A new multi-layered metalens design has been developed at the Australian National University. This concept could revolutionise portable optics in devices like smartphones, drones, and satellites. This means the tiny 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. These lenses manipulate light using nanostructures arranged on a flat surface. This is in contrast to the curved surfaces required for traditional lenses. This design makes metalenses ideal for developing compact and lightweight optical devices.
Metalenses are ultra-thin, flat lenses that use a patterned layer of sub-wavelength nanostructures called meta-atoms to manipulate light, controlling its phase, amplitude, and polarization to achieve focusing and imaging.
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.
By stacking metamaterial layers instead of relying on a single one, researchers overcame fundamental limits in focusing multiple wavelengths of light. This algorithm-driven approach produced intricate nanostructures shaped like clovers, propellers, and squares, enabling improved performance, scalability, and polarisation independence.
These improvements came about via an inverse design algorithm based on shape optimization, with parameterization that meant a lot of degrees of freedom.
To achieve this, the scientists guided 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 lens is described as easy to manufacture since it has a low aspect ratio, and each layer can be fabricated individually and then packaged together. The lens is also polarisation insensitive, and it is potentially scalable through mature semiconductor nanofabrication platforms.
The optimisation 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 nanometres tall and 1000 nanometres 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.
The researchers anticipated this new approach to manufacture metalenses, in order to collect a lot of light, will be a boon for future portable imaging systems.
The research appears in the journal Optics Express, titled “Design of multilayer Huygens’ metasurfaces for large-area multiwavelength and polarization- insensitive metalenses.”
