Browsing by Subject "Dielectric layer"
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Item 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.Item Open Access Closed-form green's function representations for mutual coupling calculations between apertures on a perfect electric conductor circular cylinder covered with dielectric layers(IEEE, 2011-06-07) Akyüz, M. S.; Ertürk, V. B.; Kalfa, M.Closed-form Green's function (CFGF) representations are developed for tangential magnetic current sources to calculate the mutual coupling between apertures on perfectly conducting circular cylinders covered with dielectric layers. The new representations are obtained by first rewriting the corresponding spectral domain Green's function representations in a different form (so that accurate results for electrically large cylinders, and along the axial line of a cylinder can be obtained). Then, the summation over the cylindrical eigenmodes is calculated efficiently. Finally, the resulting expressions are transformed to the spatial domain using a modified two-level generalized pencil of function method. Numerical results are presented showing good agreement when compared to CST Microwave Studio results.Item 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, EkmelGraphene 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.Item Open Access Long-range tamm surface plasmons supported by graphene-dielectric metamaterials(American Institute of Physics Inc., 2017) Hajian, H.; Caglayan, H.; Özbay, EkmelConsidering the Ohmic losses of graphene in the calculations and by obtaining exact dispersion relations of the modes, we theoretically study propagation and localization characteristics of Tamm surface plasmons supported by terminated graphene metamaterials. The metamaterials are composed of alternating layers of graphene and dielectric with subwavelength periods. We also examine the Tamm modes within the framework of long-wavelength approximation. It is shown that, in case the Ohmic losses of the graphene layers are taken into account, surface plasmons are not supported in a long-wavelength region, in which the graphene-dielectric multilayer structure behaves as a hyperbolic metamaterial. We prove that, when the metamaterial is truncated with air, by choosing sufficiently thick but still subwavelength dielectric layers, i.e., d = 300 nm, these surface waves will have a moderate propagation (localization) length that is comparable with those of a single layer of graphene. On the other hand, in case a miniaturized graphene metamaterial (10 < d(nm) < 100) is truncated by a thick cap layer (dcap = 5d) with εcap > εdielectric, it is possible to considerably improve the propagation and localization characteristics of the Tamm modes supported by the system within the 5.5-50 THz range of frequency, as compared to a single layer of graphene.Item Open Access Multiband one-way polarization conversion in complementary split-ring resonator based structures by combining chirality and tunneling(OSA - The Optical Society, 2015) Serebryannikov, A. E.; Beruete, M.; Mutlu, M.; Özbay, EkmelMultiband one-way polarization conversion and strong asymmetry in transmission inspired by it are demonstrated in ultrathin sandwiched structures that comprise two twisted aperture-type arrays of complementary split-ring resonators (CSRRs), metallic mesh, and dielectric layers. The basic features of the resulting mechanism originate from the common effect of chirality and tunneling. The emphasis is put on the (nearly) perfect polarization conversion of linear incident polarization into the orthogonal one and related diodelike asymmetric transmission within multiple narrow bands. Desired polarization conversion can be obtained at several resonances for one of the two opposite incidence directions, whereas transmission is fully blocked for the other one. The resonances, at which the (nearly) perfect conversion takes place, are expected to be inherited from similar structures with parallel, i.e., not rotated CSRR arrays that do not enable chirality and, thus, polarization conversion. It is found that the basic transmission and polarization conversion features and, thus, the dominant physics are rather general, enabling efficient engineering of such structures. The lowest-frequency resonance can be obtained in structures made of conventional materials with total thickness less than λ?/ 50 and up to ten such resonances can correspond to thickness less than λ ?/ 20 . ©2015 Optical Society of America.