Metasurfaces for Apochromatic Lenses

Newly developed metasurface-augmented gradient-index lenses correct for color separation by focusing blue, green, and red light to the same point. (Source: J. Nagar, PSU)

Producing the perfect color images often requires multiple, heavy lenses so that each color focuses in exactly the same plane. Now Penn State engi­neers have deve­loped a new theory that solves the problem using a single thin lens comprised of gradient index materials and meta­surface layers to properly direct the light. “If we want high perfor­mance optical systems, then we have to overcome material dispersion,” said Sawyer D. Campbell, assistant research professor in electrical engi­neering. “If we don’t, we get smeary colors, which signi­ficantly degrades image quality.”

Single apochro­matic lenses – ones that properly focus the three colors red, blue and green – that have less curva­ture and are thinner and lighter could improve cell phone cameras and allow manu­facture of thinner cell phones. They also could make lighter, better body cameras, helmet cameras, sniper scopes, thermal imaging devices and unmanned aerial vehicles or drones. In essence, anything that uses lenses to image could be made simpler and lighter. “Usually there are several lenses, but that increases the weight,” said Jogender Nagar, graduate student in electrical engi­neering. “Our aim is to reduce size and weight while increasing perfor­mance.”

The researchers thought to combine two techno­logies: that of gradient-index (GRIN) lens, and meta­surfaces – ultra-thin optical layers with sub-wavelength features that mani­pulate the wave­front in a desired manner. “Our system uses one lens,” said Nagar. “We use the curvature of the lens, the distri­bution of materials in the lens, and a meta­surface to make the lens thinner, lighter, but still focus properly.”

Most lenses use curva­ture to control the focal point, but three separate conven­tional lenses are needed to focus the three separate colors to one focal point and produce a high quality image. By spatially varying the material compo­sition inside the lens, one GRIN lens can per­fectly focus two colors. Then, by adding a metasurface to the GRIN lens, one layered lens can perfectly focus all three colors and do the work of three conven­tional lenses. “The gradient in the lens can be axial – varying along the direction of light propa­gation, or optical axis; or radial – varying out­wardly from the optical axis,” said Douglas H. Werner, professor in electrical engineering. “Or it could be more complex.”

The researchers developed a theo­retical model and simu­lation framework for creating these lenses. “We had to use some advanced tools that were specially developed in the lab” said Werner. “Tools for modeling, simulation and optimization that we created for solving such a challen­ging design problem.” The theo­retical model specifies the proper surface curvature and gradient in the GRIN lens and the proper patterning for the metasurface to satisfy the require­ments of perfect focusing of all three colors. The model opti­mizes both the lens and the meta­surface to work together.

“The theory is very general and covers a wide range of conditions,” said Werner. “Fabri­cation will be the challenge initially. We hope develop­ment of the theory will steer the fabri­cation, making it possible to produce such lenses at a low cost and high volume.” (Source: PSU)

Reference: J. Nagar et al.: Apochromatic singlets enabled by metasurface-augmented GRIN lenses, Optica 5, 99 (2018); DOI: 10.1364/OPTICA.5.000099

Link: Computational Electromagnetics and Antennas Research Lab, Pennsylvania State University, University Park, USA

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