Kirigami Reconfigurable Gradient Metasurface

The metasurface is a futuristic technology being implemented in many practical applications. These surfaces are extremely thin so they induce field discontinuities at the interface which leads to unexpected ups and downs in the amplitude, phase, and polarization of electromagnetic waves at a scale of subwavelength. Metasurfaces have exceptional capabilities to play with electromagnetic wavefront. Many fascinating devices like invisibility cloaks, wave plates, focusing lenses, imaging, and other devices have been made from Metasurfaces. The main deficiency of these metasurfaces is that they lack the ability to tune or reconfigure responses as they were composed of metallic or dielectric structures, as a result, they inbuilt only a few features when they are manufactured and this limits their applications.

To overcome the above issue, switchable or tunable components like diodes or phase-changing materials were used to get active control, this helped reconstruct the field distribution at the interface by changing the external stimulus for example the light intensity, bias voltage, etc. These metasurfaces were used in holography, active metalens, and other practical applications but getting this active control is very expensive and requires a large labor force.

Kirigami is a Chinese art that has made its way into the upcoming technologies like tunable optical devices, energy storage, etc. In Kirigami a folding pattern is created that gives the structure an overall change in shape, which means it can be reconfigured and tunable. This technique is an exceptional candidate to produce meta-devices with complex 3d and lightweight structures, by following certain folding sequences. But still, they lack the ability to exploit the potential of optical wavefront with spatially-varying elements distribution as they are restricted to tuning globally over the entire surface.

The research team at Nanjing University, including PhD candidate Yilin Zheng, Weixu Yang, Linxiao Wu, Kai Qu, Dr. Ke Chen, Junming Zhao, Tian Jiang, and led by Professor Yijun Feng have come up with an idea for kirigami-based reconfigurable gradient metasurface (KRGM), by which electromagnetic wavefront can be tuned over continuous state ranges through a sequence of folding patterns of the metasurface. To prove this concept, the authors used two different kirigami techniques, to design two meta-devices like tunable anomalous refractor and reconfigurable metalens through KRGM which are then displayed in simulations and experiments. The original research article is now published in Journal of Advanced Functional Materials.

Two different 3D kirigami structures were made from a 2D metasurface. Throughout the folding process, the phase gradient at the metasurface was modified by the stretching or pressing operation that finally resulted in the modification of electromagnetic performance of the total metasurface. This is the key design principle of the planned kirigami reconfigurable phase gradient metasurface. It was demonstrated that in the foldable metasurface based on the chosen kirigami technique the researchers changed the folding angles and were able to reconfigure the electromagnetic responses. In metalens, it was observed that changing the focal length was decreased when folding angles were increased.

In summary, it is known that traditionally chromatic dispersion occurs as the focal length increases for a planar metalens when the frequency increases. By using kirigami the focal length can be decreased when the folding angles are increased over a particular bandwidth, hence it opposes chromatic dispersion and rather mitigates it. This allowed the use of different folding states for metalens to keep the focal length unchanged over a continuous frequency band. This kirigami technique is not restricted to the particular type of meta-atoms or frequency, in fact, it can be deployed in other frequencies and different polarization incidences too. In a statement to Advances in Engineering Series, Professor Yijun Feng, the corresponding and lead authors said the new study offers an innovate way of crafting reconfigurable gradient metasurface with added mechanical properties. With the continuing development of science and technology, this kirigami reconfigurable gradient metasurface offers a vast range of applications in data technology, imaging system, medicine science, and deployable device.