Ultrathin Metalens Captures Chiral, Spectral Images Simultaneously

An ultracompact planar lens has been engineered to simultaneously form two images of the same object, with opposite helicity and within a single field-of-view. The chiral and spectral properties of the object can be simultaneously probed across the visible spectrum with only the ultrathin lens and a camera. The ultrathin surface of the lens can form an image and at the same time resolve polarization and spectral information of the object, thus integrating the functions of polarization and dispersive optical components.

Imaging with the multispectral chiral lens forms two images of the beetle, Chrysina gloriosa, on the color camera. The left image was formed by focusing left-circularly polarized light reflected from the beetle and the right image was formed from right-circularly polarized light. The left-handed chirality of the beetle's shell can clearly be seen. Courtesy of the Capasso Lab/Harvard SEAS.

To demonstrate the efficacy of the lens, researchers at Harvard University’s John A. Paulson School of Engineering and Applied Sciences (SEAS) imaged the exoskeleton of a chiral beetle, Chrysina gloriosa, which is known to exhibit high reflectivity of polarized light, with high spatial resolution limited by the numerical aperture (NA) of the planar lens. The results demonstrated the potential of a metasurface to serve as a compact and multifunctional device with significant imaging capabilities.

The multispectral chiral lens (MCHL) consists of two arrays of titanium oxide nanofins on one glass substrate, which forms two images of opposite chirality. Functionality is incorporated into an optically thin layer through a single-step lithographic process. The lens can be used for both high-quality imaging and spectroscopy of bulk and microscopic scale specimens where the spatial resolution is limited by the NA of the lens.

Optical fiber (pumped with broadband light) formed by the multispectral chiral lens. Courtesy of the Capasso Lab/Harvard SEAS.

A key feature of the lens is its size, said researcher Wei Ting Chen.

"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.

Current chiral spectroscopy and imaging techniques require cascading multiple optical components. Use of the ultrathin lens for chiral imaging overcomes the limitations of bulky and expensive equipment.

The lens consists of two arrays of titanium oxide nanofins on one glass substrate, as seen below, which forms two images of opposite chirality. Two arrays of titanium oxide nanofins, scale bar 600 nm. Courtesy of the 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 professor Federico Capasso. "We have demonstrated the potential of metalenses in realizing a compact and multifunctional device with unprecedented chiral imaging capabilities."

The research was published in Nano Letters (doi: 10.1021/acs.nanolett.6b01897)