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Metasurfaces are two-dimensional (2D), nanoengineered surfaces that interact strongly with electromagnetic waves and can control light with remarkable precision. These ultra-thin layers can be used to develop a wide range of advanced technologies, including optical photonic, sensing and communication systems.

Active metasurfaces, whose electromagnetic response can be dynamically tuned in real-time, are particularly promising for advanced real-world applications, particularly for the development of reconfigurable antennas, highly sensitive sensors and other adaptive systems. These metasurfaces can also serve as optical modulators, devices that adjust the intensity or phase of light and thus enable the encoding of information onto light beams.

While engineers have introduced various metasurface-based optical modulators over the past few years, most devices developed so far require high-voltage electrical signals to operate. This means that to noticeably change the optical response of the metasurfaces they are based on, users need to apply a strong electrical field to them.

Researchers at the University of Tokyo recently introduced a new hybrid metasurface that combines silicon nanostructures with an organic electro-optic layer, which can modulate light at very low voltages. This promising metasurface, introduced in a paper in Nature Nanotechnology, could be used to develop new low-power and high-speed optical technologies.

"Active metasurfaces incorporating electro-optic materials enable high-speed free-space optical modulators that show great promise for a wide range of applications, including optical communication, sensing and computing," Go Soma, Koto Ariu and their colleagues wrote in their paper.

"However, the limited light–matter interaction lengths in metasurfaces typically require high driving voltages exceeding tens of volts to achieve satisfactory modulation. We present low-voltage, high-speed free-space optical modulators based on silicon-organic-hybrid metasurfaces with dimerized-grating-based nanostructures."

Notably, the hybrid metasurface engineered by Soma, Ariu and their colleagues is compatible with existing complementary metal–oxide–semiconductor (CMOS) devices nanofabrication processes, which means that it could be easier to combine with existing devices and deploy in real-world settings. The metasurface effectively confines light in carefully engineered nanostructures and this ultimately enhances its modulation capabilities.

In initial tests, the metasurface was found to modulate light at remarkable speeds, while requiring low voltages to operate. Therefore, compared to previously developed metasurface-based optical modulators, the team's metasurface could significantly reduce power consumption.

"By exploiting a high-Q resonant mode, normally incident light is effectively trapped within a submicrometer-scale silicon slot region embedded with organic electro-optic material," wrote the authors. "Consequently, highly efficient modulation is obtained, enabling data transmission at 50 Mbps and 1.6 Gbps with driving voltages of only 0.2 V and 1 V, respectively. These metasurface modulators can now operate at CMOS-compatible voltage levels, allowing energy-efficient high-speed practical applications of active metasurfaces."

In the future, the recent work by Soma, Ariu and their collaborators could contribute to the further advancement of various technologies, for instance, enabling the development of new high-speed communication and sensing systems. Other research teams could draw inspiration from this study and explore similar nanoengineering strategies to improve the energy-efficiency of optical modulators.

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