Achromatic Metalens Operates Over a Continuous Visible Bandwidth

The development of a flat lens that works within a continual bandwidth of colors that are close to that of an LED could lead to novel applications in imaging, spectroscopy and sensing.

The development is based on the 2016 discovery of a planar metalens that can focus light with high efficiency within the visible spectrum, by using an ultrathin array of nanopillars to bend and focus light as it passes through. However, this lens can only focus one color at a time.

A scanning electron microscope image shows a side view of the metalens, with nanopillars optimized to focus colors without chromatic dispersion. Scale bar: 200 nm. Courtesy of the Capasso Lab/Havard SEAS).

Researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) used dispersion engineering to correct for chromatic dispersion, a phenomenon where different wavelengths of light are focused at different distances from the lens. Titanium dioxide nanopillars, tiled on a dielectric spacer layer above a metallic mirror, were optimized for shape, width, distance and height.

In experiments, the achromatic metalens (AML) demonstrated the ability to operate over a continuous bandwidth in the visible, working in reflection mode with a focal length independent of wavelength from λ = 490 to 550 nm.

The AML was designed with reverse chromatic dispersion, so the focal length increases as the wavelength increases, in contrast to conventional diffractive lenses.

SEAS researchers have developed the first flat lens that works within a continual bandwidth of colors, from blue to green. This bandwidth, close to that of an LED, paves the way for new applications in imaging, spectroscopy and sensing. Courtesy of Vyshakh Sanjeev/ Harvard SEAS.

“Traditional lenses for microscopes and cameras — including those in cellphones and laptops — require multiple curved lenses to correct chromatic aberrations, which adds weight, thickness and complexity,” said professor Federico Capasso. “Our new breakthrough flat metalens has built-in chromatic aberrations corrections so that a single lens is required.”

Correcting for chromatic dispersion would be a design requirement for any optical system that works with light of different colors.

“By harnessing chromatic aspects, we can have even more control over the light," said researcher Reza Khorasaninejad. “Here, we demonstrate achromatic flat lenses and also invent a new type of flat lens with reverse chromatic dispersion. We showed that one can break away from the constraints of conventional optics, offering new opportunities only bound by the designer’s imagination.”

The researchers believe that the ability to engineer the chromatic dispersion of metalenses could enable a wide variety of applications not previously possible, such as imaging under LED illumination, fluorescence and photoluminescence spectroscopy.

“This method for dispersion engineering can be used to design various ultrathin components with a desired performance,” said researcher Zhujun Shi. “This platform is based on single-step lithography and is compatible with high-throughput-manufacturing technique such as nano-imprinting.”

The research was published in Nano Letters (doi: 10.1021/acs.nanolett.6b05137).