Traditional glass lenses (a) focus different colors to different points resulting in color separation and image blurring. In contrast, newly developed metasurface-augmented gradient-index lenses (b) correct for color separation by focusing blue, green and red light to the same point. Image: Jogender Nagar, Penn State.Producing perfect color images often requires multiple, heavy lenses to focus the different wavelengths of visible light to exactly the same point. Now engineers at Penn State have come up with a way to do this using a single thin lens comprising gradient index materials and metasurface layers.
"If we want high performance optical systems, then we have to overcome material dispersion," said Sawyer Campbell, assistant research professor in electrical engineering at Penn State. "If we don't, we get smeary colors, which significantly degrades image quality."
Single apochromatic lenses, which properly focus the wavelengths corresponding to the three primary colors of red, blue and green, that are lighter, thinner and possess less curvature could improve cell phone cameras and allow the manufacture of thinner cell phones. They also could lead to the development of 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," explained Jogender Nagar, graduate student in electrical engineering. "Our aim is to improve SWaP – reduce size and weight while increasing performance."
To do this, the researchers combined two technologies: gradient-index (GRIN) lenses and metasurfaces – ultra-thin optical layers with sub-wavelength features that manipulate the light waves in a desired manner. The researchers report the results of their work in a paper in Optica.
"Our system uses one lens," said Nagar. "We use the curvature of the lens, the distribution of materials in the lens and a metasurface – a pattern placed on the surface – to make the lens thinner, lighter, but still focus properly."
Most lenses use curvature to control the focal point, but this means three separate conventional lenses are needed to focus the three primary colors to one focal point and produce a high quality image. However, by spatially varying the material composition inside the lens, one GRIN lens can perfectly focus two colors. Adding a metasurface to the GRIN lens then produces a single layered lens that can perfectly focus all three colors, thus doing 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 Werner, a 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 for perfectly focusing 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."
This story is adapted from material from Penn State, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.