Scientists bring metamaterial based 'perfect' lenses a step closer
19 July 2015
'Perfect' lenses* based on metamaterials may be some way off, but US scientists say they have overcome one obstacle to creating them: light absorption.
Researchers at Michigan Technological University believe they have found a way to solve one of the biggest challenges they face when trying to create a perfect lens from metamaterials - ensuring that light passes though it and is not absorbed.
The work, undertaken by Michigan Tech's Professor Durdu Güney and his team, involves the use of silver films that are 'tweaked' to produce a metamaterial that allows light to pass through, rather than being reflected, as would be expected from a metal surface.
“Aluminium and silver are the best choices so far in the visible light spectrum, not just for a perfect lens but all metamaterials,” Güney says. But they still tend to absorb light. “Loss — or the undesired absorption of light — is good in solar cells, but bad in a lens, because it deteriorates the waves,” Güney adds.
According to Güney, the solution to absorption is all in the light waves themselves, which behave strangely in metamaterials. A 'perfect' lens relies on negative index metamaterials. Most materials — positive index materials — allow only propagating light waves to pass through. Negative index metamaterials, on the other hand, don’t just allow propagating light waves to pass but also amplify decaying light waves.
“In order for the perfect lens to work, you have to satisfy a lot of electromagnetic constraints,” Güney says. “We don’t know exactly how the required optical modes need to be excited and protected in the lens for the perfect construction of an image.”
This difficulty has led researchers to try numerous modifications of the metamaterial make-up, adding bulk, mode-by-mode nit-picking and increasingly complex models. But Güney and his team propose moving away from the complications and going back to the light itself.
In their plasmon-injection scheme, the researchers take advantage of knowing which light wave crumbles as it passes through the negative index lens. They use this wave, that is destined to fail in the lens, to shield the desired light wave, allowing it to pass through 'unscathed'.
“With this approach, you can engineer this sacrificial wave,” Güney says. “It is difficult to construct this wave in other approaches."
Güney believes imaging is one of the key technologies for this work. Moving the technology forward could mean more accessible medical technology and lightweight field equipment.
*Scientists believe perfect lenses could enable nanometre-sized particles, such as viruses, to be viewed by the naked eye, rather than having to rely on more indirect technologies, such as scanning electron microscopes.