What are metamaterials?
The prefix meta indicates that the characteristics of the material are beyond what we see in nature. Metamaterials are artificially crafted composite materials that derive their properties from internal microstructure, rather than chemical composition found in natural materials.
The core concept of metamaterials is to craft materials by using artificially designed and fabricated structural units to achieve the desired properties and functionalities. These structural units – the constituent artificial 'atoms' and 'molecules' of the metamaterial – can be tailored in shape and size, the lattice constant and interatomic interaction can be artificially tuned, and 'defects' can be designed and placed at desired locations.
By engineering the arrangement of these nanoscale unit cells into a desired architecture or geometry, one can tune the refractive index of the metamaterial to positive, near-zero or negative values. Thus, metamaterials can be endowed with properties and functionalities unattainable in natural materials.
For instance, in order for invisibility cloak technology to obscure an object or, conversely, for a 'perfect lens' to inhibit refraction and allow direct observation of an individual protein in a light microscope, the material must be able to precisely control the path of light in a similar manner.
Metamaterials offer this potential.
Although metamaterials already have revolutionized optics, their performance has been limited by their inability to function over broad bandwidths of light. Designing a metamaterial that works across the entire visible spectrum remains a considerable challenge.
One hitch is that any such material needs to interact with both the electric and magnetic fields of light. Most natural materials are blind to the magnetic field of light at visible and infrared wavelengths. Previous metamaterial efforts have created artificial atoms composed of two constituents – one that interacts with the electric field, and one for the magnetic.
A drawback to this combination approach is that the individual constituents interact with different colors of light, and it is typically difficult to make them overlap over a broad range of wavelengths.
Among the most sought-after properties of metamaterials is the negative index of refraction of light and other radiation. Negative refraction is based on the equations developed in 1861 by Scottish physicist James Maxwell.
All known natural materials have a positive refractive index so that light that crosses from one medium to another gets slightly bent in the direction of propagation. For example, air at standard conditions has the lowest refractive index in nature, hovering just above 1. The refractive index of water is 1.33. That of diamond is about 2.4. The higher a material's refractive index, the more it distorts light from its original path.
In some metamaterials, however, negative refraction occurs such that light and other radiation gets bent backwards as it enters the structure.
The existence of substances with a negative refractive index was predicted as early as the middle of the 20th century. In 1976 Soviet physicist V.G. Veselago published an article that theoretically describes their properties, including an unusual refraction of light. The term metamaterials for such substances was suggested by Roger Walser in 1999.
But only in the early 2000s have researchers figured out how to create materials of any type that can achieve negative refraction. The first samples of metamaterials were made from arrays of thin wires and only worked with microwave radiation.
The realization of metamaterials and their unusual optical properties has enabled a number of novel devices, including invisibility cloaks that can completely conceal an object.
With such negative refraction materials many applications become possible in electronics manufacturing, lithography, biomedicine, insulating coatings, heat transfer, space applications, and perhaps new approaches to optical computing and energy harvesting.
The fascinating functionalities of metamaterials typically require multiple stacks of material layers, which not only leads to extensive losses but also brings a lot of challenges in nanofabrication. Many metamaterials consist of complex metallic wires and other structures that require sophisticated fabrication technology and are difficult to assemble.
The unusual optical effects do not necessarily imply the use of the volumetric (3D) metamaterials. You can also manipulate the light with the help of two-dimensional (2D) structures – so-called metasurfaces (or flat optics).
Metasurfaces are thin-films composed of individual elements that have initially been developed to overcome the obstacles that metamaterials are confronted with.
The principle of operation of metasurfaces is based on the phenomenon of diffraction. Any flat periodic array can be viewed as a diffraction lattice, which splits the incident light into a few rays. The number and direction of the rays depends on geometrical parameters: the angle of incidence, wavelength and the period of the lattice.
The structure of the subwavelength unit cell, in turn, determines how the energy of the incident light is distributed between the rays. For a negative refractive index it is necessary that all but one of the diffraction rays are suppressed, then all of the incident light will be directed in the required direction.
So far, most fabricated metasurfaces are passive, meaning that they cannot be tuned post fabrication. In contrast, active metasurfaces allow the dynamic control of its optical properties under external stimuli. They could be useful in applications ranging from free space optical communications to holographic displays, and depth sensing.
One metamaterial application of particular interest is a super lens, a device that might provide light magnification at levels that dwarf any existing technology.
The concept of a 'superlens' has attracted significant research interest in the imaging and photolithography fields since the concept was proposed back in 2000 (see the original paper by Pendry in Physical Review Letters: "Negative Refraction Makes a Perfect Lens").
A superlens allows to view objects much smaller than the roughly 200 nanometers that a regular optical lens with visible light would permit. This theoretical resolution limit (diffraction limit) of conventional optical imaging methodology was the primary factor motivating the development of higher-resolution scanning probe techniques. Though scanning electron microscopes can capture objects that are much smaller, down to the single nanometer range, they are expensive, heavy, and, at the size of a large desk, not very portable.
The superlens concept relies on the generation of surface plasmon polaritons enhancing the evanescent fields to restore the near-field components of the Fourier decomposition of the source object, hence breaking the diffraction limit.
Since superlenses have demonstrated the capability of subdiffraction-limit imaging, they have been envisioned as a promising technology for potential nanophotolithography. Already, superlens lithography is able to demonstrate the required sub-diffraction-limit resolution and high contrast performance required for cost-effective and high throughput nanopatterning mass production (Nano Letters, "High Aspect Subdiffraction-Limit Photolithography via a Silver Superlens").
Optical invisibility camouflage (or invisibility cloaking) is a technology to make an object seem invisible by causing incident light to avoid the object, flow around the object, and return undisturbed to its original trajectory.
Such sophisticated manipulation of light will probably be realistic thanks to the recent progress in the research on metamaterials. To date several research institutes have carried out the theoretical and experimental study of invisibility camouflage devices, using the extraordinary optical properties of metamaterials and the technique of transformation optics.
Optical camouflage devices designed using transformation optics have a closed region that incident light from every direction avoids. A person hiding in this region therefore seems invisible to external onlookers. However, no light can enter the cloaked region, and consequently the person hiding therein cannot be able to see outside. This is quite inconvenient for practical use. A practical camouflage device must have unidirectional transparency such that a person inside cannot be seen from the outside but can see the outside.
Akin to metamaterials that can be used to manipulate wave phenomena such as radar, sound and light, metamaterials are also able to control environmental sounds and structural vibrations, which have similar waveforms.
Finely shaped sound fields are used in medical imaging and therapy as well as in a wide range of consumer products such as audio spotlights and ultrasonic haptics.
Materials with a negative modulus or negative density can trap sounds or vibrations within the structure through local resonances so that they cannot transfer through it; they can also slow down the sound meaning that incoming sound waves can be transformed into any required sound field.
Acoustic metamaterials could be used in many applications. Large versions could be used to direct or focus sound to a particular location and form an audio hotspot. Much smaller versions could be used to focus high intensity ultrasound to destroy tumors deep within the body. Here, a metamaterial layer could be tailor-made to fit the body of a patient and tuned to focus the ultrasound waves where they are needed most.
Researchers also have developed a metamaterial that can transport sound in unusually robust ways along its edges and localize it at its corners (Nature Materials, "Observation of higher-order topological acoustic states protected by generalized chiral symmetry"). This unique property may improve technologies that use sound waves, such as sonars and ultrasound devices, making them more resistant to defects.
By Michael Berger Michael Berger – Michael is author of two books by the Royal Society of Chemistry: Nano-Society: Pushing the Boundaries of Technology and Nanotechnology: The Future is Tiny. Copyright © Nanowerk