Browsing by Subject "Negative Permittivity"

Now showing 1 - 4 of 4

Open Access
Beaming and localization of electromagnetic waves in periodic structures
(2010) Çağlayan, Hümeyra
We want to manipulate light for several applications: microscopy, data storage, leds, lasers, modulators, sensor and solarcells to make our life healthier, easier or more comfortable. However, especially in small scales manipulating light have many difficulties. We could not focus or localize light into subwavelength dimensions easily, which is the key solution to beat today’s devices both in performance and cost. Achievements in three key research fields may provide the answer to these problems. These emerging research fields are metamaterials, photonic crystals and surface plasmons. In this thesis, we investigated beaming and localization of electromagnetic waves in periodic structures such as: subwavelength metallic gratings, photonic crystals and metamaterials. We studied off-axis beaming from both a metallic subwavelength aperture and photonic crystal waveguide at microwave regime. The output surfaces are designed asymmetrically to change the beaming angle. Furthermore, we studied frequency dependent beam steering with a photonic crystal with a surface defect layer made of dimmers. The dispersion diagram reveals that the dimer-layer supports a surface mode with negative slope. Thus, a photonic crystal based surface wave structure that acts as a frequency dependent leaky wave antenna was presented. Additionally, we investigated metamaterial based cavity systems. Since the unit cells of metamaterials are much smaller than the operation wavelength, we observed subwavelength localization within these metamaterial cavity structures. Moreover, we introduced coupled-cavity structures and presented the transmission spectrum of metamaterial based coupled-cavity structures. Finally, we demonstrated an ultrafast bioassay preparation method that overcomes the today’s bioassay limitations using a combination of low power microwave heating and split ring resonator structures.
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
The left hand of electromagnetism : metamaterials
(2010) Alıcı, Kamil Boratay
Metamaterials are artificial periodic structures whose electromagnetic response is solely dependent on the constituting unit cells. In the present thesis, we studied unit cell characteristics of metamaterials that has negative permeability and permittivity. We investigated negative permeability medium elements, especially in terms of their electrical size and resonance strength. Experimental and numerical study of µ-negative (MNG) materials: multi split ring resonators (MSRRs), spiral resonators (SRs) and multi-spiral resonators are presented. The resonance frequency of the structures is determined by the transmission measurements and minimum electrical size of λ0/17 for the MSRRs and of λ0/82 for the SRs observed. We explain a method for tuning the resonance frequency of the multi-split structures. We investigated scalability of MNG materials and designed a low loss double negative composite metamaterial that operates at the millimeter wave regime. A negative pass-band with a peak transmission value of -2.7 dB was obtained experimentally at 100 GHz. We performed transmission based qualitative effective medium theory analysis numerically and experimentally, in order to prove the double negative nature of the metamaterial. These results were supported by the standard retrieval analysis method. We confirmed that the effective index of the metamaterial was indeed negative by performing far field angular scanning measurements for a metamaterial prism. Moreover, we illuminated the split-ring resonator based metamaterial flat lens with oblique incidence and observed from the scanning experiments, the shifting of the beam to the negative side. The first device was a horn antenna and metamaterial lens composite whose behavior was similar to Yagi-Uda antenna. We numerically and experimentally investigated planar fishnet metamaterials operating at around 20 GHz and 100 GHz and demonstrated that their effective index is negative. The study is extended to include the response of the metamaterial layer when the metamaterial plane normal and the propagation vector are not parallel. We also experimentally studied the transmission response of a one dimensional rectangle prism shaped metamaterial slab for oblique incidence angles and confirmed the insensitivity of split-ring resonator based metamaterials to the angle of incidence. After the demonstration of complete transmission enhancement by using deep subwavelength resonators into periodically arranged subwavelength apertures, we designed and implemented a metamaterial with controllable bandwidth. The metamaterial based devices can be listed under the categories of antennas absorbers and transmission enhancement. We studied electrically small resonant antennas composed of split ring resonators (SRR) and monopoles. The electrical size, gain and efficiency of the antenna were λ0/10, 2.38 and 43.6%, respectively. When we increased the number of SRRs in one dimension, we observed beam steerability property. These achievements provide a way to create rather small steerable resonant antennas. We also demonstrated an electrically small antenna that operates at two modes for two perpendicular polarizations. The antenna was single fed and composed of perpendicularly placed metamaterial elements and a monopole. One of the metamaterial elements was a multi split ring resonator and the other one was a split ring resonator. When the antenna operates for the MSRR mode at 4.72 GHz for one polarization, it simultaneously operates for the SRR mode at 5.76 GHz, but for the perpendicular polarization. The efficiencies of the modes were 15% and 40% with electrical sizes of λ/11.2 and λ/9.5. Finally, we experimentally verified a miniaturization method of circular patch antennas. By loading the space between the patch and ground plane with metamaterial media composed of multi-split ring resonators and spiral resonators, we manufactured two electrically small patch antennas of electrical sizes λ/3.69 and λ/8.26. The antenna efficiency was 40% for the first mode of the multi-split ring resonator antenna with broad far field radiation patterns similar to regular patch antennas. We designed, implemented, and experimentally characterized electrically thin microwave absorbers by using the metamaterial concept. The absorbers consist of i) a metal back plate and an artificial magnetic material layer; ii) metamaterial back plate and a resistive sheet layer. We investigated absorber performance in terms of absorbance, fractional bandwidth and electrical thickness, all of which depend on the dimensions of the metamaterial unit cell and the distance between the back plate and metamaterial layer. As a proof of concept, we demonstrated a λ/4.7 thick absorber of type i), with a 99.8% absorption peak along with a 8% fractional bandwidth. We have also demonstrated experimentally a λ/4.7 and a λ/4.2 thick absorbers of type ii), based on SRR and MSRR magnetic metamaterial back plates, respectively. The absorption peak of the SRR layout is 97.4%, while for the MSRR one the absorption peak is 98.4%. We conveyed these concepts to optical frequencies and demonstrated a metamaterial inspired absorber for solar cell applications. We finalized the study by a detailed study of split ring resonators at the infrared and visible band. We studied i) frequency tuning, ii) effect of resonator density, iii) shifting magnetic resonance frequency by changing the resonator shape, iv) effect of metal loss and plasma frequency and designed a configuration for transmission enhancement at the optical regime. By using subwavelength optical split ring resonator antennas and couplers we achieved a 400-fold enhanced transmission from a subwavelength aperture area of the electrical size λ2 /25. The power was transmitted to the far field with 3.9 dBi directivity at 300 THz.
Open Access
Negative refraction using true left-handed metamaterials
(2004) Aydın, Koray
Left-handed materials and negative refraction attracted a great amount of attention in recent years due to their unique physical properties. It is possible to obtain a left-handed material by combining a novel artificial structure (split ring resonator) and a wire structure periodically. We investigated the transmission and reflection properties of split ring resonators (SRR), wires and composite metamaterials consisting of SRR and wire structures. We have successfully demonstrated true left-handed behavior in free space with a high transmission peak (-1.2 dB). This is the highest transmission peak reported for a left-handed material. The left-handed transmission band coincides exactly with the region where both dielectric permittivity and magnetic permeability have negative values. We proposed and demonstrated a new method to distinguish the magnetic resonance of the SRR structures. We experimentally confirmed that composite metamaterial has a negative refractive index, at the frequencies where left-handed transmission takes place. Phase shift between consecutive numbers of layers of CMM is measured and phase velocity is shown to be negative at the relevant frequency range. Refractive index values obtained from the refraction experiments (-1.87) and the phase shift experiments (-1.78) are in good agreement.