2. Overview
- Personal Background in Metamaterials
- Introduction to Metamaterials
- Definition of Metamaterial
- How Metamaterials work
- Microwave Metamaterials
- Optical Metamaterials
- Conclusions
5. Introduction to Metamaterials
Electromagnetic waves
- Not much difference between 1kHz (λ=300km) and 1THz
(λ=0.3mm)
Why can’t 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 material’s electromagnetic properties?
- 1 method is to introduce periodic features that are electrically
small over a given frequency range, that appear “atomic” at those
frequencies
6. 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.
7. Definition of Metamaterial
- “Metamaterial” coined in the late 1990’s
- 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.
8. How Metamaterials Work
Example: How to achieve negative index of refraction
-
- negative refraction can be achieved when both µr and εr are
negative
- negative µr and εr occur in nature, but not simultaneously
-silver, gold, and aluminum display negative εr at optical
frequencies
-resonant ferromagnetic systems display negative µr at
resonance
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9. How Metamaterials Work
Example: How to achieve negative index of refraction
― What if the structures that cause this frequency variance of µr and εr 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.
10. 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 εr for all frequencies below
a certain cutoff frequency.
11. Microwave Metamaterials
― Other approaches for fabricating microwave metamaterials have also
been developed
- Transmission line models using shunt inductors for affecting εr
and series capacitors for affecting µr. This method, however, is
restrained to 1D or 2D fabrication
12. Microwave Metamaterials
― Conducting wires/posts can be replaced with loops that mimic an LC
resonating response. SRRs are still required to affect µr.
13. 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
14. 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
15. 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 SRR’s 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
16. 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 0.08λ)
17. 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.
1
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18. Optical Metamaterials
Fabrication/Design Challenges for optical metamaterials:
― Smaller wavelength = smaller features
- Coupling between elements becomes more serious
― Metal’s 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 metal’s response is known as plasmonic
- Resistive and dielectric losses become much more significant
19. 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
20. Optical Metamaterials
― Rare example of 3D optical metamaterial. Gold nanostructures with
70nm spacing between layers.
22. Conclusions
― Introduction of metamaterials in 1990’s 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
