Overlaid optical metalenses have new properties
Overlaying two film layers patterned with a nanoscale array can manipulate the propagation of light to create a capable ultrathin lens.
Ultrathin nanostructured metalens lenses 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, research at KAUST (Thuwal, Saudi Arabia) shows.1
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, graduate student Ronghui Lin and his supervisor, Xiaohang Li, discovered, at least in simulations, that new phenomena 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.
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 a 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.
For example, a two-layer simulated metalens can split a circularly polarized beam into two different helicities and focus them each at a different focus. The ratio of intensies at these focuses are different depending on the design. Unlike previous designs, this metalens design forms the two foci using light from across the whole aperture for both foci.
1. Ronghui Lin and Xiaohang Li, Optics Letters (2019); https://doi.org/10.1364/OL.44.002819.