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The fascinating world of metamaterials

The fascinating world of metamaterials

  • 12th Aug 2022
  • The Hindu Business Line

From invisibility cloaks to noise eaters, the world of magic promises to come alive with just the right 'engineering' of metamaterials

People turning invisible by cloaking themselves in a magic blanket has been the stuff of movies for long - from Indian classics like Maya Bazaar to global bestsellers like Harry Potter. But science is making such magic cloaks a reality.

The principle is simple enough, says Prof Prabhu Rajagopal of the department of mechanical engineering, IIT-Madras. You see me because light from the overhead bulb hits me, gets reflected and reaches your eye. But imagine if I were wearing a cloak that does not allow light to reflect off it and, instead, keeps the light swirling round and round within it until the light becomes heat and dies down - it never reaches your eye. I will be, literally, hidden in plain sight.

Likewise, imagine a device perched on a table in the middle of, say, a conference hall. It absorbs all the sound waves. No discussion is possible because all sounds are captured and absorbed by the device - only silence reigns in the room. Imagine a submarine fitted with such a device. It will never be detected!

Such engineered materials are called 'metamaterials', which derive their properties not from what they are made of but their structure.

Metamaterials were theoretically predicted in the late 1960s by a Soviet scientist named Victor Vaselago, who imagined materials of 'negative refractive index' - light passing through them will bend backwards. Decades later, in the 1990s, British scientist Sir John Pendry of Imperial College, London, demonstrated a cloak that could make objects invisible, first under microwaves and then, more recently, under visible light, though only if the object was static. Since then, scientists have been working on metamaterials in labs all over the world. There are many such materials in existence today, though mainly in labs; while the technology is not mature yet for commercial applications, it is getting there fast.

The engineering trick
Dr Rajagopal, who has done extensive work on ultrasonic metamaterials, believes it is just a matter of 2-3 years before metamaterials become industry-ready.

The website notes that metamaterials are functional materials that are made with conventional material such as metals and plastics - they are engineered in a manner that imparts them with special properties. The properties do not come from the base material (metals or plastics), but from the special engineered structure. As such, they are "complex structures patterned in ways that perform a special function, such as blocking light or a specific colour of light or invisibly heating a window in a car," the website says. The engineering of the material manipulates light, heat or electromagnetic ways in unusual ways.

The precise shape, geometry, size, orientation and arrangement of micro or nanoscaled features impart special electromagnetic or acoustic (ultrasonic) properties to these materials. Scientists give many examples. For instance, a surface coated with engineered gold nanoparticles would appear blue or red under light, rather than yellow, as you would expect. Note that the composition of gold has not changed. Similarly, scientists speak of the possibility of super-lenses that enable you to see objects as small as 200 nanometres (a nanometre is a billionth of a metre).

Here is an example of how a metamaterial is 'engineered'. Among several metamaterials sitting on Rajagopal's table is a junked 3D-printed part of a catalytic converter. It is a ceramic cylinder, about 8 inches long, whose inside contains about a hundred tiny tubes that are less than a millimetre in diameter. Sound from one end travels to the other through one of the tiny tubes, gets reflected and travels back down the tube only to meet an incoming wave. The two waves, being of the same frequency, merge to become a larger wave, due to 'constructive interference'. This happens repeatedly and the waves keep growing in amplitude.

Imagine using this principle in an ultrasound instrument. Ultrasound imaging uses sound waves of very short wavelengths and correspondingly high frequencies of 20 kilohertz and above. The waves are bounced off a target (imaged) object and the reflected waves tell us more about the object. With metamaterials, it is possible to image very tiny objects - such as a hairline crack in a pipeline. Just as a lens shows an object bigger than it is, this metalens, when used in ultrasound imaging, 'magnifies' the target object.

Applications of the future
Rajagopal showed Quantum a metalens, a device that was nothing more than a bunch of straws used to sip coconut water. "We have always focused on technologies that are scalable and translatable to the field; we are studying concepts that are cost-effective and also easily fabricated," Rajagopal said.

It is generally accepted in acoustic science that you cannot image an object that is smaller than half the wavelength of the sound wave. However, Rajagopal's metalens can image objects as small as one-thirty-sixth of the ultrasound wave - the finest resolution reported anywhere in the world.

Rajagopal's lab has produced an assortment of metamaterials. Hyperlens is one of these, where ultrasound waves enter channels that progressively get broader. Again, this special structure helps in securing a better picture of an imaged object. Rajagopal has also come up with a seismic metamaterial trick to protect buildings or sensitive equipment from earthquakes - a designer metal rod embedded in an autoclaved brick. This can absorb seismic waves. Yet another of Rajagopal's metamaterial inventions is a device that can focus sound waves on to a point, say, a tiny object. Just as it happens with a lens under light, these sound waves converge on the point, get converted to heat, destroying the object. Such devices can be used to kill cancer cells in a tumour.

Thus, from 'holey' materials that can bend waves around a target, to ridges or baffles on a rod that can act as a mechanical filter to damp out or enhance target frequencies of sound, to combinations of materials that could yield extraordinary transmission and focusing of waves in a target media, metamaterials offer "thrilling possibilities" for applications. Rajagopal envisions the use of metamaterials in energy harvesting and quantum computing, for which he was awarded the Government of India's prestigious Swarnajayanti Fellowship in 2020.

"A completely new type of sensors and devices for applications such as sensing and computing is emerging from phonon-photon coupling, mediated by metamaterials," he says.