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‘Flat Lens’ Masters Chromatic Aberration

Photonics.com
Feb 2015
CAMBRIDGE, Mass., Feb. 19, 2015 — Overcoming a limitation of earlier flat optics, a new achromatic metasurface is able to bend different wavelengths of light by the same amount.

The ultrathin “flat lens” is composed of a glass substrate and nanoscale silicon optical antennas. These antennas can be designed to manipulate how light passing through the lens is diffracted, potentially allowing the metasurface to generate a perfectly focused image or a twisting vortex beam.

SEM micrographs of the achromatic metasurface
SEM micrographs of the achromatic metasurface. An aperiodic array of dielectric resonators are patterned on a glass substrate. The widths of the subwavelength-spaced resonators range from 100 to 800 nm and their height is 400 nm. Images courtesy of Federico Capasso and Francesco Aieta/Harvard SEAS.


“What this now means is that complicated effects like color correction, which in a conventional optical system would require light to pass through several thick lenses in sequence, can be achieved in one extremely thin, miniaturized device,” said Harvard University professor Dr. Federico Capasso, who led the research.

Related: Dr. Federico Capasso presents webinar on flat optics

Capasso’s research group demonstrated a prototype flat lens in 2012 that corrected for some of the aberrations of conventional lenses, but would only focus light of a single wavelength. The new model uses a dielectric rather than a metal for the nanoantennas, a change which improves its efficiency and, combined with a new design approach, enables operation over a broad range of wavelengths.

“Our designs are based on low-loss dielectric resonators which introduce a dense spectrum of optical modes to enable dispersive phase compensation,” the researchers wrote in Science (doi: 10.1126/science.aaa2494).

Comparison between refractive optics and achromatic metasurfaces
Comparison between refractive optics and achromatic metasurfaces. Chromatic correction with refractive optics requires bulky designs with multiple elements of different material dispersion. The metasurface consisting of subwavelength resonators features achromatic behavior (same θ and f for multiple wavelengths) and by maintaining a flat and small footprint design.


The new approach enables the creation of two kinds of flat optical device.

The first, instead of sending different colors in different directions like a conventional diffraction grating, deflects three wavelengths of light by the same angle.

The second type of flat optical device can focus the three wavelengths to the same point. A flat lens can thus create a color image by red, green and blue, the primary colors used in most digital displays.

The team’s computational simulations also suggest that a similar architecture can be used to create a lens that collimates many different wavelengths, not just three.

Potential applications for achromatic metasurfaces include lightweight collimators for displays and chromatically-corrected imaging systems.

One organization that already has its eye on the Harvard team’s work is Google X, the technology research arm of the search engine giant.

“Last year, we challenged professor Capasso’s group to work towards a goal which was until now unreachable by flat optics,” said Bernard Kress, principal optical architect at Google X. “While there are many ways to design achromatic optics, there was until now no solution to implement a dispersionless flat optical element which at the same time had uniform efficiency and the same diffraction angle for three separate wavelengths.”

“We are very happy that Professor Capasso did accept the challenge, and also were very surprised to learn that his group actually solved that challenge within one year,” Kress said.

Harvard’s Office of Technology Development has filed for a provisional patent on the new optical technology and is actively pursuing commercial opportunities.

Funding for the research came from a Multidisciplinary University Research Initiative (MURI) grant from the U.S. Air Force Office of Scientific Research, Draper Laboratory and the National Science Foundation.

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

The Harvard team's new metasurface lenses are designed to focus light in the same spot for three different wavelengths with no need to increase the lens thickness and footprint

Ordinary refractive lenses suffer from significant chromatic aberrations as different wavelengths are focused in different spots. To compensate for this chromatic dispersion, additional lenses have to be added in an objective to compensate for chromatic aberrations as the number of wavelengths to be corrected increases. An achromatic doublet corrects for two wavelengths, an apochromat for three and a super-achromat for four wavelengths. The Harvard team's new metasurface lenses are designed to focus light in the same spot for three different wavelengths with no need to increase the lens thickness and footprint. Such lenses can be designed with large NA.



GLOSSARY
chromatic aberration
The lens aberration resulting from the normal increase in refractive index of all common materials toward the blue end of the spectrum. The change in image size from one color to another is known as lateral color or chromatic difference of magnification.
diffraction
As a wavefront of light passes by an opaque edge or through an opening, secondary weaker wavefronts are generated, apparently originating at that edge. These secondary wavefronts will interfere with the primary wavefront as well as with each other to form various diffraction patterns.  
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