image: Figure 1. Concept of active metasurface with substantial broadband tunability enabled by RME. (a) Schematic of a tunable MIM resonator for dynamic light manipulation via RME. (b) Simulated magnetic field intensity profiles of the MIM resonators controlled by RME at different wavelength regimes.
Credit: Po-Chun Hsu et al.
Metasurfaces—ultrathin structured interfaces that tailor light at subwavelength scales—are revolutionizing modern optics by enabling ultrathin and flat optical elements. Yet, a major frontier remains: how to realize dynamic metasurfaces with highly tunable optical responses. Conventional dynamic metasurfaces utilize active materials such as phase-change compounds or electrochromic polymers that provide limited optical tunability and therefore suffer from low signal-to-noise ratios (SNRs) during switching operations.
In a recent study published in Light: Science & Applications, a research team led by Professor Po-Chun Hsu at the University of Chicago reported a new paradigm—active metasurfaces driven by reversible metal electrodeposition (RME). This electrochemical process enables on-demand creation and removal of metallic layers at the nanoscale, effectively switching the metasurface between distinct optical states with remarkable tunability.
“Reversible metal electrodeposition provides gigantic optical tunability,” said Prof. Hsu. “Instead of relying on materials that change color or phase, we literally ‘grow’ and ‘erase’ metals using electricity—achieving an ideal optical contrast from transparent electrolyte to perfect metal.”
To integrate RME with metasurfaces, the researchers designed a metasurface architecture with metal–insulator–metal (MIM) resonators, in which the metal back reflector also serves the working electrode for electrodeposition. By applying different voltages, metal atoms can be uniformly and reversibly deposited and stripped around the resonators, effectively tuning the gap-surface plasmon resonances on and off (Fig. 1).
“Electrodeposition at the nanoscale can be tricky, suffering from rough morphology and poor stability. We thoroughly analyzed a wide range of RME materials and metasurface designs,” the authors noted. “Copper was chosen as our RME metal for its moderate reduction potential, which ensures ease of both reduction and oxidation. Besides, using the metal back reflector as the working electrode guarantees high electrical conductivity and lattice match. This is essential for high-quality deposition.”
As a proof of concept, the team demonstrated a dynamic beam-steering device by performing RME of Cu on a reflective gradient metasurface. In its pristine state without Cu deposition, the trapezoidal antennas of the metasurface impart a phase gradient to the reflected light, causing the reflection angle to deviate from the incident angle (anomalous reflection). Applying a small voltage (-1 V) for 700 ms deposits a ~30 nm Cu layer around the antennas, turning off their plasmonic resonances, eliminating the phase gradient, and steering the light to specular direction. Reversing the bias dissolves Cu and restores the original state. In the experiments, the RME-based metasurface achieved diffraction efficiencies exceeding 90% in both states and maintained stable performance over 3000 switching cycles (Fig. 2).
Beyond the visible spectrum, the same design principle was extended to the near- and mid-infrared regimes. Across all wavelengths, the RME-based active metasurfaces consistently exhibited record-high SNRs up to 15 dB. Such high performance is attributed to the substantial material tunability of RME systems, as predicted by their near-unity reflectivity contrast during switching (Fig. 3).
“This work bridges electrochemistry and nanophotonics,” Prof. Hsu explained. “By combining precise nanoscale design with reversible metal growth, we can dynamically control light in ways that were previously impossible. Our future efforts will focus on accelerating RME process to enable video-rate switching while maintaining its high tunability.”
The researchers envision that RME-enabled metasurfaces could lead to more reconfigurable optical and thermal devices—from adaptive beam scanners and smart windows to energy-saving radiative modulators. As the electrochemical concept is compatible with existing fabrication and battery-packaging technologies, it holds potential for future large-area and cost-effective implementations.
Journal
Light Science & Applications
Article Title
High-efficiency broadband active metasurfaces via reversible metal electrodeposition