Browsing by Author "Bulu, I."

Now showing 1 - 20 of 34

Open Access
Beaming of electromagnetic waves emitted through a subwavelength annular aperture
(Optical Society of American (OSA), 2006) Caglayan, H.; Bulu, I.; Özbay, Ekmel
We study the diffraction of electromagnetic waves from subwavelength metallic circular apertures in the microwave spectrum. The theoretical and experimental demonstration of the near- and far-field electromagnetic distributions for subwavelength circular annular apertures and circular annular apertures surrounded by concentric periodic grooves are reported here. The metallic samples had a subwavelength hole with a diameter of 8 mm and had concentric grooves with a periodicity of 16 mm. We present the angular transmission distributions from circular annular apertures, and circular annular apertures surrounded by concentric periodic grooves. At the surface-mode resonance frequency the transmitted electromagnetic waves from the subwavelength circular annular aperture surrounded by concentric periodic grooves have a strong angular confinement with an angular divergence of ±3°. This represents a fourfold reduction when compared with the angular divergence of the beam transmitted from the subwavelength circular aperture. © 2006 Optical Society of America.
Open Access
Beaming of light and enhanced transmission via surface modes of photonic crystals
(Optical Society of America, 2005-11-15) Bulu, I.; Caglayan, H.; Özbay, Ekmel
We report beaming and enhanced transmission of electromagnetic waves by use of surface corrugated photonic crystals. The modes of a finite-size photonic crystal composed of dielectric rods in free space have been analyzed by the plane-wave expansion method. We show the existence of surface propagating modes when the surface of the finite-size photonic crystal is corrugated. We theoretically and experimentally demonstrate that the transmission through photonic crystal waveguides can be substantially increased by the existence of surface propagating modes at the input surface. In addition, the power emitted from the photonic crystal waveguide is confined to a narrow angular region when an appropriate surface corrugation is added to the output surface of the photonic crystal.
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
Compact size highly directive antennas based on the SRR metamaterial medium
(Institute of Physics Publishing, 2005) Bulu, I.; Caglayan, H.; Aydin, K.; Özbay, Ekmel
In this work, we studied the far-field properties of the microwave radiation from sources embedded inside the split-ring resonator (SRR) metamaterial medium. Our results showed that the emitted power near the resonance frequency of the SRR structure was confined to a narrow angular region in the far field. The measured radiation patterns showed half-power beamwidths around 14°. The highly directive radiation is obtained with a smaller radiation surface area when compared to the previous results obtained by using photonic crystals. The reduction in the surface area is ten-fold in the case of the SRR metamaterial medium when compared to the photonic crystals. Our results provide means to create compact size highly directive antennas.
Open Access
Designing materials with desired electromagnetic properties
(Wiley, 2006) Bulu, I.; Cağlayan, H.; Özbay, Ekmel
In this work, we suggest and demonstrate a robust method to tune the plasma frequencies of wire mediums. The method we suggest involves the use of two or more wire arrangements in the unit cell. By incorporating the method we suggested it is possible to tune the plasma frequencies of wire mediums effectively by use of lower metal densities. In addition, we study the effective permittivities and permeabilities of labyrinth based metamaterials. Our results show that the effective permeability of the labyrinth based metamaterial medium is negative above a certain frequency. The results of the effective permittivity calculations for the labyrinth based metamaterial medium reveal that the labyrinth structure exhibits a strong dielectric response near the magnetic resonance frequency. Finally, we design labyrinth based left-handed mediums that have several desired properties such as simultaneous μ, ε = -1 and μ, ε = 0. © 2006 Wiley Periodicals, Inc.
Open Access
Electromagnetic wave focusing from sources inside a two-dimensional left-handed material superlens
(Institute of Physics Publishing, 2006) Aydın, K.; Bulu, I.; Özbay, Ekmel
Lenses made of negative index materials exhibit different focusing behaviours compared to positive index material lenses. Flat lens behaviour and imaging below the diffraction limit is possible with negative refractive index lenses. In this study, we employed left-handed materials (LHM) as negative index materials and experimentally investigated the focusing behaviour of such lenses. A point source is embedded inside the LHM lens. We have shown that it is possible to focus electromagnetic (EM) waves by using a planar configuration of lenses that is constructed by using two-dimensional (2D) LHMs. Flat lens behaviour is observed at 3.89 GHz, where EM waves are focused along the lateral and longitudinal directions. At 3.77 GHz, where the reflection is measured to be minimum, the focusing effect occurred at the surface of the LHM with a spot size of 0.16λ. We were able to overcome the diffraction limit with a slab-shaped LHM superlens. © IOP Publishing Ltd. and Deutsche Physikalische Gesellschaft.
Open Access
Enhanced transmission of microwave radiation in one-dimensional metallic gratings with subwavelength aperture
(American Institute of Physics, 2004) Akarca-Biyikli, S. S.; Bulu, I.; Özbay, Ekmel
We report a theoretical and experimental demonstration of enhanced microwave transmission through subwavelength apertures in metallic structures with double-sided gratings. Three different types of aluminum gratings (sinusoidal, symmetric rectangular, and asymmetric rectangular shaped) are designed and analyzed. Our samples have a periodicity of 16 mm, and a slit width of 2 mm. Transmission measurements are taken in the 10–37.5 GHz frequency spectrum, which corresponds to 8–30 mm wavelength region. All three structures display significantly enhanced transmission around surface plasmon resonance frequencies. The experimental results agree well with finite-difference-time-domain based theoretical simulations. Asymmetric rectangular grating structure exhibits the best results with ,50% transmission at 20.7 mm, enhancement factor of ,25, and ±4° angular divergence.
Open Access
Experimental demonstration of a left-handed metamaterial operating at 100 GHz
(American Physical Society, 2006) Gökkavas, M.; Güven, K.; Bulu, I.; Aydın, K.; Penciu, R. S.; Kafesaki, M.; Soukoulis, C. M.; Özbay, Ekmel
The existence of a left-handed (LH) transmission band in a bulk composite metamaterial (CMM) around 100 GHz is demonstrated experimentally. The CMM consists of stacked planar glass layers on which periodic patterns of micron-scale metallic wires and split-ring resonators are fabricated. The LH nature of the CMM band is proved by comparing the transmission spectra of individual CMM components. Theoretical investigation of the CMM by transmission simulations and an inversion scheme for the retrieval of the effective permeability and permittivity functions supports the existence of LH behavior. © 2006 The American Physical Society.
Open Access
Experimental demonstration of labyrinth-based left-handed metamaterials
(Optical Society of America, 2005-12-12) Bulu, I.; Caglayan, H.; Özbay, Ekmel
In this present work, we propose and demonstrate a resonant structure that solves two major problems related to the split-ring resonator structure. One of the problems related to the split-ring resonator structure is the bianisotropy, and the other problem is the electric coupling to the magnetic resonance of the split-ring resonator structure. These two problems introduce difficulties in obtaining isotropic left-handed metamaterial mediums. The resonant structure that we propose here solves both of these problems. We further show that in addition to the magnetic resonance, when combined with a suitable wire medium, the structure that we propose exhibits left-handed transmission band. We believe that the structure we proposed may have important consequences in the design of isotropic negative index metamaterial mediums. (c) 2005 Optical Society of America.
Open Access
Experimental observation of subwavelength localization using metamaterial-based cavities
(Optical Society of America, 2009-01-01) Caglayan, H.; Bulu, I.; Loncar, M.; Özbay, Ekmel
We report subwavelength localization of electromagnetic fields within cavities based on metamaterials. Cavity resonances are observed in the transmission spectrum of a split-ring resonator and composite metamaterials cavity structures. These cavity resonances are shown to exhibit high-quality factors. Since the unit cells of metamaterials are much smaller than the operation wavelength, subwavelength localization is possible within these metamaterial cavity structures. In the present Letter, we show that the electromagnetic field is localized into a region of λ/8, where λ is the cavity resonance wavelength. © 2008 Optical Society of America.
Open Access
Extraordinary grating-coupled microwave transmission through a subwavelength annular aperture
(Optical Society of America, 2005-03-07) Caglayan, H.; Bulu, I.; Özbay, Ekmel
We studied coupling phenomena between surface plasmons and electromagnetic waves in the microwave spectrum using circular apertures surrounded by array of grooves. We first present experimental and theoretical results of enhanced microwave transmission though a subwavelength circular aperture with concentric periodic grooves around the surface plasmon resonance frequency. This is followed by transmission studies through circular annular apertures and circular annular apertures surrounded by concentric periodic grooves. We demonstrated that 145 fold enhancement factor could be obtained with a subwavelength circular annular aperture surrounded by concentric periodic grooves. Our results show that, high transmission from a circular annular aperture with grooves is assisted by the guided mode of the coaxial waveguide and coupling to the surface plasmons. (C) 2005 Optical Society of America.
Open Access
Focusing of electromagnetic waves by a left-handed metamaterial flat lens
(Optical Society of America, 2005-10-31) Aydin, K.; Bulu, I.; Özbay, Ekmel
We present here the experimental results from research conducted on negative refraction and focusing by a two-dimensional (2D) left-handed metamaterial (LHM) slab. By measuring the refracted electromagnetic (EM) waves from a LHM slab, we find an effective refractive index of -1.86. A 2D scanning transmission measurement technique is used to measure the intensity distribution of the EM waves that radiate from the point source. The flat lens behavior of a 2D LHM slab is demonstrated for two different point source distances of ds = 0.5λ and λ. The full widths at half maximum of the focused beams are 0.36λ and 0.4λ, respectively, which are both below the diffraction limit. © 2005 Optical Society of America
Open Access
Focusing surface plasmons via changing the incident angle
(AIP Publishing LLC, 2008) Caglayan, H.; Bulu, I.; Özbay, Ekmel
We report a circular metallic aperture with a subwavelength circular slit in the microwave regime, in which we experimentally demonstrate that this aperture can excite and focus surface plasmons. Under normal illumination, there is no focusing of the surface plasmons. However, by changing the incident angle, it is possible to focus surface plasmons. We showed that under a 20° illumination angle surface plasmons focus at 4 cm away from the center on the surface of the aperture. © 2008 American Institute of Physics.
Open Access
Highly directional enhanced radiation from sources embedded inside three-dimensional photonic crystals
(Optical Society of America, 2005) Caglayan, H.; Bulu, I.; Özbay, Ekmel
We have experimentally studied emission of microwave radiation from a monopole source embedded in a three-dimensional photonic crystal. We have demonstrated enhancement of microwave radiation at the band edge and cavity mode frequencies. Furthermore, we have shown that it is possible to obtain highly directive microwave radiation sources operating at the band edge of the three-dimensional photonic crystal. We have measured half power beam widths of 13 degrees for both E and H planes, corresponding to a maximum directivity of 245. (c) 2005 Optical Society of America.
Open Access
Highly directive radiation and negative refraction using photonic crystals
(Institute of Physics Publishing, 2005) Özbay, Ekmel; Bulu, I.; Aydin, K.; Caglayan H.; Alici, K. B.; Guven, K.
In this article, we present an experimental and numerical study of certain optical properties of two-dimensional dielectric photonic crystals (PCs). By modifying the band structure of a two-dimensional photonic crystal through its crystal parameters, we show how it is possible to confine the angular distribution of radiation from an embedded omnidirectional source. We then demonstrate that the anomalous band dispersions of PCs give rise to completely novel optical phenomena, in particular, the negative refraction of electromagnetic waves at the interface of a PC. We investigate the spectral negative refraction, which utilizes a transverse magnetic (TM)-polarized upper band of a PC, in detail and show that a high degree of isotropy can be achieved for the corresponding effective index of refraction. The presence of nearly a isotropic negative refractive index leads to focusing of omnidirectional sources by a PC slab lens, which can surpass certain limitations of conventional (positive refractive) lenses. These examples indicate the potential of PCs for photonics applications utilizing the band structure.
Open Access
Highly directive radiation from sources embedded inside photonic crystals
(American Institute of Physics, 2003) Bulu, I.; Caglayan, H.; Özbay, Ekmel
In this work, we have experimentally and theoretically studied the angular distribution of power emitted from a radiation source embedded inside a photonic crystal. Our results show that it is possible to obtain highly directive radiation sources operating at the band edge of the photonic crystal. Half power beam widths as small as 6degrees have been obtained. Our results also show that the angular distribution of power strongly depends on the frequency and on the size of the photonic crystal. (C) 2003 American Institute of Physics.
Open Access
Investigation of localized coupled-cavity modes in two-dimensional photonic bandgap structures
(IEEE, 2002) Özbay, Ekmel; Bayındır, Mehmet; Bulu, I.; Cubukcu, E.
We present a detailed study of the localized coupled-cavity modes in 2-D dielectric photonic crystals. The transmission, phase, and delay time characteristics of the various coupled-cavity structures are measured and calculated. We observed the eigenmode splitting, waveguiding through the coupled cavities, splitting of electromagnetic waves in waveguide ports, and switching effect in such structures. The corresponding field patterns and the transmission spectra are obtained from the finite-difference-time-domain (FDTD) simulations. We also develop a theory based on the classical wave analog of the tight-binding (TB) approximation in solid state physics. Experimental results are in good agreement with the FDTD simulations and predictions of the TB approximation.
Open Access
Investigation of magnetic resonances for different split-ring resonator parameters and designs
(Institute of Physics Publishing, 2005) Aydin, K.; Bulu, I.; Guven, K.; Kafesaki, M.; Soukoulis, C. M.; Özbay, Ekmel
We investigate the magnetic resonance of split-ring resonators (SRR) experimentally and numerically. The dependence of the geometrical parameters on the magnetic resonance frequency of SRR is studied. We further investigate the effect of lumped capacitors integrated to the SRR on the magnetic resonance frequency for tunable SRR designs. Different resonator structures are shown to exhibit magnetic resonances at various frequencies depending on the number of rings and splits used in the resonators.
Open Access
Negative refraction and focusing of electromagnetic waves by metallodielectric photonic crystals
(American Physical Society, 2005) Bulu, I.; Caglayan, H.; Özbay, Ekmel
In this work, we have experimentally and theoretically studied negative refraction of electromagnetic waves by a metallodielectric photonic crystal. We showed that by adding a periodic dielectric perturbation to the unit cell of a metallic photonic crystal, it is possible to increase the range of incidence angles at which the electromagnetic waves are negatively refracted. We obtained negative refraction for large incidence angles by the use of a metallodielectric photonic crystal. Furthermore, as an application of the negative refraction effect, we demonstrated the focusing of electromagnetic waves by the use of a metallodielectric photonic crystal. We obtained focusing of electromagnetic waves away from the surface of the photonic crystal. The observed focusing points are found to be larger than one wavelength away from the photonic crystal interface.
Open Access
Negative refraction, subwavelength focusing and beam formation by photonic crystals
(Institute of Physics Publishing Ltd., 2007) Özbay, Ekmel; Aydin, K.; Bulu, I.; Guven, K.
We present a review of our experimental and numerical studies on the negative refraction related phenomena in two-dimensional dielectric photonic crystals (PCs). By employing photonic bands with appropriate dispersion, the propagation of the electromagnetic wave through a PC can be controlled to a large extent, and diverse and completely novel electromagnetic phenomena can be generated. We perform the spectral analysis of the negative refraction arising from a convex TM polarized photonic band of a hexagonal PC. As a consequence of negative refraction, we demonstrate a photonic crystal flat lens, which has the ability to focus electromagnetic waves and provide subwavelength resolution laterally. Finally, a photonic crystal with an embedded source is shown to provide a highly directional beam, which can be utilized in certain antenna applications.