Metamaterials are a class of materials that interact with energy and light in ways not found in nature. To fully harness the functionalities of metamaterials, multiple stacks of material are typically required, which comes with challenges in fabrication. Thus, metasurfaces, thin 2D structures created with metamaterials with similar functionalities, can be used instead for the manipulation of light.

Professor Min Seok Jang from the School of Electrical Engineering collaborated with Professor Victor W. Brar from the University of Wisconsin-Madison in creating a method for metasurface design capable of 360° active phase modulation without causing a significant change in light amplitude, thus surpassing the limits of existing methods.

Metasurfaces could prove to be very useful in various future technologies. However, to fully work with potential applications, metasurfaces need to have 360° active optical phase modulation: controlling the phase and intensity of the reflected and transmitted light by manipulation of individual meta-atoms. However, in contrast to static implementations, dynamic systems have faced various obstacles. Previous methods for 360° phase shift relied on multiple control parameters, which resulted in complex operation methods as well as small and non-uniform light amplitudes.

Dynamic systems require tuning of localized material that changes the phases of optical wavefronts, but the tuning also affects the magnitude of reflected and transmitted light. This leads to a non-uniform amplitude of light. To further complicate the issue, most tunable materials cannot cover the 360° range. Metasurfaces generally function by interacting with incident light that results in optical resonances of electrons inside the metasurface. To achieve the 360° phase shift, the optical resonances must change while the linewidth remains constant, but increasing the optical resonance increases the linewidth, leading to a limited range.

The research team circumvented those limitations by using special optical resonances: one with decoupled phase and amplitude characteristics, which allows large and uniform amplitude even with phase change, and another with a large phase modulation range allowing 360° control. The research team designed a technique for combining the two optical resonances to create a metasurface with a 360° range and uniform amplitude. By applying this technique, they created a metasurface based on graphene that modulated a 540° phase range with uniform amplitude.

Having functionalities that have applications in many futuristic technologies from LIDAR to holograms, metasurfaces are gaining a lot of attention. Professor Jang expressed hope that this development will aid researchers in implementing various future technologies. In addition, metasurfaces are in the micro or nanometer scale, which opens up the possibility of integration into electronic circuits.