Phase-Shifted Flat Lens Creates Perfect Image

By moving the phase shifting of light to the surface of a lens instead of creating phase delays as it propagates through a material, Harvard engineers have created an ultrathin, flat lens that could potentially replace bulk components in optical systems.

Conventional lenses impart distortions and optical aberrations, including the “fish eye” effect, astigmatism and coma aberrations.

An artist’s rendition of a new ultrathin, flat lens focuses light without imparting the optical distortions of conventional lenses. (Image: Francesco Aieta)

The new 60-nm-thick flat lens, developed by physicists at the Harvard School of Engineering and Applied Sciences (SEAS), is completely accurate and does not require complex corrective techniques. It operates at telecom wavelengths and is scalable from the near-infrared to the terahertz spectrum bands.

The device is made of an ultrathin silicon wafer plated with a nanometer-thin layer of gold. Parts of the gold layer are stripped away, leaving behind an array of V-shaped structures, which are evenly spaced across the surface. When a laser is shone on the flat lens, the V-shaped structures act as nanoantennas that capture the incoming light and hold onto it for a short time before releasing it.

From left, Francesco Aieta, Federico Capasso and Patrice Genevet. (Image: Eliza Grinnell, SEAS Communications)

“Our flat lens opens up a new type of technology,” said principal investigator Federico Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS. “We’re presenting a new way of making lenses. 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.”

(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. (Image: Francesco Aieta)

Modifying the size, spacing and angle of the array of nanoantennas, dubbed a “metasurface,” enables the array to be tuned for specific wavelengths of light.

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

The study appeared in Nano Letters.

For more information, visit: www.seas.harvard.edu