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 MetamaterialsElectromagnetic 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

Introduction to MetamaterialsWhats in a name?- Meta- means altered, changed or higher, beyondWhy 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 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.

How Metamaterials WorkExample: How to achieve negative index of refraction- - negative refraction can be achieved when both r and r are negative

- negative r and r occur in nature, but not simultaneously-silver, gold, and aluminum display negative r at optical frequencies-resonant ferromagnetic systems display negative r at resonance

How Metamaterials WorkExample: How to achieve negative index of refraction

What if the structures that cause this frequency variance of r and r 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 r for a resonant band of frequencies.

Conducting posts are polarized by the electric field, generating the desired r 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 r and series capacitors for affecting r. 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 r.

Microwave MetamaterialsProven 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 MetamaterialsProven 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 MetamaterialsTuneable 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

Microwave MetamaterialsPlanar microwave focusing lensResearchers 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 LensJ.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)

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.

Optical MetamaterialsFabrication/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

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 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

FinQuestions???