Flat lens offers a perfect image for telecom systems

Ultrathin wafer of silicon and gold focuses telecom wavelengths without distortion
August 27, 2012

Left: A micrograph of the flat lens (diameter approximately 1 mm) made of silicon. The surface is coated with concentric rings of gold optical nanoantennas (inset) which impart different delays to the light traversing the lens. Right:The colored rings show the magnitude of the phase delay corresponding to each ring. (Credit: Francesco Aieta)

Applied physicists at the Harvard School of Engineering and Applied Sciences (SEAS) have created an ultrathin, flat lens that focuses light without imparting the distortions of conventional lenses.

At a mere 60 nanometers thick, the flat lens is essentially two-dimensional, yet its focusing power approaches the ultimate physical limit set by the laws of diffraction.

Operating at telecom wavelengths (from near-infrared to up to terahertz wavelengths, the range commonly used in fiber-optic communications — but not visible light), the new device is completely scalable and simple to manufacture.

“We’re presenting a new way of making lenses,” says principal investigator Federico Capasso, Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS.

“Instead of creating phase delays as light propagates through the thickness of the material, you can create an instantaneous phase shift right at the surface of the lens.”

Capasso and his collaborators at SEAS create the flat lens by plating a very thin wafer of silicon with an nanometer-thin layer of gold. Next, they strip away parts of the gold layer to leave behind an array of V-shaped structures, evenly spaced in rows across the surface. When Capasso’s group shines a laser onto the flat lens, these structures act as nanoantennas that capture the incoming light and hold onto it briefly before releasing it again. Those delays, which are precisely tuned across the surface of the lens, change the direction of the light in the same way that a thick glass lens would, with an important distinction.

The flat lens eliminates optical aberrations such as the “fish-eye” effect that results from conventional wide-angle lenses. Astigmatism and coma aberrations also do not occur with the flat lens, so the resulting image or signal is completely accurate and does not require any complex corrective techniques.

The array of nanoantennas, dubbed a “metasurface,” can be tuned for specific wavelengths of light by simply changing the size, angle, and spacing of the antennas. “They may become particularly interesting in the mid-infrared, the terahertz, and those ranges of frequencies where transparent refractive materials are harder to find, compared to the near-infrared and the visible,” according to the researchers.

“In the future we can potentially replace all the bulk components in the majority of optical systems with just flat surfaces,” says lead author Francesco Aieta, a visiting graduate student from the Università Politecnica delle Marche in Italy.

UPDATE 8/27/2012 from Federico Capasso: “The lens is scalable to single visible-light wavelengths (colors), but we would use a metal with lower optical losses than gold, such as silver. So far, the lens is not broadband in the sense of negligible chromatic aberrations, but with other designs we have in mind it should also be possible to use it for broadband light (white light). For a pattern with very high spatial frequencies (“super oscillation” or lots of detail in the image) one could go, say, a factor of 2 below the Rayleigh limit (diffraction limit) in spatial resolution at visible wavelengths.

“The lens can also be easily scalable in the far IR (up to 100 to 300 microns). Because there are not too many good refractive materials for ordinary lenses in these wavelength ranges, our approach can also have a major impact there.”