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8/13/2019 Learing Basics of MEMS & Metamaterials 1
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OUTLINES
• EM spectrum
• Why Metamaterials?
• Introduction to Metamaterials
•
Properties of Metamaterials
• Why MEMS?
• How to actuate the MEMS structure?
• Principles of electrothermal actuation of MEMS
•
Design of MEMS structure for tunablemetamaterials
• How to Design Metamaterials?
• Split Ring Resonators (SRR)
• Tunable Metamaterials
•Applications of Metamaterials
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EM spectrum
•
All EM waves except the EM waves in the range of THz respond to naturalmaterials.
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Why Metamaterials?
• The band of frequency in EM spectrum betweenradio waves and IR light is THz gap. Its is defined as0.1 to 10THz.
• EM waves in this range does not respond to naturalmaterials.
• Thus we have to develop metamaterials, which canshow unusual natural material properties, so that itcan respond to the THz range of EM waves.
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Introduction to Metamaterials
What’s 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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Properties of Metamaterials
•
Metamaterial is a periodic material that derives its properties fromits structure rather than its components
• Depending on the structure , metamaterials may have refractiveindex less than 1 and even negative
• They are assemblies of multiple individual elements fashioned from
conventional microscopic materials such as metals or plastics, butthe materials are usually arranged in periodic patterns
• Their precise shape, geometry, size, orientation and arrangementcan affect the waves of light or sound in an unconventional manner,creating material properties which are unachievable withconventional materials
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• In negative index metamaterials (NIM), both
permittivity and permeability are negative resulting ina negative index of refraction.
• When a negative index of refraction occurs,
propagation of the electromagnetic wave is reversed.
• Resolution below the diffraction limit becomes
possible. This is known as Sub wavelength imaging.
Properties of Metamaterials
Negative index of refraction, Backwardwaves and Sub-wavelength imaging
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Left Handed Materials (LHMs)if E is along the positive x direction and H along the positive y direction,
the wave will propagate along the negative z direction in a LHM
Conventional materials Left Handed materials
Properties of Metamaterials
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How to Design Metamaterials?
• Designing each unit cell using MEMS structure
• Periodic arrangement of the unit cell structure leadto the pattern for metamaterial
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a
a
p
p
schematics of the elementary cell.
d
•Mostly the open ring resonator canbe considered as an LC circuit
•The incident wavelength, λi > 1m
•From the figure,
P→ pitch of the cell
a→ metamaterials dimension
Unit Cell Size
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Split Ring Resonators (SRR)
• one of the most common elements used tofabricate metamaterials
• designed to mimic the magnetic response of atoms and tohave a stronger magnetic coupling than is found in nature
• Each unit can be designed to have its own magnetic
response and the overall effect reduces power requirements
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• A metamaterial with a variable response to anincident electromagnetic wave.
• The response includes the capability to determine
whether the EM wave is transmitted, reflected, orabsorbed
• Allow arbitrary adjustments to frequency changes inthe refractive index.
•the lattice structure of the tunable metamaterial isadjustable in real time, making it possible toreconfigure a metamaterial device during operation
Tunable metamaterials
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Applications of Metamaterials
•
There'd be plenty of applications in the civilian world aswell, even for rudimentary cloaking devices.
• For example, to create receptacles to shield sensitivemedical devices from disruption by MRI scanners, or buildcloaks to route cellphone signals around obstacles.
• Potential applications of metamaterials are diverse:
• remote aerospace applications,
• sensor detection and infrastructure monitoring,
• smart solar power management,
• public safety, high-frequency battlefield communication .
• lenses for high-gain antennas, improving ultrasonic sensors,and even shielding structures from earthquakes.
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LIGHT
SOURCE
OBJECT
METAMATERIAL
LIGHT
RAYS
A 3D Possibility
Invisibility using metamaterials
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WhyMEMS?•
To control the transmission and reflectionproperties of metamaterials
• More drastic change of THz transmissionperformance by geometrically changing the
metamaterial unit cells
How to actuate the MEMSstructure?•
Electrostatics• Electrothermal
• Electromagnetic
• Piezoelectric
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Principles of electrothermalactuation of MEMS
Material A
Material B
• Coefficient of Thermal Expansion (CTE) measures the
fractional change in length per degree change in temperature
at a constant pressure of a solid substance.
• Due to CTE) of the material, substrate will bend towards the
opposite direction of the expanded material
• If both the materials are thermally expanded, depending on
the CTE, the material will bend to the side of material with
less CTE.
For example: refer to figure 1CTE of Material A = 27nm/K
CTE of Material B = 0.1nm/K
The cantilever tends to bend downwards since
the CTE of Material A is more than Material B.Figure 1
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Design of MEMS structure fortunable metamaterials
• A cantilever structure is to be designed and fabricated to act
as an actuator
• The structure will be tuned with an AC input and it will beactuated.
• This cantilever actuated MEMS structure will be the unit cell
for the metamaterials
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