High performance imaging systems need to focus many wavelengths to the same position in space; unfortunately, no single conventional lens can do this. Three or more lenses are often needed to improve image quality, because conventional lenses focus different colors to different points.

The performance of a conventional optic is governed by its shape, but there are many other tools optical engineers can use to control light. GRadient INdex (GRIN) lenses mix two materials with different refractive indices such that light bends as it travels through the GRIN lens itself, instead of solely at the surface. A surface covered by an array of nanostructures, or metasurface, can also focus light. Challenges with multicolor focusing still exist with GRIN and metasurface lenses, because both the index of refraction of a material and scattering properties of nanostructures vary across the electromagnetic spectrum. 

Researchers led by Douglas Werner at The Pennsylvania State University have now discovered a method for significantly improving imaging across visible and infrared wavelengths. For visible light, Werner’s method would achieve superior focusing of three different colors when compared to conventional and state-of the art techniques. Werner’s results, published recently in Optica, could be used to improve designs for many imaging applications.

“The goal of our work [was] to overcome challenges with conventional materials that limit performance in imaging systems,” says Sawyer Campbell, a co-author of the article. Campbell and his colleagues devised a set of equations to describe a hybrid lens where a metasurface is fabricated directly on one side of a GRIN optic. Based on their equations, the researchers predicted that this single optical component, called a metasurface-augmented GRIN, or MS-GRIN, lens could focus red, green, and blue light to the same point. The theoretical MS-GRIN lens outperformed conventional optical design techniques currently in use.  

Three-color focusing “would typically require three simple lenses to achieve [this] performance,” says Jogender Nagar, a graduate student in Werner’s group and lead author of the article, “but we could achieve this with just one.” Previous results have demonstrated two-color focusing with a single optical component; however, three color focusing is ideal for providing distortion-free multicolor images. While the theoretical lens still needs to be fabricated, it was not clear before the development of these equations that a single optic could achieve this performance.

The study showed that the GRIN lens was integral to achieving three-color focusing. The GRIN lens introduced multiple degrees of freedom: optical designers can control how the light bends as it travels through the material of the lens and refracts off of its surface. This would not be possible if the metasurface was combined with a conventional lens.

The formulas outlined in this article will help scientists designing optical systems understand how to combine various components strategically to create the same effect while taking up less space. This work “encourages people to think about a new design space and provides tools to aid design within that space,” says Rebecca Dylla-Spears of Lawrence Livermore National Laboratory. Researchers developing new materials or fabrication techniques could also use these formulas to recognize new uses for their work. Dylla-Spears is not affiliated with the publication in Optica.

There are many potential applications for lenses designed with the recipe described in this work. Many applications could be improved by using a single MS-GRIN lens to perform a job that typically requires three or more conventional lenses. Without sacrificing image quality, MS-GRIN lenses could be used to create lighter night vision goggles or reduce the weight of the imaging optics in unmanned aerial vehicles. These hybrid lenses could even be used in commercial systems such as cell phone cameras when fabrication costs are reduced. “This is just scratching the surface,” Werner says. “There will be a lot more designs coming out in the future combining GRIN and metasurface lenses.”

Read the article in Optica.