Will transformation optics be the next optical revolution?

Transformation optics and metamaterial,which can make sci-fi cloaking in the lab,promise to achieve many previously unseen optical feats--the big challenge now is to turn the new concepts from the ivory tower into reality.
Metamaterials have changed the rules of optics.Nanostructured materials can control light at subwavelengths,affecting everything from negative refraction to invisibility cloaks that had long been considered infeasible.Over the past 12 years,metamaterials,which had previously been overreacting,have developed into one of the most exciting fields of photonics.
The promise of metamaterials and transformation optics,however,stems from the fact that they have slowly been extended into practical applications.Invisibility cloaks can be demonstrated in a laboratory,but usually only small objects hidden in monochromatic light can be seen from a specific Angle.The main effect depends on the resonance,so they do not work in broadband lighting.Nanostructures with wavelengths much smaller than light are difficult to manufacture precisely,and even harder to manufacture in bulk.Structural materials that can interact with light waves in an ideal way are difficult to find.The challenge now is to overcome these limitations and develop practical applications.
The development of metamaterials
Metamaterials were first demonstrated at microwave frequencies,developed from early work on artificial media.The basic idea is to assemble arrays of many subwavelength elements(including conductors and media)into bulk structures that would otherwise not have metamaterial properties,particularly refractive index.
In conventional optics,the refractive index n is usually defined as the ratio of the speed of light traveling in a vacuum to the speed in the material.However,the potential physical significance depends on two more fundamental data--permittivityεand permeabilityμ,n is actually equal to
N=plus or minus epsilon mu
In a vacuum,εandμare both defined as 1,so n=1 in a vacuum.In a medium such as glass,both of these numbers are positive;But conductors in the visible band have negative permittivity and positive permeability,so their complex refractive index has a large negative component,and therefore the metal has a large attenuation.
Natural materials with uniform composition have a uniform refractive index because light waves only"see"the bulk material,not the atoms.Similarly,metamaterials have a uniform refractive index for light waves,because many of the same,evenly spaced units are much smaller than the wavelength.However,the magnetic permeability and dielectric constants of such constructed metamaterials can be engineered to achieve refractive indices that are impossible in conventional optics,such as bending light backwards as it enters the metamaterial at n=-1.
The effectiveness of the permittivity and permeability of a metamaterial depends on how light waves interact with their internal components.Regular wire arrays produce effective permittivity,which can vary from positive to negative numbers depending on size,spacing,and arrangement.Similarly,adjusting the design of the open-ring elements can produce a wide range of permeability.The optical effects are similar to those of radio waves equipped with subwavelength antenna arrays.
Transformation optics
Metamaterials that consist of uniform arrays of subwavelength units are essentially custom-made materials designed for unique properties.However,when the design is extended to include non-uniform arrays of subwavelength elements,more options will be created,opening the door to a new field of transformation optics that will go beyond geometric optics to manipulate electromagnetic fields in metamaterials.
"Our intuitive understanding of light is that it approximates rays and is closely related to our vision.For our eyes,light behaves like a stream of particles.""John Pendry of Imperial College London,a metamaterial pioneer,wrote in the journal Science.The standard ray approximation assumes that light passes through an object in a straight line,resulting in a uniform refraction coefficient.However,in the subwavelength range,the light image fails,and the structural design can change the propagation of electric and magnetic field lines in arbitrary ways,which is not possible in conventional block optics.This is the domain of transformation optics.
Fermat's principle is used in traditional optics to describe how the change of refractive index affects the propagation path of light."The emerging field of variable optics allows us to solve the problem in reverse,that is,how to achieve specific optical paths by designing the properties of various materials."Liu Yongming and Zhang Xiang wrote.So in order to hide an object using an invisibility cloak,they can specify the light path they want to guide the light,and then use transform optics to design the metamaterials needed to follow the light that follows that light path.
Designing a two-dimensional invisibility cloak requires bypassing the light path of a hidden object;Transformation optics has been used to design such metamaterial cloaking.Transformation optics can also be used in the design and manufacture of various lenses,beam rotators,beam shifters and metamaterial structural units used in optical illusions.
Metamaterial building blocks
The transformation optics building blocks are essentially the same subwavelength units used in other metamaterials.In addition to metals,there are surface plasmons generated by dielectric,metal-dielectric interface.The size and shape of these units vary by the desired deformation of the metamaterial.In visible light,individual units have to be less than 400,right?700 nanometers,or something more than a thousand atoms wide in a solid.
Researchers are exploring more directions.The strong magnetic reactions of metals can be used to produce unique interactions,but there is a large loss in the near infrared and visible wavelengths.Metals react differently at optical and infrared wavelengths.The resonance effect of metal structure can produce strong interaction,but the effect peak is beyond the limit range.Dielectric can provide more wide band response and lower loss.
Among the possibilities being explored,dielectric nanocavity resonance has a high refractive index,a property that provides lower losses than metals and a smaller scale than vacuum wavelengths.For example,Lei Shi and colleagues at the Polytechnic Institute of Valencia(Madrid,Spain)studied near-infrared resonance in silica gel as small as 250 nanometers.They are keen on metamaterials that use liquids such as air and water.
Flat metamaterial surfaces
The fabrication of optical metamaterials has faced challenges,and some researchers are working on planar structures."The possibility of controlling light with surface confining,planar components is fascinating."Vladimir Shalaev of Purdue University(Bloomington,IN)and colleagues recently wrote."Metamaterial surfaces cause new physics and phenomena to differ markedly from the aspects corresponding to three-dimensional structures.They are also compatible with on-chip nanophotonics.
These metamaterial surfaces deposit thin metal and dielectric patterns on a substrate.The response is not consistent with the reflection and refraction of the bulk material and depends on the light dispersion in the subwavelength layer.So far,demonstration experiments have been done on the negative refractive surface antenna arrays including in the infrared band and the plane chirality effect in the optical band.Metamaterial surfaces also produce 3D effects without the use of solid materials,and the Purdue team hopes they can also control the phase,polarization and frequency of light.
Phase change materials can be integrated into flat semiconductor metamaterial surfaces for optical opening,beam control,pulse shaping,or modulation,the Purdue team wrote.They also propose the possibility of fabricating hyperbolic metamaterial surfaces,similar to mass fabricating hyperbolic metamaterials through strong anisotropy and hyperbolic dispersion generated by metal-dielectric interactions.Bulk material plates can dramatically change the behavior of light,but they are difficult to manufacture because of losses.However,Purdue's team believes that quasi-two-dimensional hyperbolic metamaterials have lower surface losses and can be used with standard semiconductor technologies.
Transformation optics has become a powerful paradigm for exploiting the application potential of optical metamaterials."The key benefit of this approach for electromagnetism is the physical implications,"writes Pendry.He predicts that"transformation optics will become the design tool of choice in electromagnetic theory.""
The challenges may be as great as the potential prospects.At this very moment,the metamaterial revolution is under way in many research directions in laboratories around the world.New breakthroughs are needed in the design and manufacture of nanostructures.There is no consensus on the best shapes and materials to use to make metamaterials.The application area is also uncertain.Like lasers half a century ago,today's metamaterials seem to be a solution,looking for a problem,but we know how to achieve it.