Ordinary refractive lenses (left) 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 2 wavelengths, an apochromat for 3 and finally a so called super-achromat for four wavelengths. The Harvard team’s new metasurface lenses (right) are designed to focus light in the same spot for 3 different wavelengths with no need to increase the lens thickness and footprint. (Image courtesy of Patrice Genevet, Federico Capasso, and Francesco Aieta, Harvard SEAS.)
NO NEED FOR COLOR CORRECTION—HARVARD PHYSICISTS’ FLAT OPTICS, USING NANOTECHNOLOGY, GETS IT RIGHT THE FIRST TIME
Most lenses are, by definition, curved. After all, they are named for their resemblance to lentils, and a glass lens made flat is just a window with no special powers.
But a new type of lens created at the Harvard School of Engineering and Applied Sciences(SEAS) turns conventional optics on its head.
A major leap forward from a prototype device demonstrated in 2012, it is an ultra-thin, completely flat optical component made of a glass substrate and tiny, light-concentrating silicon antennas. Light shining on it bends instantaneously, rather than gradually, while passing through. The bending effects can be designed in advance, by an algorithm, and fine-tuned to fit almost any purpose.
With this new invention described today in Science, the Harvard research team has overcome an inherent drawback of a wafer-thin lens: light at different wavelengths (i.e., colors) responds to the surface very differently. Until now, this phenomenon has prevented planar optics from being used with broadband light. Now, instead of treating all wavelengths equally, the researchers have devised a flat lens with antennas that compensate for the wavelength differences and produce a consistent effect—for example, deflecting three beams of different colors by the same angle, or focusing those colors on a single spot.
“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 principal investigator Federico Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at Harvard SEAS.
Bernard Kress, Principal Optical Architect at Google [X], who was not involved in the research, hailed the advance:
“Google [X], and especially the Google Glass group, is relying heavily on state-of-the-art optical technologies to develop products that have higher functionalities, are easier to mass produce, have a smaller footprint, and are lighter, without compromising efficiency,” he said. “Last year, we challenged Professor Capasso’s group to work towards a goal which was until now unreachable by flat optics. 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.”
Read more: Perfect colors, captured with one ultra-thin lens
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