Browsing by Author "Caglayan, H."

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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
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
Bright off-axis directional emission with plasmonic corrugations
(OSA - The Optical Society, 2017) Sattari, H.; Rashed, A. R.; Özbay, Ekmel; Caglayan, H.
In this work, a new plasmonic bulls-eye structure is introduced to efficiently harvest the emitted light from diamond nitrogen vacancy (NV) centers. We show that the presence of a simple metal sub-layer underneath of a conventional bulls-eye antenna, separated by a dielectric layer, results in the spontaneous emission enhancement and increment in out-coupled light intensity. High Purcell factor is accessible in such a structure, which consequently boosts efficiency of the radiated light intensity from the structure. The structure shows considerable enhancement in far-field intensity, about three times higher than that of a one-side corrugated (conventional) optimized structure. In addition, we study for the first time asymmetric structures to steer emitted beams in two-axis. Our results show that spatial off-axial steering over a cone is approachable by introducing optimal asymmetries to grooves and ridges of the structure. The steered light retains a level of intensity even higher than conventional symmetric structures. A high value of directivity of 16 for off-axis steering is reported. © 2017 Optical Society of America.
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
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
Comparison of back and top gating schemes with tunable graphene fractal metasurfaces
(American Chemical Society, 2016) Aygar, A. M.; Balci, O.; Cakmakyapan, S.; Kocabas, C.; Caglayan, H.; Özbay, Ekmel
In this work, fractal metasurfaces that consist of periodic gold squares on graphene are used to increase light-graphene interaction. We show by simulations and experiments that higher level fractal structures result in higher spectral tunability of resonance wavelength. This is explained by higher field localization for higher level fractal structures. Furthermore, spectral tunability of fractal metasurfaces integrated with graphene is investigated comparing two different schemes for electrostatic gating. Experiment results show that a top-gated device yields more spectral tunability (8% of resonance wavelength) while requiring much smaller gate voltages compared to the back-gated device. © 2016 American Chemical Society.
Open Access
Composite chiral metamaterials with negative refractive index and high values of the figure of merit
(Optical Society of America, 2012) Li, Z.; Caglayan, H.; Alici, K. B.; Kafesaki, M.; Soukoulis, C. M.; Özbay, Ekmel
A composite chiral metamaterial (CCMM) is designed and studied both numerically and experimentally. The CCMM is constructed by the combination of a continuous metallic wires structure and a purely chiral metamaterial (CMM) that consists of conjugated Rosettes. For the CMM, only very small, useful bands of negative index can be obtained for circularly polarized waves. These bands are all above the chiral resonance frequencies because of the high value of the effective parameter of relative permittivity epsilon. After the addition of the continuous metallic wires, which provide negative permittivity, the high value of epsilon can be partially compensated. Thus, a negative index band for the left circularly polarized wave that is below the chiral resonance frequency is obtained for the CCMM. At the same time, a negative index band for the right circularly polarized wave that is above the chiral resonance frequency is also obtained. Furthermore, both negative index bands correspond to the transmission peaks and have high values of the figure of merit. Therefore, the CCMM design that is proposed here is more suitable than the CMM for the construction of chiral metamaterials with a negative index. (C) 2012 Optical Society of America
Open Access
Coupling effect between two adjacent chiral structure layers
(Optical Society of America, 2010-03-01) Li, Z.; Caglayan, H.; Colak, E.; Zhou, J.; Soukoulis, C. M.; Özbay, Ekmel
A pair of mutually twisted metallic cross-wires can produce giant optical activity. When this single chiral layer is stacked layer by layer in order to build a thick chiral metamaterial, strong coupling effects are found between the two adjacent chiral layers. We studied these coupling effects numerically and experimentally. The results show that the existing coupling between chiral layers can make the chiral properties of a two-layered chiral metamaterial different from the constituting single chiral layers. It is explained qualitatively that the coupling effects are generated from the coupling of metallic cross-wires belonging to different chiral layers. Our experimental results are in good agreement with the simulation results. ©2010 Optical Society of America
Open Access
Coupling enhancement of split ring resonators on graphene
(Pergamon Press, 2014-12) Cakmakyapan, S.; Caglayan, H.; Özbay, Ekmel
Metallic split ring resonator (SRR) structures are used in nanophotonics applications in order to localize and enhance incident electromagnetic field. Electrically controllable sheet carrier concentration of graphene provides a platform where the resonance of the SRRs fabricated on graphene can be tuned. The reflectivity spectra of SRR arrays shift by applying gate voltage, which modulates the sheet carrier concentration, and thereby the optical conductivity of monolayer graphene. We experimentally and numerically demonstrated that the tuning range can be increased by tailoring the effective mode area of the SRR and enhancing the interaction with graphene. The tuning capability is one of the important features of graphene based tunable sensors, optical switches, and modulator applications. © 2014 Elsevier Ltd. All rights reserved.
Open Access
Enhanced transmission and beaming via a zero-index photonic crystal
(American Institute of Physics Inc., 2016) Hajian, H.; Özbay, Ekmel; Caglayan, H.
Certain types of photonic crystals with Dirac cones at the Γ point of their band structure have a zero effective index of refraction at Dirac cone frequency. Here, by an appropriate design of the photonic structure, we obtain a strong coupling between modes around the Dirac cone frequency of an all-dielectric zero-index photonic crystal and the guided ones supported by a photonic crystal waveguide. Consequently, we experimentally demonstrate that the presence of the zero-index photonic crystal at the inner side of the photonic crystal waveguide leads to an enhancement in the transmission of some of the guided waves passing through this hybrid system. Moreover, those electromagnetic waves extracted from the structure with enhanced transmission exhibit high directional beaming due to the presence of the zero-index photonic crystal at the outer side of the photonic crystal waveguide. © 2016 Author(s).
Open Access
Enhanced transmission and directivity from metallic subwavelength apertures with nonuniform and nonperiodic grooves
(AIP Publishing LLC, 2008) Li, Z.; Caglayan, H.; Colak, E.; Özbay, Ekmel
Nonuniform and nonperiodic grooves are used to enhance the transmission and directivity of emissions from a single metallic subwavelength aperture. By using nonuniform and nonperiodic grooves, the amplitude and phase of the diffracted power flow from each groove can be adjusted properly. As a result, the transmission and emission directivity can be further improved when compared to apertures with uniform and periodic grooves. Our experimental results are in good agreement with the finite difference time domain simulation results.
Open Access
Enhanced tunability of V-shaped plasmonic structures using ionic liquid gating and graphene
(Elsevier Ltd, 2016) Ozdemir, O.; Aygar, A. M.; Balci, O.; Kocabas, C.; Caglayan, H.; Özbay, Ekmel
Graphene is a strong candidate for active optoelectronic devices because of its electrostatically tunable optical response. Current substrate back-gating methods are unable to sustain high fields through graphene unless a high gate voltage is applied. In order to solve this problem, ionic liquid gating is used which allows substrate front side gating, thus eliminating the major loss factors such as a dielectric layer and a thick substrate layer. On the other hand, due to its two dimensional nature, graphene interacts weakly with light and this interaction limits its efficiency in optoelectronic devices. However, V-shaped plasmonic antennas can be used to enhance the incident electric field intensity and confine the electric field near graphene thus allowing further interaction with graphene. Combining V-shaped nanoantennas with the tunable response of graphene, the operation wavelength of the devices that utilize V-shaped antennas can be tuned in situ. In the present paper, we demonstrate a graphene-based device with ionic liquid gating and V- shaped plasmonic antennas to both enhance and more effectively tune the total optical response. We are able to tune the transmission response of the device for up to 389 nm by changing the gate voltage by 3.8 V in the mid-infrared regime.
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
Experimental validation of strong directional selectivity in nonsymmetric metallic gratings with a subwavelength slit
(American Institute of Physics, 2011-02-02) Cakmakyapan, S.; Caglayan, H.; Serebryannikov, A. E.; Özbay, Ekmel
Strong directional selectivity is theoretically predicted and experimentally validated at the microwave frequencies in the beaming regime for a single subwavelength slit in nonsymmetric metallic gratings with double-side corrugations. The operation regime can be realized at a fixed angle of incidence when the surface-plasmon assisted transmission is significant within a narrow range of observation angles, if illuminating one of the grating interfaces, and tends to vanish for all observation angles, if illuminating the opposite interface. The studied effect is connected with asymmetry (nonreciprocity) in the beaming that occurs if the surface plasmon properties are substantially different for the two interfaces being well isolated from each other.
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
The focusing effect of graded index photonic crystals
(AIP Publishing LLC, 2008) Kurt, H.; Colak, E.; Cakmak, O.; Caglayan, H.; Özbay, Ekmel
We describe an approach to implement graded index (GRIN) structures using two-dimensional photonic crystals (PCs). The lattice spacing along the transverse direction to propagation is altered and we show, both theoretically and experimentally, that such a spatial perturbation is an effective way to obtain GRIN PC. The response of the structure to spatially wide incident beams is investigated and strong focusing behavior is observed. The large spot size conversion ratio can be attainable and is mainly limited by the finite size of the structure. The designed GRIN PC shows promise for use in optical systems that require compact and powerful focusing elements compared to the traditional bulky lenses.
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.