Elsevier Ltd. All rights reserved.
Open Access
Plasmon dispersion and damping in double-layer electron systems
(Elsevier Science Ltd, Exeter, United Kingdom, 2000) Davoudi, B.; Tanatar, Bilal
We use dynamical local-field corrections to study the plasmon dispersion and damping in double-layer electron systems. The wave vector and frequency-dependent local-fields describing the exchange-correlation effects are obtained within the quantum version of self-consistent field approach. The calculated plasmon dispersions are modified by the dynamic local-fields at intermediate wave vectors (i.e. q to approximately kF). The plasmons are damped outside the single-particle excitation region.
Open Access
Properties of a microjoule-class fiber oscillator mode-locked with a SESAM
(IEEE, 2011) Lecaplain, C.; Ortac, Bülend; MacHinet G.; Boullet J.; Baumgartl, M.; Schreiber, T.; Cormier, E.; Hideur, A.
Energy scaling of ultrafast Yb-doped fiber oscillators has experienced rapid progress largely driven by many applications that require high average power femtosecond pulses. The fundamental challenge for ultrafast fiber lasers relies on the control of excessive nonlinearity, which limits pulse energy. The development of all-normal dispersion laser cavities based on large-mode-area photonic crystal fibers (PCFs) has enabled significant energy scaling [1-3]. In particular, up to microjoule energy levels have been achieved from rod-type fiber-based oscillators [2-3]. In such lasers, pulse shaping is dominated by the strength of the mode-locking mechanism which determines the pulse properties. In this contribution, we report the generation of high-energy sub-picosecond pulses from a highly normal dispersion fiber laser featuring an Yb-doped rod-type PCF and a large-mode-area PCF [Fig.1(a)]. Passive mode-locking is achieved using saturable absorber mirrors (SAMs). We study the influence of the SAM parameters on performances obtained in this new class of fiber oscillators. The structures exhibit 20 % modulation depths and 500 fs relaxation time with resonant and antiresonant designs. The antiresonant SAM structures ensure absorption bandwidths 45 nm while the resonant structures exhibit 20 nm bandwidths. Stable mode locking with average powers as high as 15 μW at 15 MHz repetition rate, corresponding to microjoule energy level are obtained with all the structures. However, pulse properties and pulse shaping mechanism distinguish between resonant and antiresonant designs. Using a broadband antiresonant SAM leads to generation of highly-chirped pulses with 30 ps duration and 10 nm spectral width [Fig.1(b)]. The output pulses are extra-cavity dechirped down to 550 fs duration. By increasing the strength of the mode-locking mechanism through the combination of the SAM with the NPE process, we obtain shorter pulses with slightly boarder spectra. Indeed, the output pulse duration is decreased from 30 ps to 13 ps by adjusting the wave-plates settings. The dechirped pulse duration is then shortened to 450 fs. We note that the current laser performances are limited to 1 J by the available pump power. Using a resonant SAM structure, the output pulse duration is decreased to 7 ps [Fig.1(b)]. This pulse shortening results from the spectral filtering induced by the limited SAM bandwidth. All these results are in good agreement with numerical simulations which will be discussed in this communication. © 2011 IEEE.
Open Access
Rayleigh-bloch waves in CMUT arrays
(Institute of Electrical and Electronics Engineers Inc., 2014) Atalar, Abdullah; Köymen, Hayrettin; Oğuz, H. K.
Using the small-signal electrical equivalent circuit of a capacitive micromachined ultrasonic transducer (CMUT) cell, along with the self and mutual radiation impedances of such cells, we present a computationally efficient method to predict the frequency response of a large CMUT element or array. The simulations show spurious resonances, which may degrade the performance of the array. We show that these unwanted resonances are due to dispersive Rayleigh-Bloch waves excited on the CMUT surface-liquid interface. We derive the dispersion relation of these waves for the purpose of predicting the resonance frequencies. The waves form standing waves at frequencies where the reflections from the edges of the element or the array result in a Fabry-Pérot resonator. High-order resonances are eliminated by a small loss in the individual cells, but low-order resonances remain even in the presence of significant loss. These resonances are reduced to tolerable levels when CMUT cells are built from larger and thicker lates at the expense of reduced bandwidth. © 2014 IEEE.
Open Access
Reducing the dispersion errors of the finite-difference time-domain method for multifrequency plane-wave excitations
(Taylor & Francis, 2003) Oğuz, U.; Gürel, Levent
We demonstrate the applications of discrete-time signal-processing (SP) techniques for the purpose of generating accurate plane waves in the finite-difference time-domain (FDTD) method. The SP techniques are used either to reduce the high-frequency content of the source excitation or to compute more precise incident-field values in the computational domain. The effects of smoothing windows of various lengths, digital lowpass filters of various bandwidths and characteristics, and polynomial interpolation schemes of various orders are investigated. Arbitrary signals with multifrequency content are considered.
Open Access
Spatial filters based on EBG structures with anisotropic-like dispersion
(IEEE, 2010) Serebryannikov, A.E.; Cakmak, A.O.; Çolak, Evrim; Özbay, Ekmel
Bandpass and bandstop spatial filters based on the dielectric-rod EBG structures are proposed and validated for the frequency range from 18 to 25 GHz. The obtained experimental results are well consistent with the theoretical predictions. The exploited mechanism utilizes, in particular, anisotropic-like dispersion, which can occur in the conventional EBG structures made of isotropic materials.
Open Access
Terahertz time-domain study of silver nanoparticles synthesized by laser ablation in organic liquid
(IEEE Microwave Theory and Techniques Society, 2016-07) Koral, C.; Ortaç, B.; Altan, H.
We report the investigation of laser-synthesized Ag nanoparticles (Ag-NPs) in an organic liquid environment by using terahertz time-domain spectroscopy (THz-TDS) technique. Colloidal Ag-NPs with an average diameter of 10 nm in two-propanol solution through nanosecond pulsed laser ablation were synthesized. THz-TDS measurements were performed on different volumetric concentration of Ag-NPs suspensions placed in 2-mm path length quartz cuvette. Due to the dispersive and highly absorptive nature of the nano liquids, an approach based on extracting the optical properties through the changes in amplitude and phase solely around the main peak of THz waveform is developed. This approach allowed for an accurate estimation of the complex refractive index of the Metallic-NPs suspension for the different prepared volumetric concentrations. In addition, using Maxwell-Garnett theory, the NP concentration is also extracted. This method shows that the time-domain nature of the THz pulse measurement technique is extremely useful in instances where slight variations in highly dispersive samples need to be investigated.
Open Access
Wide-angle and simultaneously wideband blazing (deflection) enabling multifunctionality in metagratings comprising epsilon-near-zero materials
(Optica Publishing Group (formerly OSA), 2023-05-01) Serebryannikov, Andriy E.; Skigin, Diana C.; Hajian, Hodjat; Özbay, Ekmel
This paper investigates diffractions by gratings made of a dispersive material in an epsilon-near-zero (ENZ) regime and having one-side corrugations, and those by two-component dielectric-ENZ gratings with the inner corrugations and flat outer interfaces. The goal is to achieve wideband and simultaneously wide-angle −1st order blazing (deflection) that may enable wideband spatial filtering and demultiplexing in reflection mode. Several typical scenarios are discussed, which differ in the maximum magnitude of the blazed wave and size of the blazing area observed on the frequency-incidence angle plane, as well as the contribution of the ranges of positive and negative permittivity in the vicinity of zero. The high capability of ENZ and dielectric-ENZ gratings in asymmetric reflection is demonstrated for three different levels of losses for the dispersive material.