Exploring the theoretical limits of metalenses

Ultra-thin flat lenses are currently attracting attention in both industry and academia.

A new paper published in Optica, “Focusing on bandwidth: achromatic metalens limits,” details the finding of some fundamental limits on so-called “metalenses,” ultra-thin flat lenses which have shown great promise in their ability to function as ultracompact optical systems for focusing and imaging. 

Federico Presutti, Ph.D. student in applied engineering and physics, and Francesco Monticone, assistant professor in electrical and computer engineering are the paper’s authors.

“Think of any optical system with lenses for focusing and imaging: a camera, microscope, a lens antenna,” Monticone said. “All these lenses could in principle be replaced by ultra-thin flat structures —metalenses — with huge savings in terms of size, weight and cost of fabrication.”

The paper’s theoretical results will allow research groups working on metasurfaces to assess and compare the performance of different devices, and may offer fundamental insight into how to design better achromatic broadband metalenses for different applications.

“Typically, different colors are focused at different points, leading to what are known as chromatic aberrations,” Monticone explained. “Our results determine the maximum range of colors that can be focused by the metalens at the same focal point.”

A lens with chromatic aberrations does not focus all colors to the same point, leading to noticeable blurring and rainbow effects in images. Super-achromatic lenses could be replaced with thin metalenses, which allow more freedom to directly control how different colors interact with the structure. 

There are, however, limits to how well metalenses can eliminate chromatic aberrations having to do with their thickness and refractive-index contrast. This paper establishes these limits for the first time.

Dozens of academic research groups in the U.S. alone are working in this area, a field also known as “flat optics.” Startup companies, often spin-offs from academic research, as well as larger corporations are showing increased interest in applications of metalens technology due to the material and cost savings.

Some are working on compact imaging optics for consumer electronics such as cellphone cameras or for medical devices like optical endoscopes. Others are creating compact lenses with an ultra-wide field of view for applications in augmented and virtual reality. 

Optica is published by the Optical Society of America and focused on high-impact, pioneering research across the entire spectrum of optics and photonics. 

The figure shown above is a delay-line model of a thin broadband metalens.

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