Ultrathin lens resolves chirality and color

Imaging with the multispectral chiral lens forms two images of the beetle, Chrysina gloriosa, on the color camera (Source: Capasso Lab / Harvard SEAS)

Imaging with the multispectral chiral lens forms two images of the beetle, Chrysina gloriosa, on the color camera (Source: Capasso Lab / Harvard SEAS)

Many things in the natural world are geome­trically chiral, meaning they cannot be super­imposed onto their mirror image. Even light and certain molecules are chiral, including DNA and amino acids. In chiral light, the direction of oscil­lation of the electro­magnetic wave rotates clockwise or counter­clockwise. Materials made of chiral molecules reflect light diffe­rently depending on the nature of their chirality. The artificial sweetener Aspartame, for example, is chiral. One chira­lity of the molecule is sweet, the other bitter. Drugs can also be chiral. Perhaps the most infamous example of chirality is the Thali­domide molecule. One version of the molecule acts as a sedative to pregnant women, the other caused terrible birth defects in thousands of children. As a result, being able to observe and analyze the chira­lity of an object is very important in many fields.

However, current chiral imaging techniques to resolve polari­zation and spectral infor­mation require multiple cascading components, leading to bulky and expensive pieces of equipment. Now, researchers at the Harvard John A. Paulson School of Engi­neering and Applied Sciences SEAS have developed an ultra-compact, flat lens that can simul­taneously capture both spectral infor­mation and the chirality of an object.

Two arrays of titanium oxide nanofins, scale bar 600 nm (Source: Capasso Lab / Harvard SEAS)

Two arrays of titanium oxide nanofins, scale bar 600 nm (Source: Capasso Lab / Harvard SEAS)

“For the first time, chiroptical properties can be probed across the entire visible spectrum using only a single planar lens and a camera without the addition of other optical compo­nents,” said Federico Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engi­neering. “We have demon­strated the potential of meta­lenses in realizing a compact and multi­functional device with unprecedented chiral imaging capa­bilities.”

The lens consists of two arrays of tita­nium oxide nanofins on one glass substrate, which forms two images of oppo­site chira­lity. To demonstrate the efficacy of the lens the team imaged the chiral exo­skeleton of a beetle. A key feature of the lens is its size, said Wei Ting Chen, a post­doctoral fellow in the Capasso Lab. “The chiral lens is very compact, only 3 mm in diameter and thinner than hair, allowing us to integrate it into portable devices,” said Chen.

“Our work is an unpre­cedented demon­stration of a multi­functional metasurface,” said Reza Khora­saninejad, a postdocteral fellow in the Capasso Lab. “We showed that an ultra thin surface can both form an image and simul­taneously resolve both polari­zation and spectral information of a bio­logical specimen.” (Source: SEAS)

Reference: M. Khorasaninejad et al.: Multispectral Chiral Imaging with a Metalens, Nano Lett., online 07 June 2016; DOI: 10.1021/acs.nanolett.6b01897

Link: Harvard John A. Paulson School of Engineering and Applied Sciences (F. Capasso), Harvard University, Cambridge, USA

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