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
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 atan atomic scale could be replicated on a larger scale?― To appear homogeneous, the structures would have to be electricallysmall and spaced electrically close― The concept of metamaterials was first proven in the microwavespectrum.
10 Microwave Metamaterials ― Early metamaterials relied on a combination of Split-ring resonators(SSRs) and conducting wires/posts― SSRs used to generate desired µrfor a resonant band of frequencies.― Conducting posts are polarized bythe electric field, generating thedesired εr for all frequencies belowa certain cutoff frequency.
11 Microwave Metamaterials ― Other approaches for fabricating microwave metamaterials have alsobeen 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 LCresonating response. SRRs are still required to affect µr.
13 Microwave Metamaterials Proven areas of Microwave Metamaterials:― Microwave cloaking bybending EM rays usinggraded indices of refraction― Currently limited to relativelynarrow bandwidths andspecific polarizations― Limited by resonant frequencyresponse
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 canalso be adjusted, from fully closedto fully “open” (see Fig. right)― Capable of achieving phasemodulation 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.
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 higherfrequencies.- 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 theoreticalstage- Mathematically characterizing nanoscale plasmonice effects.- Computer simulations of proposed designs.― Relatively little work has been done with physically realized opticalmetamaterials
20 Optical Metamaterials ― Rare example of 3D optical metamaterial. Gold nanostructures with70nm spacing between layers.
21 Optical Metamaterials ―Experimental measurements of the previous optical metamaterialparallel polarized wavesperpendicular polarized waves
22 Conclusions― Introduction of metamaterials in 1990’s opened new possibilities inelectromagnetics.― Successful implementation of metamaterial technology in themicrowave 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