Propagation of Electromagnetic Waves in Structured Metamaterials and System of Two-Level Atoms
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Date
2017-05
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Addis Ababa University
Abstract
Metamaterials are artificially engineered composites that exhibit superior properties
not observed in nature or in the constituent materials. Because of the sub-wavelength
periodicity of their constituent materials, metamaterials appear as homogeneous entity
to an incident wave and can be described by effective parameters, such as electric
and magnetic material properties, that are controlled by the unit cells geometry and
the type of its constituent parts. These controllable material properties, in turn,
makes structured metamaterials (SMMs) more interesting for diverse potential device
applications. Consequently, it is of vital importance to investigate the responses
of SMMs to electromagnetic waves (EMWs).
In this dissertation, the dispersion properties of SMMs consisting of strips of a
copper wire (electron subsystem) and square copper split-ring-resonators (magnetic
subsystem) with different and coinciding resonant frequencies are studied. In a narrow
frequency band above the resonant frequency of the electron subsystem, the structured
metamaterial is described by a negative refractive index. In addition to this,
there are some peculiar properties observed in these metamaterials. Among these
properties is the nonanalytic behavior of the real part of the refractive index as a
function of the frequency with a discontinuity of its derivative in the metamaterial
with two resonances. It is also shown that the superluminal, slow, and backward
microwaves can exist in the structured metamaterials. However, in the absence of
gain components, only the slow microwaves can propagate considerably.
In addition, we investigated the propagation of narrow packets of EMWs in frequency
dispersive medium with the consideration of the complex refractive index. It
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is shown that taking into account of the dispersion of the complex refractive index
within the context of the conventional expression of the group velocity of narrow wave
packets of EMWs propagating in a dispersive medium results in the appearance of
additional constraints on the group velocity, which dictates that the physically acceptable
group velocity can only be realized in the case of a negligible imaginary part
of the group index. The conditions that allow one to realize the physically acceptable
group velocity are formulated and analyzed numerically for the relevant model of the
refractive index of a system of two-level atoms in the optical frequency range. It is
shown that in the frequency band where superluminal light propagation is expected,
there is a strong dispersion of the refractive index that is accompanied with strong
absorption, resulting in a strongly attenuated superluminal light
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Keywords
Propagation of Electromagnetic Waves