Browsing by Subject "Metamaterials"

Now showing 1 - 20 of 107

Open Access
Accurate modeling of metamaterials with MLFMA
(ESA Publications, 2006) Ergül, Özgür; Ünal, Alper; Gürel, Levent
Electromagnetic modelling of large metamaterial (MM) structures employing multilevel fast multipole algorithm (MLFMA) is reported. MMs are usually constructed by periodically embedding unit cells, such as split-ring resonators (SRRs), into a host medium. Without utilizing any homogenization techniques, we accurately model large numbers of unit cells that translate into very large computational problems. By considering all of the electromagnetic interactions, the resulting dense matrix equations are solved iteratively with the accelerated matrix-vector products by MLFMA. To increase the efficiency, we also employ parallel computing in the solutions of large SRR problems.
Open Access
Aerospace metamaterials and functional coatings
(2025-01) Astarlıoğlu, Aziz Taner
Optically transparent and electrically conductive thin-film coatings are widely used to functionalize surfaces of various high-technology platforms, including mobile phones, displays, detectors, and LEDs. Their integration into aviation transparencies, such as canopies, windshields, and windows, is widely known and used for de-icing purposes. However, there are limited reports or information available about using such thin-film coatings for electromagnetic interference (EMI) shielding, low observability (LO), and solar irradiation protection features. This thesis aims to study and demonstrate optically transparent aviation structures possessing the properties of EMI shielding, LO features, and solar radiation protection altogether. To this end, in this thesis, we specifically addressed the problem of achieving high EMI shielding and solar protection, which require high electrical conductivity, resulting in a trade-off reducing the optical quality and LO performance. Transparent engineering polymers are widely used in structural parts in aviation thanks to their enhanced mechanical performance. However, good-quality films require high-temperature processes, which is not applicable to transparent aviation structures. Therefore, the architecture of layered films can be applied to meet the requirements of well-featured aviation transparencies. For these purposes, in the thesis we also designed monolithic and laminated aviation transparencies with surface modification based on stratified films and their patterned ones using numerical and experimental methods. We developed numerical approaches for the design of aircraft transparencies, including both the optical and electromagnetic requirements and validated our results. We successfully conducted experimental studies for uniform large-area thin-film coatings onto aviation transparencies. The results revealed that EMI shielding and solar control performance were achieved with minimal optical losses for planar structures. LO requirement was incorporated into prototypes built on curved or laminated transparent structures instead of monolithic ones to sustain optical, solar protection, and EMI shielding performance to a possible extent. We showed that the low observability performance of such patterned structures, the metamaterials, is enhanced in terms of bandwidth and attenuation compared to the planar thin-film-coated monolithic counterparts. These multi-functional thin-film coatings are essential in aviation, especially for high-performance 5th-generation fighter jets and other civil applications. This thesis paves the way for thin-film-coated transparent aviation structure designs across different domains, including visible, infrared, and microwaves, to enable their multi-functionality at large scales. The experimental large-area coating method guides the coating of a large and complex area to remove the limitation of metamaterial applications at the industrial level. We believe that our findings in this thesis will help to replace traditional planar thin film coatings with metamaterials at the industry scale, aiming to outperform traditional counterparts.
Open Access
All ceramic-based metal-free ultra-broadband perfect absorber
(Springer, 2019-06) Soydan, Mahmut Can; Ghobadi, Amir; Yıldırım, Deniz Umut; Ertürk, Vakur Behçet; Özbay, Ekmel
In this paper, we scrutinize unprecedented potential of transition metal carbides (TMCs) and nitrides (TMNs) for realization of light perfect absorption in an ultra-broad frequency range encompassing all of the visible (Vis) and near infrared (NIR) regions. For this purpose, two different configurations which are planar and trapezoidal array are employed. To gain insight on the condition for light perfect absorption, a systematic modeling approach based on transfer matrix method (TMM) is firstly utilized. Our modeling findings prove that the permittivity data of these TMCs and TMNs are closely matched with the ideal data. Thus, they can have stronger and broader absorption behavior compared to metals. Besides, these ceramic materials are preferred to metals due to the fact that they have better thermal properties and higher durability against erosion and oxidation than metals. This could provide the opportunity for design of highly efficient light harvesting systems with long-term stability. Numerical simulations are conducted to optimize the device optical performance for each of the proposed carbides and nitrides. Our findings reveal that these ceramic coatings have the broadest absorption response compared to all lossy and plasmonic metals. In planar configuration, titanium carbide (TiC) has the largest absorption bandwidth (BW) where an absorption above 0.9 is retained over a broad wavelength range of 405–1495 nm. In trapezoid architecture, vanadium nitride (VN) shows the widest BW covering a range from 300 to 2500 nm. The results of this study can serve as a beacon for the design of future high-performance energy conversion devices including solar vapor generation and thermal photovoltaics where both optical and thermal requirements can be satisfied.
Open Access
All-silicon ultra-broadband infrared light absorbers
(Nature Publishing Group, 2016) Gorgulu, K.; Gok, A.; Yilmaz, M.; Topalli, K.; Blylkll, N.; Okyay, Ali Kemal
Absorbing infrared radiation efficiently is important for critical applications such as thermal imaging and infrared spectroscopy. Common infrared absorbing materials are not standard in Si VLSI technology. We demonstrate ultra-broadband mid-infrared absorbers based purely on silicon. Broadband absorption is achieved by the combined effects of free carrier absorption, and vibrational and plasmonic absorption resonances. The absorbers, consisting of periodically arranged silicon gratings, can be fabricated using standard optical lithography and deep reactive ion etching techniques, allowing for cost-effective and wafer-scale fabrication of micro-structures. Absorption wavebands in excess of 15 micrometers (5-20 μm) are demonstrated with more than 90% average absorptivity. The structures also exhibit broadband absorption performance even at large angles of incidence (θ = 50°), and independent of polarization.
Open Access
The almost magical world of metamaterials
(IEEE, 2008-11) Özbay, Ekmel
In recent years, there has been a burgeoning interest in rapidly growing field of metamaterials due to their unprecedented properties unattainable from ordinary materials. Veselago pointed out that a material exhibiting negative values of dielectric permittivity (epsiv) and magnetic permeability (mu) would have a negative refractive index [1]. Generally speaking, the dielectric permittivity (epsiv) and the magnetic permeability (mu) are both positive for natural materials. In fact, it is possible to obtain negative values for epsiv and mu by utilizing proper designs of metamaterials. Left-handed electromagnetism and negative refraction are achievable with artificially structured metamaterials exhibiting negative values of permittivity and permeability simultaneously at a certain frequency region. The first steps to realize these novel type of materials were taken by Smith et al., where they were able to observe a left-handed propagation band at frequencies where both dielectric permittivity and magnetic permeability of the composite metamaterial are negative [2]. Soon after, left-handed metamaterials with an effective negative index of refraction are successfully demonstrated by various groups.
Open Access
Analysis of double-negative materials with surface integral equations and the multilevel fast multipole algorithm
(IEEE, 2011) Ergül O.; Gürel, Levent
We present a fast and accurate analysis of double-negative materials (DNMs) with surface integral equations and the multilevel fast multipole algorithm (MLFMA). DNMs are commonly used as simplified models of metamaterials at resonance frequencies and are suitable to be formulated with surface integral equations. However, realistic metamaterials and their models are usually very large with respect to wavelength and their accurate solutions require fast algorithms, such as MLFMA. We consider iterative solutions of DNMs with MLFMA and we investigate the accuracy and efficiency of solutions when DNMs are formulated with two recently developed formulations, namely, the combined tangential formulation (CTF) and the electric and magnetic current combined-field integral equation (JMCFIE). Numerical results on canonical objects are consistent with previous results in the literature on ordinary objects. © 2011 IEEE.
Open Access
Applications of electromagnetic phenomena in periodic structures
(2012) Çakmak, Atilla Özgür
The field of Electromagnetics encompasses several research areas and finds itself applications in all frequency ranges starting from very low frequencies up to optical wavelengths. Periodic structures offer a vast research area in Electromagnetics. Amongst these periodic configurations metamaterials and photonic crystals have been investigated in this study. Metamaterials (MTMs) are artificial materials which are carefully engineered to give outstanding electromagnetic responses, e.g. negative phase velocity, negative refraction. On the other hand, Photonic Crystals (PhCs) offer band stopping and full reflection at certain wavelengths and they are highly favored due their particular properties. PhCs are even commercially available nowadays in optical communication. In the first part of the study, we have been concentrated on the enhanced transmission through subwavelength apertures with the incorporation of the MTMs. Resonators that are inspired from the MTM research field are placed in the vicinity of the subwavelength apertures such that a near-field electromagnetic wave localization at the output side can be observed. The considered subwavelength apertures have poor transmission figures on the order of 1/10,000 (in terms of intensity) on the average throughout the investigated frequency band, which dramatically limits the propagation. We show that once the subwavelength resonators are allowed to interact with such subwavelength pertures, astonishingly high transmission enhancement figures (typically ranging from 30 dB up to 50 dB) can be attained, which in turn results in electromagnetic wave localization in the near-field. On top of these single aperture related studies, we investigated the propagation of the electromagnetic waves in aperture arrays. We explicitly distinguish the working mechanisms of the presently studied aperture arrays which also cause a transmission enhancement below the cutoff frequency of the regarding apertures. The transmission enhancement has been shown to be accompanied by the left handed propagation, which is a characteristic of the MTMs. We show that both right and left handed transmission channels can be opened simultaneously in these periodically stacked aperture arrays. Alternatively, we have worked on the graded-index PhCs (GRIN PhCs) in order to tailor the course of the propagating electromagnetic waves. It has been demonstrated that modified Gauss-Hermite modes are available in GRIN PhCs similar to the those in conventional homogenous GRIN structures that are already in use in the field of optics. The underlying physics of the propagation has been discussed and a focusing lens has been proposed, which is based on the GRIN PhCs. The proposed lens has been shown to provide improved input and output coupling figures for the waveguide configurations. The input coupling efficiency has been boosted by a factor of 8 dB, while the GRIN PhC at the output side of the waveguide achieved a collimated beam with 7 degrees of half power beam width according to the results of the far-field measurements. Lastly, PhC based gratings have been utilized to search for an optical diode. It has been shown that such a PhC grating can offer an extremely good contrast (on the order of 1000) between the intensities of the electromagnetic waves that are approaching from different sides of the proposed design. The working mechanism of the optical diode has been shown to rely on the gratings that opened higher order diffraction channels as the zeroth order diffractions are suppressed.
Open Access
Asymmetric chiral metamaterial circular polarizer based on four U-shaped split ring resonators
(Optical Society of America, 2011-04-28) Mutlu, M.; Akosman, A. E.; Serebryannikov, A. E.; Özbay, Ekmel
An asymmetric chiral metamaterial structure is constructed by using four double-layered U-shaped split ring resonators, which are each rotated by 90° with respect to their neighbors. The peculiarity of the suggested design is that the sizes of the electrically and magnetically excited rings are different, which allows for equalizing the orthogonal components of the electric field at the output interface with a 90° phase difference when the periodic structure is illuminated by an x-polarized wave. As a result, left-hand circular polarization and right-hand circular polarization are obtained in transmission at 5:1 GHz and 6:4 GHz, respectively. The experiment results are in good agreement with the numerical results.
Open Access
AVBVICVII ferroelectrics as novel materials for phononic crystals
(Taylor and Francis Inc., 2017) Palaz S.; Oltulu, O.; Mamedov, A. M.; Özbay, Ekmel
In the present work the acoustic band structure of a two-dimensional (2D) phononic crystal (PC) containing a semiconducting ferroelectric - AVBVICVII (A = Sb, Bi; B = S, Se, Te; C = I, Br, and Cl) was investigated theoretically and numerically by the plane-wave-expansion (PWE) method. Two-dimensional PC with square lattices composed of semiconducting ferroelectric cylindrical rods embedded in the organic/inorganic matrix is studied to find the existence of stop bands for the waves of certain energy. This phononic bandgap - forbidden frequency range - allows sound to be controlled in many useful ways in structures that can act as sonic filters, waveguides or resonant cavities. Phononic band diagram ω = ω(k) for a 2D PC was plotted versus the wavevector k along the Г-X-M-Г path in the square Brillouin zone (BZ). The band diagram shows four stop bands in the wide frequency range. The unusual properties of matrix and ferroelectric properties of AVBVICVII give us ability to control the wave propagation through the PC in over a wide frequency range. We study the 2D composites by solving the basic acoustic wave equation and use Bloch wave analysis to identify the band gaps.
Open Access
Beaming and enhanced transmission through a subwavelength aperture via epsilon-near-zero media
(Nature Publishing Group, 2017) Hajian, H.; Özbay, Ekmel; Caglayan, H.
We numerically validate and experimentally realize considerable funneling of electromagnetic energy through a subwavelength aperture that is covered with an epsilon-near-zero metamaterial (ENZ). The epsilon-near-zero metamaterial is composed of two layers of metasurfaces and operates at microwave frequencies. We demonstrate that the presence of the metamaterial at the inner and outer sides of the aperture not only lead to a significant enhancement in light transmission, but also cause a directional emission of light extracting from this hybrid system. In addition to these experimental results, we theoretically demonstrate the same concept in mid-IR region for a subwavelength gold aperture with indium tin oxide as an epsilon-near-zero material. Moreover, we found that using a dielectric spacer in-between the sunwavelength aperture and the ENZ medium, it is possible to red-shift the enhancement/directional frequency of the system.
Open Access
Bismuth-based metamaterials: From narrowband reflective color filter to extremely broadband near perfect absorber
(De Gruyter, 2019) Ghobadi, Amir; Hajian, Hodjat; Gökbayrak, Murat; Bütün, Bayram; Özbay, Ekmel
In recent years, sub-wavelength metamaterials-based light perfect absorbers have been the subject of many studies. The most frequently utilized absorber configuration is based on nanostructured plasmonic metals. However, two main drawbacks were raised for this design architecture. One is the fabrication complexity and large scale incompatibility of these nano units. The other one is the inherent limitation of these common metals which mostly operate in the visible frequency range. Recently, strong interference effects in lithography-free planar multilayer designs have been proposed as a solution for tackling these drawbacks. In this paper, we reveal the extraordinary potential of bismuth (Bi) metal in achieving light perfect absorption in a planar design through a broad wavelength regime. For this aim, we adopted a modeling approach based on the transfer matrix method (TMM) to find the ideal conditions for light perfect absorption. According to the findings of our modeling and numerical simulations, it was demonstrated that the use of Bi in the metal-insulator-metal-insulator (MIMI) configuration can simultaneously provide two distinct functionalities; a narrow near unity reflection response and an ultra-broadband near perfect absorption. The reflection behavior can be employed to realize additive color filters in the visible range, while the ultra-broadband absorption response of the design can fully harvest solar irradiation in the visible and near infrared (NIR) ranges. The findings of this paper demonstrate the extraordinary potential of Bi metal for the design of deep sub-wavelength optical devices.
Open Access
Cavity formation in split ring resonators
(Elsevier BV, 2008-12) Caglayan, H.; Bulu, I.; Loncar, M.; Özbay, Ekmel
We report that it is possible to obtain a cavity structure by the deformation of a unit cell of an split ring resonator (SRR) structure. We presented the Q-factor of the cavity resonance as 192 for an SRR-based single cavity. Subsequently, we brought two and three cavities together with an intercavity distance of two metamaterial unit cells and investigated the transmission spectrum of SRR-based interacting 2-cavity and 3-cavity systems. The splitting of eigenmodes due to the interaction between the localized electromagnetic cavity modes was observed. Eventually, in taking full advantage of the effective medium theory, we modeled SRR-based cavities as 1D Fabry–Perot reflectors (FPRs) with a subwavelength cavity at the center. Finally, we observed that at the cavity resonance, the effective group velocity was reduced by a factor of 67 for an SRR-based single cavity compared to the electromagnetic waves propagating in free space.
Open Access
Chiral metamaterials with negative refractive index based on four "U" split ring resonators
(American Institute of Physics, 2010-08-23) Li, Z.; Zhao, R.; Koschny, T.; Kafesaki, M.; Alici, K. B.; Colak, E.; Caglayan, H.; Özbay, Ekmel; Soukoulis, C. M.
A uniaxial chiral metamaterial is constructed by double-layered four "U" split ring resonators mutually twisted by 90°. It shows a giant optical activity and circular dichroism. The retrieval results reveal that a negative refractive index is realized for circularly polarized waves due to the large chirality. The experimental results are in good agreement with the numerical results.
Open Access
Chiral metamaterials: From negative index to asymmetric transmission
(IEEE, 2013) Mutlu, Mehmet; Li, Zhaofeng; Özbay, Ekmel
Chiral metamaterials are attractive for their intriguing properties such as negative refractive index, optical activity and circular dichroism, and asymmetric transmission. In this paper, we review the research we have conducted for the purpose of investigating these exciting properties. © 2013 EurAAP.
Open Access
Chiral Structures: Manipulation of Asymmetric Transmission in Planar Chiral Nanostructures by Anisotropic Loss (Advanced Optical Materials 7/2013)
(2013) Li, Z.; Gokkavas, M.; Özbay, Ekmel
Asymmetric transmissions of circularly polarized optical waves can be achieved when the waves are incident normal to planar chiral structures, provided that the structures are anisotropic and lossy. In order to clarify how the factor of loss affects the asymmetric transmission, Z. Li, M. Gokkavas, and E. Ozbay studied a typical planar chiral structure by using an optical lumped element model. On page 482, they found that the anisotropy of loss, instead of the whole loss, plays a crucial role for achieving asymmetric transmission. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Open Access
Color generation and enhancement using large-scale compatible metamaterial design architectures
(2022-01) Köşger, Ali Cahit
Metamaterials are a type of artificial matt that can impose exotic functionalities beyond natural materials. These specifically designed sub-wavelength structures acquire these functionalities from their collective geometric arrangement rather than their individual single-unit properties. As a result, metamaterials have shown promising applications, including negative refraction, artificial magnetism, asymmetric transmission, lasing, and cloak of invisibility. Among all these applications, the concept of color generation and enhancement using metamaterial designs have attracted much attention in recent years. We can achieve color generation from two primary sources: i) filtering white light, and ii) generating light from emitting materials such as quantum dots. In color generation using white light, a metamaterial design reflects or transmits a narrow portion of the incident spectrum. Thus, the design acts as a color filter. However, the source is already a narrowband color light in the second category. Thus metamaterials merely amplify the color intensity rather than manipulate its spectral response. In this thesis, metamaterial structures are designed, fabricated, and characterized in both categories mentioned above; The content of this thesis consists of two parts; i) In the first part, we generated additive red-green-blue (RGB) colors in reflectance mode with near-unity amplitude. For this purpose, we designed a multilayer structure made of metal-insulator-metal-semiconductor-insulator (MIMSI) stacks to achieve >0.9 reflection peaks with full-width-at-half-maximum (FWHM) values <0.3λpeak. The proposed design also shows near-zero reflection in off-resonance spectral ranges, which, in turn, leads to high color purity. Finally, we fabricated the optimized designs and verified the simulation and theoretical results with characterization findings. This work demonstrates the potential of multilayer tandem cavity designs in realizing lithography-free large-scale compatible functional optical coatings. ii) In the second part, we utilized a large-scale compatible plasmonic nanocavity design platform to achieve almost an order of magnitude photoluminescence enhancement from light-emitting quantum dots. The proposed design is multi-sized/multi-spacing gold (Au) nano units that are uniformly wrapped with thin aluminum oxide (Al2O3) layer as a foreign host to form a metal-insulator-semiconductor (MIS) cavity, as we coated them with semiconductor quantum dots (QDs). Our numerical and experimental data demonstrate that, in an optimal insulator layer thickness, the simultaneous formation of broadband Fabry-Perot (FP) resonances and plasmonic hot spots leads to enhanced light absorption within the QD unit. This improvement in absorption response leads to the PL enhancement of QDs. This work demonstrates the potential and effectiveness of a host comprised of random plasmonic nanocavities in the realization of lithography-free efficient emitters. Overall, this thesis presents an alternative perspective on applying large-scale compatible metamaterials in color generation. Furthermore, the proposed designs and routes can be extended toward other functional photoelectronic designs, where high performances can be acquired in scaleable architectures.
Open Access
Compact left-handed metamaterial based on double-layer planar metal strip arrays
(Optical Society of America, 2006) Guven, K.; Çalışkan, Deniz; Özbay, Ekmel
The existence of a left-handed transmission peak of a metamaterial consisting of double-layer planar metal strip arrays at 15 GHz is demonstrated. This design is very suitable to submicron scales required at communication wavelengths. © 2006 Optical Society of America.
Open Access
Computational analysis of complicated metamaterial structures using MLFMA and nested preconditioners
(IEEE, 2007-11) Ergül, Özgür; Malas, Tahir; Yavuz, Ç; Ünal, Alper; Gürel, Levent
We consider accurate solution of scattering problems involving complicated metamaterial (MM) structures consisting of thin wires and split-ring resonators. The scattering problems are formulated by the electric-field integral equation (EFIE) discretized with the Rao-Wilton- Glisson basis functions defined on planar triangles. The resulting dense matrix equations are solved iteratively, where the matrix-vector multiplications that are required by the iterative solvers are accelerated with the multilevel fast multipole algorithm (MLFMA). Since EFIE usually produces matrix equations that are ill-conditioned and difficult to solve iteratively, we employ nested preconditioners to achieve rapid convergence of the iterative solutions. To further accelerate the simulations, we parallelize our algorithm and perform the solutions on a cluster of personal computers. This way, we are able to solve problems of MMs involving thousands of unit cells.
Open Access
Continuously tunable terahertz metamaterial employing magnetically actuated cantilevers
(Optical Society of American (OSA), 2011) Ozbey, B.; Aktas O.
Terahertz metamaterial structures that employ flexing microelectromechanical cantilevers for tuning the resonance frequency of an electric split-ring resonator are presented. The tuning cantilevers are coated with a magnetic thin-film and are actuated by an external magnetic field. The use of cantilevers enables continuous tuning of the resonance frequency over a large frequency range. The use of an externally applied magnetic field for actuation simplifies the metamaterial structure and its use for sensor or filter applications. A structure for minimizing the actuating field is derived. The dependence of the tunable bandwidth on frequency is discussed. © 2011 Optical Society of America.
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.