Metamaterials are engineered materials or artificially structured composite materials that are made to possess properties absent in conventional materials. The internal microstructure of these materials is what makes these stand apart from naturally occurring materials. The properties of these materials depend on the structure rather than the composition of the material. The structure of the material can be tailored and tuned on a microscale level to suit the required application. The material is arranged in a recurring fashion at scales smaller than the wavelength of the waves used in that particular application. Upon precise engineering, the material can be made to interact with waves in a way not observed in bulk materials. Some of the applications of metamaterials:
Negative refractive index - Refractive index refers to the ability of a material to deflect light. Conventional materials have a positive refractive index. In negative refractive index materials, light bends in a direction opposite to that of the incident beam after entering the medium. The negative value of the refractive index is due to the negative permeability and permittivity of the material and is known as double negative materials (DNG). The structure comprises of periodically arranged split-ring resonators. Unlike regular materials, these materials follow the left-hand rule of electromagnetism. These materials have unusual electromagnetic properties and hence makes a "perfect lens." A metamaterial with a refractive index of -1 can focus light with minimum change in the resolution even if it is the form of a parallel-sided slab.
Imaging below 200nm is not possible using optical methods, but with the help of superlens, which is a metamaterial application, imaging below the diffraction limit, i.e., subdiffraction-limit imaging, is possible. The superlens can be a more cost-effective and high-resolution alternative to electron microscopes.
Figure 1: a) Positive Refractive Index material; b) Negative refractive Index material; c) A parallel-sided slab with a Negative Refractive Index behaves as a perfect lens.
Optical Cloaking - Cloaking refers to the usage of metamaterials as an invisibility cloak. Conventional material scatters light and hence is visible to the human eye. An object can be said to be "invisible" if it does not absorb, scatter, or reflect any electromagnetic radiation, i.e., it does not disturb the existing electromagnetic field. A cloaking device bends radiation around the object, making it undetectable. Metamaterials can act as cloaking devices over certain electromagnetic ranges. But the application in this field is limited as a fully functional cloaking device in the visible light range has not yet been achieved.
Figure 2: Left) An object placed in an electric field disturbs the field lines; Right) A cloaked object does not disturb the field lines making it "invisible."
Seismic metamaterials are earthquake dampeners, which dampen the effect of seismic waves on infrastructure and other objects. They work on a similar principle as the electromagnetic metamaterials, i.e., they make the object invisible to the seismic waves. These materials, when cloaking around an object, deflect the waves away from the object. A densely populated forest is a natural example of a seismic metamaterial.
Figure 3: An illustrated representation of a structure being made "invisible" to seismic waves with metamaterials
As the name implies, acoustic metamaterials can control and manipulate sound waves. Similar to electromagnetic metamaterials, acoustic metamaterials have negative values for bulk modulus and mass density, which does not allow sound wave propagation. This property can be exploited to cancel or concentrate on sound waves. The periodic arrangement of unit cells gives the material new bulk properties, as mentioned above. They can trap and slow down sound waves as used in sound isolation. This method can be widely used for medical purposes. E.g.: - Metamaterials can be utilized to focus high-intensity ultrasound on destroying tumours in inaccessible areas in the body. Acoustic metamaterials are a good substitute for conventional acoustic materials at low-frequency ranges since, at low frequencies, the wavelength is higher than the effective depth of the material, thus reducing the performance of regular materials. Acoustic metamaterials are further being implemented in medical imaging, soundproofing and noise cancellation, sonar dampening, stealth technology, cloaking, and frequency filters.
Figure 4: An illustration of an acoustic metamaterial used to trap and slow down sound waves
Research in metamaterials is an emerging interdisciplinary field and has tremendous potential in various fields in the forthcoming years. From camouflage and stealth in military operations to noise isolation to even something like the invisibility cloak from Harry Potter can be envisioned with the aid of metamaterials. Something which was only witnessed in science fiction movies and novels is closer to reality than we imagine.