Tiny new lenses, smaller than a hair, could transform phone and drone cameras

The design uses layers of metamaterials to simultaneously focus a range of wavelengths from an unpolarized source and over a large diameter, overcoming a major limitation of metalenses, said the first author of the paper reporting the design, Mr Joshua Jordaan, from the Research School of Physics at the Australian National University and the ARC Centre of Excellence for Transformative Meta-Optical Systems (TMOS).

"Our design has a lot of nice features that make it applicable to practical devices."

"It's easy to manufacture because it has a low aspect ratio and each layer can be fabricated individually and then packaged together, it's also polarisation insensitive, and is potentially scalable through mature semiconductor nanofabrication platforms," Mr Jordaan said.

The project was led by researchers from the Friedrich Schiller University Jena in Germany as part of the International Research Training Group Meta-ACTIVE. The paper reporting their design is published in Optics Express.

Metalenses have thickness mere fractions of the width of a hair, which is orders of magnitude thinner than conventional lenses. They can be designed to have properties such as focal lengths that would be impossibly short for conventional optics.

Initially the team attempted to focus multiple wavelengths with a single layer, but they hit up against some fundamental constraints, Mr Jordaan said.

"It turns out the maximum group-delay attainable in a single-layer metasurface has physical limitations, and these in turn set upper bounds on the product of the numerical aperture, physical diameter and operating bandwidth."

"To work at the wavelength range we needed, a single layer would either have to have a very small diameter, which would defeat the purpose of the design, or basically have such a low numerical aperture that it's hardly focusing the light at all," he said.

"We realized we needed a more complex structure, which then led to a multi-layer approach."

With the design shifted to incorporating several metalens layers, the team approached the problem with an inverse design algorithm based on shape optimization, with parameterization that meant a lot of degrees of freedom.

They guided the software to search for metasurface shapes that, for a single wavelength, created simple resonances in both the electric and magnetic dipole, known as Huygens resonances. By employing resonances, the team were able to improve on previous designs by other groups, and develop metalens designs that were polarization independent, and had greater tolerances in manufacturing specifications - crucial in the quest to scale fabrication to industrial quantities.

The optimization routine came up with a library of metamaterial elements in a surprising range of shapes, such as rounded squares, four-leaf clovers and propellers.

These tiny shapes, around 300 nm tall and 1000 nm wide, spanned the full range of phase shifts, from zero to two pi, enabling the team to create a phase gradient map to achieve any arbitrary focusing pattern - although they were initially just aiming for a simple ring structure of a conventional lens.

"We could, for example, focus different wavelengths into different locations to create a colour router," Mr Jordaan said.

However, the multilayer approach is limited to a maximum of around five different wavelengths, Mr Jordaan said.

"The problem is you need structures large enough to be resonant at the longest wavelength, without getting diffraction from the shorter wavelengths," he said.

Within these constraints, Mr Jordaan said the ability to make metalenses to collect a lot of light will be a boon for future portable imaging systems.

"The metalenses we have designed would be ideal for drones or earth-observation satellites, as we've tried to make them as small and light as possible," he said.