Due to the growth in mobile wireless systems, electrically small antennas (ESAs) which are efficient and have significant bandwidth are in great demand. But these requirements are contradictory. ESAs are known to be highly capacitive in nature. As a result of this, matching a power source to the ESA requires a matching network which increases the cost in terms of manufacturing as well as real estate. In recent years a new class of materials called metamaterials has emerged. These manmade materials with their unusual constitutive parameters possess immense potential to solve the problem of size reduction. This study seeks to validate, using Finite Difference Time Domain (FDTD) technique, a new metamaterial construct to achieve the desired objectives. FDTD code is developed for a cylindrical metamaterial wrapped around a modified biological antenna. The metamaterials are modeled using a Drude constitutive parameter model to simulate frequency dispersion. Epsilon Negative (ENG) as well as Double Negative (DNG) metamaterials are taken into consideration. Results show that the ESA using a metamaterial wraparound is found to have a quality factor lower than the theoretical Chu limit. Both ENG as well as DNG metamaterials exhibit their potential. The resonant frequency of the metamaterial antenna is reduced over the classical design while the radiation pattern of the antenna remains virtually unchanged.

Zugl.: München, Techn. Univ., Diss., 2008

Metamaterials are artificial materials engineered to have properties that may not be found in nature. The most fundamental properties of metamaterials are the periodic and sub-wavelength pattern structure. Metamaterials that affect electromagnetic waves in optic wavelengths are called optical metamaterials. The goal of this Thesis was to lay the foundation for design and development of optical metamaterials as potential biomedical sensor. Current biomedical sensors can made of some sensitive biological element (e.g. microorganism and antibodies). The limitations of the biological elements can be due to their susceptibility to extraneous factors such as PH value and temperature. The metamaterials made of gold, aluminum, glass and quartz can bave very high biological stability. Understanding and design of metalmaterials to modulate electromagnetic properties was the focus of the PhD and this was enabled by the use of CST Microwave Studio® software.This PhD made a distinct contribution to the design of two Optical metamaterials. Circular Dichroism (CD) has been widely used to gain information about biomolecules, DNA and organic compounds but, due to problems with accuracy, some indistinct features cannot be detected. The first is Circular Dichroism (CD), which is based on the negative refractive index. The material architectures modeled consisted of a glass substrate and an aluminum nano-ring on top. The research methodology involved adjusting the dimensions of the ring and modelling the impact on CD spectrum. The research in this Thesis shows that controllable CD filter design using optical metamaterial techniques could make it possible to amplify the target signals or unlock jammed signals. The other optical metamaterial designed in this Thesis is a core-shell structure. As a unit cell, the single core-shell and core-shell chain can be built into a biomedical sensor. The aim of this type of sensor is to control the position and value of the Fano dip by regulating the structural parameters of the optical metamaterial. To accomplish this, a concentric spherical structure that consists of a silver core and a quartz (SiO2) shell was designed. The thickness of the shell and the diameter of the core was adjusted independently. The Fano signal was found in the extinction cross section and could be easily identified. This peculiarity allowed the single or multiple optical scales to be marked in the visible spectrum. This can improve efficiency and precision when compared with traditional methods for the detection of biological macromolecules. This work opens new opportunities for fano resonance engineering in plasmonic metamaterials and nanostructures.

In the thesis, we propose an intelligent metamaterial to reduce the specific absorption rate (SAR) of cellular phone to phantom. We call our design intelligent metamaterial because it will not affect the antenna performance greatly. When the antenna is close to the phantom, the metamaterial acts like a stopband filter, thus reducing the SAR. We design a small metamaterial cell resonating at GSM 900 band, having 2 cm x 2cm x 0.4 cm in size. We use three cells to shield the PIFA antenna and the SAR is reduced by 29.12 % at a distance of 15 mm away from the phantom. Then we design a metamaterial cell having 1 cm x 1.95 cm x 0.4 cm in size,operating in GSM 1800 band. We use four cells to shield the PIFA antenna and the SAR reduced by 40.2 % at a distance of 15 mm away from the phantom. We also propose a dualband (GSM 900/1800) metamaterial having 2 cm x 4 cm x 0.8 cm in size. We use nine metamaterial cells to shield the dualband antenna and the ground¡Athe SAR reduced by 25 % at 900 MHz and 32.6 % at 1800 MHz at a distance of 15 mm away from the phantom.

In the thesis, we designed a metamaterial radome to increase the antenna gain. Owing to the need of high-directivity radiation in fix-point communications, antenna array and reflective antenna had been used to increase the directivity of antenna traditionally. Complicated feed and huge antenna size are the disadvantages of these techniques. We proposed a simpler metamaterial radome to increase the antenna gain. We find the subwavelength-hole structure formed by four Jerusalem cross structures can collimate electromagnetic wave originally spreading out from the holes. With the same size, multiple subwavelength holes in metamaterial radome can further enhance the antenna gain. We showed that metamaterial radome with 9 subwavelength holes can improve the gain by about 3.5 dB. In addition, we applied the concept of Fabry-Perot Cavity (FPC) to find the suitable distance between the radome and the antenna. When the resulting electromagnetic waves are in-phase, the radome can increase the antenna gain effectively. Recently, high-directivity radiation in fix-point communications is required and in the meantime multi-mode communication systems have become more and more popular. For practical purposes, we designed a coplanar dual-band metamaterial radome to be operated at 2.5 GHz and 3.5 GHz for WiMAX. This structure allows adjustment of its characteristics independently at each band. This coplanar dual-band radome can enhance the antenna gain by about 1.74 dB and 2.08 dB at 2.5 GHz and 3.5 GHz, respectively.

A study is made of the problem of metamaterial homogenization, which is the attempt to represent an artificially fabricated inhomogeneous periodic structure as a homogeneous medium with an electromagnetic response described by a number of constitutive parameters (permittivity, permability, etc.) In particular, the importance of spatial dispersion in metamaterials and the need to characterize metamaterials with wavevector dependent constitutive parameters is explained an examined. A brief survey of important previous attempts at metamaterial homogenization is presented. This is followed by a discussion of spatial dispersion in metamaterial crystals. The importance of spatial dispersion in metamaterials is justified and some manifestations of spatial dispersion described. In particular the little known phenomenon of bianisotropy in centrosymmetric crystals due to spatial dispersion is explained. Also, the effects of spatial dispersion on physical quantities such as energy flux and dissipation are identified. We then describe a new method for solving for the free eigenmodes of a metamaterial crystal with a complex wavevector eigenvalue simulation. Next, two different theoretical attempts by the author at metamaterial homogenization are described, both accompanied by tests of the calculated constitutive parameters and critical examination of the strengths and weaknesses of each approach. Finally, strong evidence of the presence and importance of spatial dispersion in metamaterials is presented. / text

This dissertation examines the use of metamaterial structures in millimeter-wave communication systems. Metamaterials, which are composite structures that have electromagnetic properties not found in nature, have been an area of explosive growth in academic research, including applications such as electrically small antennas, sub-wavelength imaging, and negative phase velocity transmission lines. In this dissertation, several potential applications of metamaterials are investigated, including new ideas related to negative forces. The design of highly directive antennas and their use in high data rate communication systems are emphasized. At millimeter-wave frequencies, specifically in a frequency band around 60 GHz, there is an enormous amount of available unlicensed worldwide spectrum available for data transmission. These systems may benefit from the knowledge of metamaterials and their integration with antenna systems. Although there are many challenges with working at such high frequencies and the metamaterials themselves are inherently dispersive and lossy, it will be demonstrated that useful structures can be designed and fabricated at these frequencies. Metamaterial-based artificial magnetic conductors were designed and it has been shown that they can handle 'gigabit per second' data rates. Moreover, superstrate structures were also designed to achieve near zero-index of refraction properties and, as a result, highly directive 60 GHz antenna systems. These metamaterial superstrate-based patch antennas were built and tested successfully with actual 'gigabit per second' data rates. Design and practical fabrication challenges associated with these millimeter-wave applications were addressed and will be reviewed.

Thesis advisor: Willie J. Padilla / Realization of an ideal electromagnetic absorber has long been a goal of engineers and is highly desired for frequencies above the microwave regime. On the other hand, the desire to control the blackbody radiation has long been a research topic of interest for scientists--one particular theme being the construction of a selective emitter whose thermal radiation is much narrower than that of a blackbody at the same temperature. In this talk, I will present the computational and experimental work that was used to demonstrate infrared metamaterial absorbers and selective thermal emitters. Based on these work, we further demonstrate an electrically tunable infrared metamaterial absorber in the mid-infrared wavelength range. A voltage potential applied between the metallic portion of metamaterial array and the bottom ground plane layer permits adjustment of the distance between them thus altering the electromagnetic response from the array. Our device experimentally demonstrates absorption tunability of 46.2% at two operational wavelengths. Parts of this thesis are based on unpublished and published articles by me in collaboration with others. The dissertation author is the primary researcher and author in these publications. The text of chapter two, chapter five, and chapter seven is, in part, a reprint of manuscript being prepared for publication. The text of chapter three is, in part, a reprint of material as it appears in Physical review letters 104 (20), 207403. The text of chapter four is, in part, a reprint of material as it appears in Physical Review Letters 107 (4), 45901. The text of chapter six is, in part, a reprint of material as it appears in Applied Physics Letters 96, 011906 / Thesis (PhD) — Boston College, 2013. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

The original work presented in this thesis pertains to the design and characterisation of resonant-cavity-based acoustic metamaterials, with a focus on airborne sound. There are five separate experimental chapters, each with a unique approach to the design of periodic structures that can support and manipulate air-bound acoustic surface waves via diffractive coupling between resonant-cavities. The first two chapters concern measurement of the acoustic transmission though various kinds of periodic slit-arrays, whilst the latter three chapters utilise a near-field imaging technique to directly record and characterise the dispersion of trapped acoustic surface waves. The first experimental chapter investigates the effect that thermodynamic boundary layers have on the Fabry-Perot-like cavity resonances that are so often utilised in acoustic metamaterial design. At audio frequencies, these boundary layers have a decay length that is typically more than two orders of magnitude smaller than the width of the resonating slit-cavities, hence it may naively be assumed that their effect can be ignored. However, by studying in detail the effect that reducing slit-cavity width has on the frequency of the measured cavity-resonance, for both a single slit cavity and a slit-cavity array, it is found that these boundary layer effects become significant on a far larger scale than their characteristic thickness. This is manifested in the form of a reduction in the resonant frequency as the slit-width is narrowed. Significant attenuation of the resonance and a 5% reduction in the effective speed of sound through the cavity is measured when the boundary layers form only 5% of the total width of each slit. Hence, it is both shown that the prevalent loss free treatment of acoustic slit-cavities is unrealistic, and that one may control the effective speed of sound through the slit-cavities with a simple change in slit-width. The second chapter explores the effect of ‘compound’ grating structure on trapped acoustic surface waves, a compound grating having a basis comprised of more than one resonating element. The angle dependent acoustic transmission spectra of four types of aluminium slit-array are recorded, and for the compound gratings, it is found that sharp dips appear in the spectra that result from the excitation of a ‘phase-resonance’. This occurs as new degrees-of-freedom available to the acoustic near-field allow the fields of adjacent cavities within a unit-cell to be both out-of-phase and strongly enhanced. By mapping the transmission spectra as a function of in-plane wavevector, the dispersions of the modes supported by each sample are determined. Hence, the origin of the phase-resonant features may be described as acoustic surface waves that have been band-folded back into the radiative regime via diffraction from higher in-plane wavevectors than possible on a simple grating. One of the samples is then optimised via numerical methods that account for thermodynamic boundary layer attenuation, resulting in the excitation of a sharp, deep transmission minimum in a broad maximum that may be useful in the design of an acoustic filter. The third chapter introduces the near-field imaging technique that can be utilised to directly characterise acoustic surface waves, via spatial fast Fourier transform algorithms of high-resolution pressure field maps. The acoustic response of a square-lattice open-ended hole array is thus characterised. It is found that over a narrow frequency band, the lattice symmetry causes the acoustic surface power flow to be channelled into specific, predictable directions, forming ‘beams’ with a well defined width. In chapter four, the existence of the ‘acoustic line mode’ is demonstrated, a type of acoustic surface wave that may be supported by a simple line of open-ended hole cavities. The near-field imagine technique is again used to extract the mode dispersion. This acoustic line mode may be readily manipulated, demonstrated by arrangement of the line of holes into the shape of a ring. The existence of this type of mode offers a great deal of potential for the control of acoustic energy. Chapter five explores the effect of ‘glide-symmetry’ on a pair of acoustic line modes arranged side-by-side. A control sample not possessing glide- symmetry is first characterised, where measurement of the acoustic near- fields show that this sample supports two separate modes at different frequencies, with their phase either symmetric or anti-symmetric about the mirror plane between the lines of holes. One of these lines is then shifted along its periodicity by half of a grating pitch, thus creating glide-symmetry. The resulting sample is found to support a single hybrid mode, capable of reaching a much larger in-plane wavevector than possible on a simple grating with no gaps in its band-structure, and displaying a region of negative dispersion. The third sample demonstrates how one may increase the coupling strength between the two lines of holes via manipulation of the cavity shape, thus enhancing the glide-symmetry effect. The thesis concludes with preliminary investigations into other possible ways of manipulating acoustic surface waves, such as with the use of ‘screw-symmetry’.

Stuttgart, Univ., Diss., 2008.