Overview of Metamaterials and their

Radar and Optical Applications

Jay B Bargeron

Overview

- Personal Background in Metamaterials

- Introduction to Metamaterials

- Definition of Metamaterial

- How Metamaterials work

- Microwave Metamaterials

- Optical Metamaterials

- Conclusions

Personal Background

Introduction to Metamaterials

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

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.

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.

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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1))(())(( 2/2/2/1 jjjjj

rr eeeee

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.

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.

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

Microwave Metamaterials

― Conducting wires/posts can be replaced with loops that mimic an LC resonating response. SRRs are still required to affect µr.

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

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

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

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)

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.

1n

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

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

Optical Metamaterials

― Rare example of 3D optical metamaterial. Gold nanostructures with 70nm spacing between layers.

Optical Metamaterials

―Experimental measurements of the previous optical metamaterial

parallel polarized waves perpendicular polarized waves

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

Fin

Questions???