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
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 εrare 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 negativeµr at resonance
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.
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 perpendicular polarized waves parallel 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