Image of wafer-scale membranes made up of 1 cm × 1 cm chips, each containing 7 × 7 metasurfaces. EPFL BIOS CC BY SA 4.0
EPFL researchers have developed ultra-thin silicon structures that can rapidly tune their interaction with light. The process overcomes a photonics challenge for advanced communications, sensing, and quantum technologies.
Light in the mid-infrared (mid-IR) portion of the electromagnetic spectrum plays a key role in modern sensing. Because molecules interact with this kind of light in specific ways, researchers use technologies like mid-IR spectroscopy to identify biological materials, drugs, and pollutants. Mid-IR light can also serve as a carrier of information in free-space optical communications, where data are transmitted through the air without using cables or fibers. Better control of mid-IR light could therefore lead to more sensitive detectors and faster communications.
Metasurfaces are one of the solutions scientists are exploring to achieve this kind of control. These ultra-thin structures are built using nanoscale patterns that can shape and direct light waves. Unfortunately, most metasurfaces developed thus far are static: once fabricated, their optical properties are fixed, which limits their use in real-world photonic systems.
Now, researchers led by Hatice Altug in the Bionanophotonic Systems Laboratory (BIOS) in EPFL's School of Engineering have overcome this bottleneck with metasurfaces based on suspended membranes of crystalline silicon. By inducing mobile electrical charges within the silicon itself, the researchers can change how the metasurfaces respond to light in real time, without changing their physical structure. The devices also achieve record optical performance, with more than an order of magnitude better performance in key metrics compared to previous mid-IR platforms based on similar materials.
"By using crystalline silicon with the nanostructured layer suspended in air, we were able to greatly reduce optical losses that typically limit conventional mid-infrared platforms," Altug explains. The work has been published in Nature Communications.
From milliseconds to billionths of a second
As BIOS senior scientists Ivan Sinev explains, the team demonstrated two ways of tuning the metasurfaces without altering their nanostructure.
"In one approach, we use integrated electrodes as micro-heaters to induce changes in the silicon's optical properties thousands of times per second via an electrical current," he says. "In another, we used ultrafast laser pulses to induce these changes, allowing the metasurfaces to respond on billionths of a second (nanosecond) timescales."
This rapid, real-time manipulation of light signals could help communication systems transmit information more efficiently, and enable sensors that adapt quickly to changing conditions.
"Importantly, our platform's record optical performance does not come at the expense of scalability, because we use manufacturing techniques that are already compatible with large-scale semiconductor production," says first author Felix Brikh.
A platform for future mid-infrared photonics
Rather than creating a single device, the work establishes a versatile platform for future mid-IR technologies. In particular, the platform could enable compact, low-power devices for free-space optical communications, and highly selective and adaptive chemical and biological sensing. It could also help develop active radiative cooling technologies to help surfaces release heat, for example in refrigeration or satellite thermal management applications.
Looking further ahead, the combination of high optical performance and fast tunability could support advanced photonics applications that rely on precise control of light, such as quantum spectroscopy.
