Overview%20of%20Metamaterials

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Overview of Metamaterials and their Radar and
Optical Applications Jay B Bargeron
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Overview

• - Personal Background in Metamaterials
• - Introduction to Metamaterials
• - Definition of Metamaterial
• - How Metamaterials work
• - Microwave Metamaterials
• - Optical Metamaterials
• - Conclusions

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Personal Background
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Introduction to Metamaterials
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Introduction to Metamaterials
Electromagnetic waves - Not much difference
between 1kHz (?300km) and 1THz (?0.3mm) Why
cant optical light (Terahertz frequency) go
through walls like microwaves? - Material
response varies at different frequencies -
Determined by atomic structure and arrangement
(10-10 m). How can we alter a materials
electromagnetic properties? - 1 method is to
introduce periodic features that are electrically
small over a given frequency range, that appear
atomic at those frequencies
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Introduction to Metamaterials
Whats in a name? - Meta- means altered,
changed or higher, beyond Why are they called
Metamaterials? - Existing materials only exhibit
a small subset of electromagnetic properties
theoretically available - Metamaterials can
have their electromagnetic properties altered to
something beyond what can be found in nature. -
Can achieve negative index of refraction, zero
index of refraction, magnetism at optical
frequencies, etc.
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Definition of Metamaterial

• - Metamaterial coined in the late 1990s
• - According to David R. Smith, any material
  composed of periodic, macroscopic structures so
  as to achieve a desired electromagnetic response
  can be referred to as a Metamaterial
• -(very broad definition)
• -Others prefer to restrict the term Metamatetial
  to materials with electromagnetic properties not
  found in nature
• - Still some ambiguity as the exact definition
• - Almost all agree the Metamaterials do NOT rely
  on chemical/atomic alterations.

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How Metamaterials Work

• Example How to achieve negative index of
  refraction
• -
• - negative refraction can be achieved when both
  µr and er are negative
• - negative µr and er occur in nature, but not
  simultaneously
• -silver, gold, and aluminum display negative er
  at optical frequencies
• -resonant ferromagnetic systems display negative
  µr at resonance

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How Metamaterials Work
Example How to achieve negative index of
refraction ? What if the structures that cause
this frequency variance of µr and er at an
atomic scale could be replicated on a larger
scale? ? To appear homogeneous, the structures
would have to be electrically small and
spaced electrically close ? The concept of
metamaterials was first proven in the microwave
spectrum.
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Microwave Metamaterials
? Early metamaterials relied on a combination of
Split-ring resonators (SSRs) and conducting
wires/posts ? SSRs used to generate desired µr
for a resonant band of
frequencies. ? Conducting posts are polarized
by the electric field,
generating the desired er
for all frequencies below a
certain cutoff frequency.
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Microwave Metamaterials
? Other approaches for fabricating microwave
metamaterials have also been developed -
Transmission line models using shunt inductors
for affecting er and series capacitors for
affecting µr. This method, however, is
restrained to 1D or 2D fabrication
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Microwave Metamaterials
? Conducting wires/posts can be replaced with
loops that mimic an LC resonating
response. SRRs are still required to affect µr.
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Microwave Metamaterials
Proven areas of Microwave Metamaterials ?
Microwave cloaking by bending EM rays using
graded indices of refraction ? Currently
limited to relatively narrow bandwidths and
specific polarizations ? Limited by
resonant frequency response
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Microwave Metamaterials
Proven areas of Microwave Metamaterials ?
Sub-wavelength antennas - n 0 in
metamaterial - capable of directionality - same
antenna can be used for multiple frequency
bands - currently used in Netgear wireless
router (feat. right) and the LG Chocolate
BL40
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Microwave Metamaterials
Tuneable metamaterials ? Consider a 2-D
metamaterial, with series capacitance to affect
its EM response - This capacitance can be tuned
via ferroelectric varactors, affecting the index
of refraction of the material ? The size of the
split in SRRs can also be adjusted, from
fully closed to fully open (see Fig.
right) ? Capable of achieving phase
modulation of up to 60 degrees ? Applications in
phased-arrays, beam forming, and beam
scanning
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Microwave Metamaterials
Planar microwave focusing lens ?Researchers at
University of Colorado have achieved a planar
array for focusing microwave radar -Though not
touted as metamaterial, meets the requirements
under the broad definition of metamaterials. The
Perfect Lens ?J.B. Pendry theoretically described
how a rectangular lens with n -1 could make a
perfect lens capable of resolving
sub-wavelength features. -Researchers in China,
using a planar Transmission Line type of
metamaterial to focus a point source (480 MHz) ,
managed to achieve sub-diffraction focusing down
to 0.08?)
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Faster than light transmission lines?
Could this be possible? - recall that v c / n,
where v is the phase velocity. - if
then phase velocity will be greater than
c! Reality Law of Causilty - We cannot see into
the future OR even the present - While phase
velocity can exceed c, group velocity cannot -
Any change in energy/frequency will propagate
through the metamaterial slower than c.
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Optical Metamaterials
Fabrication/Design Challenges for optical
metamaterials ? Smaller wavelength smaller
features - Coupling between elements becomes
more serious ? Metals response to
electromagnetic waves changes at higher
frequencies. - Metal no longer behaves as
perfect electrical conductors (dielectric
losses need to be taken into account) - A
frequency is eventually reached where the energy
of the oscillating, excited electrons becomes
comparable to the electric field. When this
occurs, the metals response is known as
plasmonic - Resistive and dielectric losses
become much more significant
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Optical Metamaterials
? Most research on optical metamaterials has been
at the theoretical stage - Mathematically
characterizing nanoscale plasmonice effects. -
Computer simulations of proposed designs. ?
Relatively little work has been done with
physically realized optical metamaterials
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Optical Metamaterials
? Rare example of 3D optical metamaterial. Gold
nanostructures with 70nm spacing between
layers.
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Optical Metamaterials
?Experimental measurements of the previous
optical metamaterial
perpendicular polarized waves
parallel polarized waves
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Conclusions
? Introduction of metamaterials in 1990s opened
new possibilities in electromagnetics. ?
Successful implementation of metamaterial
technology in the microwave spectrum. ?
Inherent difficulties exist in fabricating
optical metamaterials ? Most work to date
related to modeling proposed designs ? Little
work, so far, on successful application of
optical metamaterials
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Fin
Questions???