Reconfigurable Intelligent Surface Offers Precision Wave Control

An international team of researchers has developed a mechanical reconfigurable intelligent surface that features a high reconfiguration degree of freedom, is low cost, and is characterized by its low energy consumption. The device operates at microwave frequencies and uses a robust control method to determine the rotation angle of each meta-atom.

Reconfigurable intelligent surfaces (RISs) are programmable structures that can be used to control the propagation of electromagnetic waves by changing the electric and magnetic properties of the surface. They provide a new approach to improving the performance of wireless communications systems: changing the propagation environment rather than adapting to it.

The integration of metallic resonators and electronic-driven elements, such as PIN diodes and varactor diodes, has advanced RIS research to a stage at which RISs can manipulate electromagnetic waves with subwavelength resolution. When combined with a field-programmable gate array, these RISs can be switched dynamically among many different functions in real time, simply by changing the coding sequences.

Schematics of the RIS supercell and the meta-atom. Right: Geometric phase control in each supercell is achieved by transmitting the torque from a stepping motor to the meta-atoms through a set of gears. Left: Pink and sky-blue dials schematically depict the geometric phase control resolution and variation gradients for different circular polarizations. Courtesy of Quan Xu et al.

Still, pain points persist in these diode-based RISs. First, the reconfiguration degree of freedom in the unit element scale is limited by the working principle of PIN/varactor diodes, as most are binary. Also, the typical power dissipation of a single diode is around several hundred milliwatts; maintaining functionality requires a continuous power supply. Such energy consumption entails trade-offs between the size or number of unit element and the overall RIS size, hindering large-scale and long-term applications.

The device developed by the team consists of a 20 × 20 supercell covering an area of 870 × 870 mm. Each supercell is composed of a stepping motor, a set of transmission gears, and a 4 × 4 array of meta-atoms. Each meta-atom can be mechanically rotated to achieve the desired phase control. This reprogrammable function enables a continuous and arbitrary phase control pattern over the entire RIS with high efficiency and uniform amplitude.

The researchers showcased dynamic and efficient control of the impinging wavefront on the RIS by reconfiguring its operation in real time across a number of functionalities including metalensing, focused vortex beam generation, and holographic imaging by a proper design of the PB phase distribution. They demonstrate that the quasi-continuous phase tunability significantly improves the wavefront controllability.

The mechanically reprogrammable modules and meta-atoms can be flexibly attached/detached. Plus, the system maintains designated functionality without consuming electricity, so it offers a new energy-saving and environmentally friendly alternative.

“This RIS promises to achieve multidimensional manipulation of electromagnetic waves by incorporating different gear sets and different meta-atoms, which may bring RIS-related research to the next level,” said senior author Weili Zhang, professor of engineering at Oklahoma State University.

The research was published in Advanced Photonics (www.doi.org/10.1117/1.AP.4.1.016002).