Since ancient times, people have experimented with lighting, cherishing shiny metals like gold and cutting gemstones to brighten their sparkles. Today we are far more advanced in how we work with this ubiquitous energy.
Starting with 19 th-century experimentation, we began to explore controlling how light is working with matter.
Combining multiple materials in complex structures let us use light in new ways. We crafted lenses and mirrors to attain telescopes to peer out into the universe, and microscopes to explore the world of the small.
Today this work continues, on a much more detailed level. My own research into what are called metamaterials explores how we can construct materials in ways that do amazing and previously impossible things.
We can build metamaterials to react in particular ways to certain frequencies of illuminate. For example, we can create a smart filter for infrared cameras that allows the user to easily determine if the white powder in an envelope is baking soda or anthrax, determine if a skin melanoma is benign or malignant and find the sewer pipe in your cellar without transgressing through the cement. These are just a few applications for one device; metamaterials in general are far more powerful.
Working With Light
What scientists call light is not just what we can see, but all electromagnetic radiation from low-frequency radio waves to high-frequency X-rays.
Normally, light moves through training materials at a slower velocity. For example, visible light travels through glass about 33 percent slower than it does through air. A materials fundamental resistance to the transmission of illuminate at a specific frequency is called its indicator of refraction. While this number changes with the illuminates frequency, it starts at 1 the index of refraction for a vacuum and goes up. The higher the index, the slower the light moves, and the more its track bendings. This can be seen when looking at a straw in a beaker of water( see below) and is the basis of how we attain lenses for eyeglasses, telescopes and other optics.
Scientists have long wondered if they could make a material with a negative indicator of refraction at any devoted frequency. That would mean, for example, that light would bend in the opposite direction when entering the material allowing for new types of lenses to be made. Nothing in nature fits into this category. The properties of such a material were it to exist were predicted by Victor Veselago in 1967.
These odd materials have properties that look very strange compared with our everyday experiences. In the picture below, we ensure two cups of water, each with a straw in it. The scene on the left is what happens ordinarily the section of the straw in the water seems unplugged from the part of the straw that is in the air. The image is displaced because air and water refract light differently.
The image on the right indicates what the straw would look like if the fluid were training materials with a negative index of refraction. Since the light bends in the opposite direction, the image is reversed, generating the observed illusion.
At left: normal refraction. At right: with simulated negative refraction. Water glass with straw( normal) from shutterstock.com
While Veselago could imagine these materials in the late 1960 s, he could not conceive of a way to generate them. It took an additional 30 years before John Pendry published newspapers in 1996, 1998 and 1999 describing how to make a composite man-made material, which he called a metamaterial.
An early metamaterial utilizing repeating elements of copper split-rings and copper wires. D. R. Smith et al ., Left-handed Metamaterials, in Photonic Crystals and Light Localization, ed. C. M. Soukoulis( Kluwer, Netherlands, 2000 ).~ ATAGEND, CC BY-ND
Making Metamaterials
This work was followed up experimentally by David R. Smiths group in 2000, which created a metamaterial utilizing copper split-rings on circuit board and lengths of copper wires as recurring elements. The image below proves one such example being developed by his group. The size and shape of the split-rings and copper posts determines what frequency of light the metamaterial is tuned to. The combining of these components interacts with the incident sun, creating a region with an fully engineered effective indicator of refraction.
At present, we are only able to construct metamaterials that manage interactions with very specific parts of the electromagnetic spectrum.
The electromagnetic spectrum, indicating all types of illumination, including the narrow band of visible light. Philip Ronan, CC BY-SA
Smiths group ran initially in the microwave section of the spectrum, because working with larger wavelengths makes metamaterial building easier, as multiple copies of the split-rings and pins must fit into the space of one wavelength of the sun. As researchers work with shorter wavelengths, metamaterial components required to much smaller, which is more challenging to build.
Since the first experimentations, multiple research groups have stimulated metamaterials that work in the infrared; some are skirting the fringe of the visible portion of the spectrum. For these short wavelengths, circuit board, copper wires and pins are far too large. Instead the structures have to use micro- and nano-fabrication techniques similar to what is used to attain computer chips.
Creating Invisibility
Soon after the first metamaterials were fabricated, researchers began engineering applications for which they would be useful. One application that got a lot of press was the creation of an invisibility cape.
Normally if a microwave radar were aimed at an object, some of the radiation would assimilate and some would reflect off. Sensors can detect those disorders and rebuild what the object must have looked like. If an object is surrounded by the metamaterial shawl, then the radar signal bends around the object, neither being absorbed nor reflected as if the object were never there.
By creating a metamaterial layer on the surface area of an object, you can change what happens to the illuminate that hits the object. Why is this important? When you look at a still pond of water, it is not surprising to see your reflection. When you point a flashlight at a pond at night, some of that light beam ricochets off onto the trees beyond.
Now imagine you could coat the surface of that pond with a metamaterial that worked for all the visible spectrum. That would remove all reflection you wouldnt see your own reflection , nor any light ricochetting into the woods.
This type of control is very useful for determining specifically what type of light can enter or exit a material or a device. For instance, solar cells could be coated with metamaterials that would admit only specific( e.g ., visible) frequencies of lighting for conversion to electricity, and would reflect all other sunlight to another device that collects the remaining energy as heat.
The Future Of Wave Engineering
Engineers are now generating metamaterials with what is called a dynamic response, meaning its properties vary depending on how much energy is passing through it, or what light is aimed at it. For example, a dynamic metamaterial filter might let passage of light only in the very near infrared, until electricity is applied, at which point it lets through merely mid-infrared light. This ability to tune the responsiveness of metamaterials has great potential for future applications, including uses we cant yet imagine.
The amazing thing about all of the wondrous possibilities of metamaterials’ interaction with illuminate is that the principle works much more broadly. The same maths that predict the structure required to produce these effects for light can be applied to the interaction of materials with any type of waves.
A group in Germany has successfully generated a thermal shawl, preventing an area from heating by bending the heat flowing around it just as an invisibility cape bends light. The principle has also been used for sound waves and has even been discussed for seismic vibrations. That opens the potential for making a building invisible to earthquakes! We are only beginning to discover how else we might use metamaterials and their underlying principles.
Thomas Vandervelde, Associate Professor of Electrical and Computer Engineering, Tufts University
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