Flat Lens With a Twist

Illustration of a metasurface, which comprises an array of nanofins and behaves like an ultrathin bifocal lens. (Source: KAUST)

Ultrathin nano­structured films that control light propagation provide a way of inte­grating optical components into portable and wearable electronic devices. Twisting a stack of such films offers a simple means to control their behavior and perfor­mance, KAUST research shows. A surface patterned with an array of nanometer-scale structures can change the properties of light passing through it. Each element in the array behaves like a tiny antenna that controls the light’s local phase; that is the relative position of the light wave in its oscil­latory cycle. These ultrathin meta­lenses can focus light just like a conventional, albeit much thicker, glass lens, while being more functional.

“This techno­logy can arbitrarily shape the light pixel by pixel, which is impossible for conven­tional lenses because of fabri­cation limi­tations,” says Ronghui Lin. “The metalens tech­nology has the potential to replace the huge lens assemblies used in professional reflex cameras with a lens as thin as a postcard.” One challenge in the development of multi­functional metalenses is their limited effi­ciency. One possible way to improve this is to stack the metalens. By doing this, Lin and his supervisor, Xiaohang Li, discovered that new pheno­menon can be enabled when one metalens is laid on top of another.

The team looked at a metalens with a surface covered in an array of fins or cylinders with an elliptical cross section. By varying the relative orienta­tion of these fins, the lens can add a geometric phase to incoming circu­larly polarized light. “Consider the rotation of the hands of a clock, which return to the same place every day,” explains Lin. “The rotation angle of these nanofins works in a similar manner. When light passes through these structures, its phase or ‘time’ is changed.” The degree of change depends on the nanofin rotation. This is a powerful tool to mani­pulate circu­larly polarized light.

Lin and Li used finite-difference time-domain simu­lations to model light propa­gation in a metalens system comprising two stacked phase elements. Their results showed that by twisting the relative alignment of the two layers, a pheno­menon similar to the Moiré effect can be observed. The team used this pheno­menon to develop a bifocal metalens with control­lable focal length and inten­sity ratio. “We believe this multi­layer metalens archi­tecture could also apply to other systems and achieve more complicated func­tionalities,” says Lin. (Source: KAUST)

Reference: R. Lin & X. Li: Multifocal metalens based on multilayer Pancharatnam–Berry phase elements architecture, Opt. Lett. 44, 2819 (2019); DOI: 10.1364/OL.44.002819

Link: Advanced Semiconductor Laboratory, King Abdullah University of Science and Technology KAUST, Thuwal, Saudi Arabia

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