Browsing by Subject "beaming"

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Open Access
Chiral metamaterial and high-contrast grating based polarization selective devices
(2013) Mutlu, Mehmet
The utilization of purposely designed artificial media with engineered electromagnetic responses enables the obtaining of intriguing features that are either impossible or difficult to realize using readily available natural materials. Here, we focus on two classes of artificial media: metamaterials and high-contrast gratings. Metamaterials and high-contrast gratings are designed within the subwavelength periodicity range and therefore, they are non-diffractive. We exploit the magnetoelectric coupling effect in chiral metamaterials to design several structures. Firstly, we design a linear to circular polarization convertor that operates for x-polarized normally incident plane waves. Then, we combine the chirality feature and the electromagnetic tunneling phenomenon to design a polarization insensitive 90◦ polarization rotator that exhibits unity transmission and crosspolarization conversion efficiencies. Subsequently, we combine this polarization rotator with a symmetric metallic grating with a subwavelength slit for the purpose of enabling the one-way excitation of spoof surface plasmons and achieving a reversible diodelike beaming regime. Then, we exploit the asymmetric transmission property of chiral metamaterials and show that a polarization angle dependent polarization rotation and a strongly asymmetric diodelike transmission is realizable. Afterwards, a brief waveguide theory is provided and eventually, the dispersion relations for a periodic dielectric waveguide geometry are derived. Then, using these relations and considering the finiteness of the waveguide length, we show the theoretical description of high-contrast gratings. Finally, we theoretically and experimentally show that the achievement of a broadband quarter-wave plate regime is possible by using carefully designed high-contrast gratings.
Open Access
Novel approaches to control the propagation of electromagnetic waves : metamaterials and photonic crystals
(2007) Bulu, İrfan
Applications in areas such as photonics, antennas, imaging and optics require the control of propagation of electromagnetic waves and the control of emission of electromagnetic radiation. Achievements in three key research fields may provide the answer to these problems. These emerging research fields are metamaterials, photonic crystals and surface plasmons. This thesis will be about our work on metamaterials and photonic crystals. Metamaterials are a new class of artificial structures whose electromagnetic response can be described by effective permeability and permittivity functions that may attain negative values. I will present our results on the properties of a metamaterial structure that we proposed recently, the labyrinth structure. I will demonstrate that the labyrinth structure can be used to design a medium that exhibits negative permeability values within a certain frequency range. Moreover, I will explore the possibility of negative refraction and sub-wavelength focusing of electromagnetic waves by two and threedimensional labyrinth structure based left-handed mediums. Novel applications such as metamaterial based compact size antennas, ultra-small high-Q cavities will be also discussed. Another type of artificial electromagnetic structures are the photonic crystals. Photonic crystals can be described by a periodic modulation of the permittivity and/or the permeability of a medium. I will discuss two phenomena arising from the dispersion properties of photonic crystals and their possible applications. One of these phenomena is the existence of surfacebound electromagnetic modes and the other is the negative refraction effect. I will further show that the surface-bound modes can be used for applications such as beaming of electromagnetic waves and enhancement of transmission through sub-wavelength apertures. In addition, I will demonstrate that the negative refraction effect can be utilized to focus electromagnetic waves emitted from a finite size source.