A fresh approach to creating multicolor lenses could lead to an exciting new generation of compact, affordable, and high-performance optics for portable devices like smartphones and drones.
This innovative design utilizes layers of metamaterials to focus a range of wavelengths from an unpolarized light source across a large diameter. This overcomes a key limitation of current metalenses, according to Mr. Joshua Jordaan, the lead author 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 offers several appealing features that make it suitable for practical applications,” he noted.
“It’s easy to manufacture thanks to its low aspect ratio, allowing individual layers to be created separately and then combined. Moreover, it’s polarization-insensitive and has potential for scalability using established semiconductor nanofabrication techniques,” Mr. Jordaan explained.
The project was spearheaded by researchers at Friedrich Schiller University Jena in Germany, as part of the International Research Training Group Meta-ACTIVE. Their findings were published in Optics Express.
Metalenses can be incredibly thin—thinner than a human hair—while achieving focal lengths that would be impractically short for traditional optics.
Initially, the team explored focusing multiple wavelengths with a single layer but encountered fundamental limitations, according to Mr. Jordaan.
“We discovered that a single-layer metasurface has physical constraints that set limits on the relationship between numerical aperture, physical diameter, and operating bandwidth,” he remarked.
“To achieve the required wavelength range, a single layer would have to be either too small—defeating the purpose of the design—or possess such a low numerical aperture that it barely focuses light,” he added.
“We understood that a more complex structure was necessary, leading us to adopt a multilayer approach.”
With this shift, the team employed an inverse design algorithm based on shape optimization, allowing for a wide range of design possibilities.
They guided the software to search for metasurface shapes that produced specific resonances in both electric and magnetic dipoles, known as Huygens resonances. This approach improved upon previous designs, resulting in polarization-independent metalenses that offered greater manufacturing tolerance—essential for scaling up production.
The optimization process yielded a variety of metamaterial shapes, including rounded squares, four-leaf clovers, and propellers.
These tiny structures, about 300 nm tall and 1000 nm wide, provided a complete range of phase shifts from zero to two pi, enabling the team to create a phase gradient map for arbitrary focusing patterns—initially, they aimed for a simple ring shape like a traditional lens.
“For instance, we could direct different wavelengths to different locations, creating a color router,” Mr. Jordaan explained.
Still, the multilayer design is limited to focusing a maximum of around five different wavelengths, he said.
“The challenge is that structures need to be large enough to resonate at the longest wavelength without causing diffraction at shorter wavelengths,” he clarified.
Despite these constraints, Mr. Jordaan believes that their designed metalenses could significantly enhance future portable imaging systems.
“These metalenses would be perfect for drones or Earth-observation satellites, as we’ve aimed to keep them as small and lightweight as possible,” he concluded.
Summary: Researchers have developed a revolutionary multilayer metalens design that overcomes limitations in traditional optics, making it ideal for compact devices like smartphones and drones. This innovative approach combines resonant shapes in metamaterials, enabling effective focusing of multiple wavelengths.
