Browsing by Subject "Refraction (Optics)"

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Open Access
Characterization and applications of negative-index metamaterials
(2008) Aydın, Koray
Metamaterials offer novel electromagnetic properties and promising applications including negative refraction, flat-lenses, superlenses, cloaking devices. In this thesis, we characterized the negative-index metamaterials that is composed of periodic arrangements of split-ring resonators (providing negative permeability) and thin wire (providing negative permittivity) arrays. The resonances of split-ring resonators (SRR) are investigated experimentally and theoretically. By combining SRR and wire arrays together, we observed a transmission band where both permittivity and permeability are simultaneously negative, indicating a left-handed behavior. Reflection measurements reveal that the impedance is matched to the free space at a certain frequency range. The lefthanded metamaterial is also shown to exhibit negative refractive index by using three different experimental methods namely, refraction from a wedge-shaped negative-index metamaterial (NIM), beam-shift from a slab-shaped NIM and phase shift from NIMs with different lengths. Flat-lens behavior is observed from a slabshaped negative-index metamaterial based microwave lenses. Furthermore, we demonstrated subwavelength imaging and subwavelength resolution by using thin superlenses constructed from SRR-wire arrays with an effective negative index. We have been able to image a point source with a record-level, λ/8 resolution. SRRand wire arrays exhibit negative index provided that the wave propagates parallel to the plane of SRR structure which makes it hard to fabricate at higher frequencies. An alternative structure called fishnet metamaterial however could yield negative index with wave propagation normal to the structure. We observed left-handed transmission and negative phase velocity in fishnet type metamaterials. Finally, we studied enhanced transmission from a single subwavelength aperture by coupling incident electromagnetic wave to a single SRR placed at the near-field of the aperture.
Open Access
Waveguiding of electromagnetic waves and investigation of negative phase velocity in photonic crystals and metamaterials
(2012) Çolak, İlyas Evrim
Electromagnetic wave propagation is characterized in periodic dielectric and metallic structures: Photonic Crystals (PCs) and Metamaterials, respectively. The applications of these structures are demonstrated in the Microwave regime. In the first application, Graded Index (GRIN) PC is used to focus the incoming waves into a small spot. Speaking in terms of PC period a, for an incident beam with Full Width Half Maximum of 9.20a the power of the focusing behavior is quantified by looking at the spot size conversion ratio, which is around 3.9. PCs can act as an efficient input coupler for the PC Waveguide (PCW). The GRIN PC has been experimentally shown to yield a coupling efficiency of 5 dB over the single PCW at 18 GHz. This method can be applied to provide a solution for the input coupling losses between PC structures and other lightwave circuits. PCs can also be used to achieve dual-bandpass and bandstop spatial filtering by proper adjustments of the lattice parameters and the frequency range. For the plane-wave excitation, a wideband spatial filtering is shown to exist due to the specific Fabry-Perot type resonances, which are nearly independent on the angle of incidence. The effect of the finite angular distribution of the Gaussian-beam excitation is also demonstrated. The spatial filtering in the incidence and observation angle domains has been discussed both numerically and experimentally for the non-plane-wave excitations under the light of calculated iso-frequency contours. In addition to bandstop characteristics, the dispersion relation of the PCs can be modified with the proper arrangement, namely by employment of the dimer layer. This surface layer supports the surface waves and serves like a waveguide for the electromagnetic waves. At higher frequencies above the lightline, surface waves radiate into air in the form of backward leaky waves and frequency dependent steering is reported from 0 º to 70º for the outgoing beam. The leaky wave behavior and backward radiation is similar to that is seen in Left-Handed (LH) Metamaterials. Metallic fishnet layers are used to demonstrate negative refractive index (NRI) in conjunction with the left-handed behavior in this class of metamaterial. A wedge structure formed by fishnet layers is used to measure the NRI which was also verified by the retrieval analysis. The limits of homogenization are discussed. The dependence of the LH properties on the fishnet parameters is investigated parametrically. For example, the NRI changes from -1.8 to -1.3 as the interseperation distance of the layers varies from as=λ/10.5 (2mm) to as=λ/4.2 (4mm) at magnetic resonance frequency around 14.3 GHz (ωm). It is also shown that the fishnet layers behave as an LC resonator as well as a TEM waveguide and a 1D transmission line at ωm.