Flat lens opens a broad world of colour


Rebecca Pool

Thursday, February 9, 2017 - 15:00
SEM image of metalens with nanopillars optimized to focus colours without chromatic dispersion. [Capasso Lab/Harvard SEAS]
US-based researchers have unveiled the world's first flat lens that works with a continuous bandwidth of colours across the visible light spectrum.
This bandwidth, close to that of an LED, opens the door to new applications in fluorescence microscopy as well as fluorescence and photoluminescence spectroscopy.
The breakthrough from Professor Federico Capasso, Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS), and colleagues is based on a previously-developed metalens that comprises an ultrathin array of nanopillars to bend and focus incoming light. 
But while this optics breakthrough focuses light with high efficiency within the visible spectrum, it can only focus one wavelength at a time.
Schematic of the achromatic lens with optimised titanium dioxide nanopillars [Vyshakh Sanjeev/ Harvard SEAS] 
To develop a flat metalens that operates over a continuous bandwidth, Capasso and colleagues needed to create a lens design that could correct chromatic abberations; an achromatic lens.
“Traditional lenses for microscopes and cameras - including those in cell phones and laptops - require multiple curved lenses to correct chromatic aberrations, which adds weight, thickness and complexity,” says Capasso. “But our new breakthrough flat metalens has built-in chromatic aberrations corrections so that only a single lens is required.”
To design the achromatic lens, the researchers optimised the shape, width, distance, and height of the titanium dioxide nanopillars - the dielectric phase shifters - that make up the heart of the metalens.
This structure allows the metalens to focus wavelengths from 490 nm to 550 nm, from blue to green, without any chromatic dispersion.
SEM image reveals a side-view of the metalens; nanopillars are optimised to focus colours without chromatic dispersion; scale bar: 200 nm. [Capasso Lab/Harvard SEAS]
The researchers also went on to design a further flat lens with reverse chromatic dispersion.
Here the focal length of the lens increases as light wavelength increases, contrary to conventional diffractive lenses.
As Capassos's PhD student, Zhujun Shi, highlights: “This method for dispersion engineering can be used to design various ultrathin components with a desired performance. This platform is based on single step lithography and is compatible with high throughput manufacturing techniques such as nano-imprinting.”
"The ability to engineer the chromatic dispersion of metalenses at will enables a wide variety of applications that were not previously possible," concludes Capasso. "In particular, for the achromatic metalens design, we envision applications such as imaging under LED illumination, fluorescence, and photoluminescence spectroscopy."
Harvard’s Office of Technology Development has filed patent applications on a portfolio of flat lens technologies and is working closely with Capasso and members of his research group to catalyse commercialisation of this technology through a startup company.
Research is published in Nano Letters.
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