Using visiblelight magnified through a compound series of lenses to image small objects,standard optical microscopes have been with us for many centuries. Whilst continually being improved, the result of these many advances ofoptics and image-capturing techniques means that many high-end optical microscopes havenow reached the limit of magnification possible as they push the resolutionproperties of light itself. In an attempt to resolve this issue, scientists atthe University of Buffalo (UB) have created a prototype visible light "hyperlens" that may help image objects once only clearly viewable throughelectron microscopes.
The resolution limit for images captured by an optical microscope system is due to the diffractionof light from a viewed object. Put simply, as light passesthrough the circular aperture of a microscope lens, the light waves from very smallpoints of light interfere with each other on the way through, causing the imageto blur.
Thediffraction problem is due to a phenomenon known as the "Rayleigh criterion", which specifies the minimum separation distance between two observed objects that canbe resolved into distinct objects. As the size of the aperture used in relationto the wavelength of light is inherent in the criterion’s formula, then thesmaller the aperture and the closer in size an object is to the wavelength oflight itself, the greater the diffraction and the more the image is blurred.
UB researchers working on metamaterials– that is, artificial materials engineered with properties not yet found in nature – claim to have overcome this diffraction limitproblem by creating a photonic hyperlens that they say changes evanescent waves of lightinto propagating waves. In other words, they use these lenses to alter theproperties of light from that which loses intensity rapidly (evanescent waves)to those that are increased in intensity (propagating waves).
The metamaterialhyperlenses first developed were made of silver and a dielectric insulatingmaterial arranged in concentric rings. Whilst this type of hyperlens worked verywell at specific wavelengths of light, it suffered from large losses atresonant frequencies.
To help improve on this, UBresearchers arranged minute slices of gold and PMMA (a clear thermoplastic)into a radiating semi-circular shape that the researchers point out looks likea very tiny Slinky suspended in its movement. This new shape turned out to be a much improved one, as it effectively ameliorates the diffraction limit on objects viewed in thevisible light range.
An immediate use for such adevice, the team believes, is that it could be combined with an opticalwaveguide to produce a hyperlens-based medical endoscope. As evenhigh-resolution endoscopes can only resolve images of objects around 10,000nanometers in size, a hyperlens-equipped endoscope could potentially increasethat resolution to at least 250 nanometers or more, and may provide medicalpractitioners with that ability to locate tiny, hard-to-find cancers that couldhelp catch the disease before it has time to spread.
"Thereis a great need in healthcare, nanotechnology and other areas to improve ourability to see tiny objects that elude even the most powerful opticalsystems," said Natalia Litchinitser, PhD, professor of electricalengineering at UB. "The hyperlens we are developing is, potentially, agiant step toward solving this problem."
The researchers also believethat the hyperlens may even eventually be capable of imaging single moleculesin visible light, which has enormous implications for research in many fields, particularly chemistry and biology. In the field of physics, such a lens may also helpsuch things as optical nanolithography, where light is shone through a mask to createa pattern on polymer or graphene films for integrated circuits, along withdevelopments in the next generation of optoelectronic electronics, includingsensors and data storage drives.
Details of this research were recently published in the journal Nature Communications.
Source: University ofBuffalo