ADVANCES Harvard's nanoscale cure for chromatic aberration

01 December 2018

SIG: Imaging Science

Scanning electron micrograph (SEM) of nanopillars in a metacorrector layer. The image spans 2.3 microns (0.0023 mm) and is magnified about 132,000 times. Image courtesy of the Capasso Lab/Harvard SEAS.

For centuries, one of the biggest problems in lens design has been chromatic aberration, the way a lens brings different colours of light to focus in different places.

A simple convex lens brings different colours of light to focus at different points, blue closest and red farthest. Image by Gary Evans ASIS FRPS.

The traditional cure has been using multiple lenses of different types of glass, the aberrations in one lens slightly counteracting the aberration from another. This has led to even simple lenses being made of numerous elements, needing very precise polishing and alignment and resulting in lenses that are bulky, heavy and expensive.

But now a team led by Professor Federico Capasso at Harvard University's School of Engineering and Applied Sciences (SEAS) is changing all that. They have developed a 'metacorrector', a single-layer coating of microscopic columns, or nanopillars, that are smaller than the wavelength of visible light. Each nanopillar is 'tuned' to control its effective refractive index, enabling the metacorrector suface to focus all wavelengths of visible light to the same point. The nanopillars, seen in the SEM image above, are each typically 70 to 140 nanometres wide, arranged in rows about 350 nanometres across. For comparison, a typical human hair is about 17,000 nanometres wide.

"You can imagine light as different packets being delivered at different speeds as it propogates in the nanopillars," explains Wei Ting Chen, one of the authors of the paper describing the breakthrough. "We have designed the nanopillars so that these packets arrive at the focal spot at the same time and with the same temporal width."

Microscope test target imaged with an uncorrected lens (left) and a metacorrected lens (right). The scale bar is 25 micrometres. By courtesy of the Capasso Lab/Harvard SEAS.

Tests have been performed on optics used in high-resolution optical microscope objectives. The results are astonishing. But this isn't the end of the road for applications of this new technology. "Our metacorrector technology can work in tandem with traditional refractive optical components to improve performance while significantly reducing the complexity and footprint of the system, for a whole range of high-volume applications," says Professor Capasso.

With a patent application filed for this new technology, we wonder how long it will be before manufacturers of smartphone cameras and other high-tech optics are beating a path to the doors of Harvard University.

The original press release can be seen here.

"Broadband Achromatic Metasurface-Refractive Optics" by Wei Ting Chen, Alexander Y. Zhu, Jared Sisler, Yao-Wei Huang, Kerolos M. A. Yousef, Eric Lee, Cheng-Wei Qiu and Federico Capasso was published in Nano Letters, 13 November 2018.