A silicon-lithium niobate metasurface controls light through air at gigahertz speeds, advancing compact systems for optical communication and signal routing.
(Nanowerk Spotlight) Optical systems that transmit and manipulate light through air—rather than confining it within fibers or waveguides—are essential for technologies like wireless communication, high-resolution sensing, and augmented reality. These so-called free-space systems are crucial for directing and reshaping light in applications where physical connections aren’t possible or desirable. But the components needed to control light in this setting face difficult tradeoffs.
Devices that adjust light’s intensity or direction are often too slow, inefficient, or bulky to meet modern performance demands. A key challenge has been designing compact elements that can rapidly and precisely modulate light while it travels through air.
Various previous designs rely on materials whose optical properties can be tuned with electric signals. Some use liquid crystals, others employ materials that change phase or conductivity. These approaches offer some control but often suffer from slow switching speeds, high energy loss, or limited modulation range.
Lithium niobate—a transparent material with strong electro-optic behavior—is widely used in integrated photonics because it responds quickly to electric fields. However, producing substantial modulation in lithium niobate typically requires long optical paths or high voltages, which makes miniaturized, efficient free-space systems difficult to build.
One way to overcome these limitations is to amplify small refractive index changes—the way a material bends or slows light—into large optical effects. This can be done using resonant nanostructures that trap light temporarily at specific wavelengths. When carefully designed, these resonances become highly sensitive to small electrical changes, allowing for sharp, controllable responses to voltage inputs in a compact format.
In a study published in Advanced Materials (“GHz‐Speed Wavefront Shaping Metasurface Modulators Enabled by Resonant Electro‐Optic Nanoantennas”), researchers present a metasurface that uses this strategy to achieve fast, voltage-controlled modulation of light propagating through air. By combining patterned silicon with a thin layer of lithium niobate, they build an array of resonant nanoantennas that respond to small electrical signals. The result is a tunable optical surface that can modulate both the intensity and direction of transmitted light at gigahertz speeds. This device offers a promising foundation for thin, reconfigurable optics suited to communication, sensing, and imaging applications.
a) Schematic of metasurface device. Incident light (kinc) with an x-polarized electric field transmits (ktrans) through the metasurface, upon which the intensity of the transmitted light is modulated. b) Schematic of nanoantenna unit cell, consisting of silicon nanoantenna, LNO thin film, and gold electrodes, where p = 680 nm, w = 100 nm, d = 40–100 nm, s = 50–350 nm, wSi = 400 nm, tSi = 300 nm, tLNO = 300 nm, tAu = 70 nm, wAu = 200 nm, and wp = 1600 nm. c) Principle of operation: the shift in resonant wavelength due to the change in refractive index of the LNO results in the intensity of the transmitted light being modulated. Simulated results are from a metasurface with a perturbation depth, d, of 40 nm, and electrode spacing, s, of 300 nm, applying ±5 V. d) Optical micrograph of the fabricated metasurface devices. e) False-color scanning electron micrograph of a fabricated device. (Image: reprinted with permission from Wiley-VCH Verlag) (click on image to enlarge)
The metasurface developed by the team consists of an array of silicon nanobars—elongated, rectangular structures—mounted on a lithium niobate film. Each silicon bar acts as a miniature antenna, guiding light along its length and scattering it in a controlled way. When small geometric features are added to the bar’s surface, these structures support guided-mode resonances, which are optical modes that concentrate light within the nanobar for short periods of time. Because a portion of the optical field extends into the lithium niobate layer beneath the silicon, changes in the refractive index of that layer—induced by an applied voltage—can shift the resonance and modulate how much light passes through the device.
This design enables precise electrical control over the optical properties of each antenna. The researchers demonstrated that applying a voltage of ±5 volts could produce a 7.1% absolute change in light transmittance at the resonance, a level of performance that exceeds many previous devices based on lithium niobate or similar materials. The modulation efficiency was measured at 0.71% per volt, meaning small voltage changes could produce clear optical effects.
To explore how different design parameters affect performance, the team fabricated multiple versions of the metasurface with varying degrees of surface perturbation—small indentations introduced into each silicon nanobar. Shallower indentations produced higher quality resonances, meaning the optical response became sharper and more sensitive to small shifts in refractive index. Devices with these higher-quality resonances showed stronger modulation performance. Quality factors above 2000 were observed in some designs, indicating strong confinement of light within the nanostructure.
The researchers also found that their device’s performance was not limited by the quality of the optical resonance but rather by the electrical properties of the system—specifically, the capacitance of the electrodes used to apply the voltage. By shrinking the overall area of the metasurface, they reduced this capacitance and extended the modulation bandwidth. A smaller device, measuring 40 micrometers on each side, achieved a bandwidth of 890 megahertz.
A larger variant designed as a beam splitter—redirecting light into two directions instead of transmitting it forward—reached a bandwidth of 1.03 gigahertz. These speeds place the technology in a range suitable for many modern optical communication systems.
The team then demonstrated the ability to control not just the intensity of light but also its direction. They designed a metasurface where adjacent antennas had different geometries, producing opposite phase shifts in the scattered light. On resonance, these phase differences caused the light to interfere in a way that redirected it into two symmetric diffraction orders—effectively splitting the beam into two paths. By applying a voltage, the researchers were able to control the balance between these directions, modulating the distribution of light in space. This function is essential for wavefront shaping, where the shape of the light wave itself is controlled to focus, steer, or pattern the beam in specific ways.
To characterize this functionality, they used a setup that allowed precise measurement of light intensity at various angles. When operated near the resonant wavelength, the device split about 12% of the incoming light into the two side directions. Although this number could be improved through further design optimization, it confirms that the concept of phase-controlled beam steering using voltage-tunable resonant antennas is viable.
The team also investigated how fast this beam steering function could operate. By applying alternating voltage signals at increasing frequencies, they measured the modulation response of the diffracted beams. The beam-splitting metasurface maintained modulation above one gigahertz, with some designs reaching 1.22 gigahertz before electrical effects in the electrodes began to limit performance. These effects included the possible excitation of acoustic waves in the lithium niobate layer, caused by periodic electric fields interacting with the crystal structure.
Importantly, the optical design itself did not impose a speed limit. The resonance quality remained sufficient even in miniaturized devices, suggesting that further gains in speed and efficiency could be achieved by improving the electrical connections or adopting more advanced electrode geometries. The authors note that the same platform could be adapted to other tuning methods, including those that take advantage of the material’s nonlinear optical or mechanical responses.
The result is a metasurface platform that offers a practical way to achieve fast, voltage-controlled modulation of light traveling through air. By combining resonant optical design with a sensitive electro-optic material and maintaining compatibility with standard fabrication methods, the researchers present a solution that bridges a long-standing gap in optical engineering. The demonstrated performance—gigahertz bandwidth, subwavelength control, and independent addressability of each antenna—makes the system suitable for applications in advanced optical displays, spatial light modulators, compact LiDAR systems, and secure free-space communication.
Rather than relying on bulky components or slow material changes, this approach uses sharp optical resonances and nanoscale engineering to produce a fast and tunable interaction between electricity and light. With further refinements, such metasurfaces could serve as building blocks for fully reconfigurable optical systems that are both thin and highly functional.
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