'First flat lens opens a broad world of colour'

Harvard researchers have developed the first flat lens that works within a continual bandwidth of colours - from blue to green - close to that of an LED, paving the way for new applications in imaging, spectroscopy and sensing.
One of the major challenges in developing a flat, broadband lens has been correcting for chromatic dispersion, the phenomenon where different wavelengths of light are focused at different distances from the 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," said Federico Capasso from Harvard University in the US.

"Our new breakthrough flat metalens has built-in chromatic aberrations corrections so that a single lens is required," said Capasso.
Correcting for chromatic dispersion - known as dispersion engineering - is a crucial topic in optics and an important design requirement in any optical systems that deals with light of different colours.
The ability to control the chromatic dispersion of flat lenses broadens their applications and introduces new applications that have not yet been possible.

"By harnessing chromatic aspects, we can have even more control over the light," said Reza Khorasaninejad from Harvard University.
"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," said Khorasaninejad.

To design an achromatic lens - a lens without chromatic dispersion - the team optimised the shape, width, distance, and height of the nanopillars that make up the heart of the metalens. As in previous research, the researchers used abundant titanium dioxide to create the nanoscale array.
This structure allows the metalens to focus wavelengths from 490 nanometre (nm) to 550 nm, basically from blue to green, without any chromatic dispersion.
"This method for dispersion engineering can be used to design various ultrathin components with a desired performance," said Zhujun Shi from Harvard University.
"This platform is based on single step lithography and is compatible with high throughput manufacturing technique such as nano-imprinting," said Shi.

The study was published in the journal Nano Letters.