Antennas receive and transmit electromagnetic waves, delivering information to our radios, televisions, cellphones and more. Researchers in the McKelvey School of Engineering at Washington University in St. Louis imagines a future where antennas reshape even more applications.
Their new metasurfaces, ultra-thin materials made of tiny nanoantennas that can both amplify and control light in very precise ways, could replace conventional refractive surfaces from eyeglasses to smartphone lenses and improve dynamic applications such as augmented reality/virtual reality and LiDAR (light detection and ranging).
While metasurfaces can manipulate light very precisely and efficiently, enabling powerful optical devices, they often suffer from a major limitation: Metasurfaces are highly sensitive to the polarization of light, meaning they can only interact with light that is oriented and traveling in a certain direction. While this is useful in polarized sunglasses that block glare and in other communications and imaging technologies, requiring a specific polarization dramatically reduces the flexibility and applicability of metasurfaces.
To overcome this obstacle, a team led by Mark Lawrence, an assistant professor in the Preston M. Green Department of Electrical & Systems Engineering, demonstrated polarization-independent and highly resonant metasurfaces that maintain high accuracy and efficiency. The results are published in Nano Letters.
“When we combine these tiny antennas to shape light waves, we can move away from relying on shaped glass or other refractive materials,” Lawrence said. “We can shrink our devices down, design and shape them however we like, and still manipulate light accurately and efficiently.”
Lawrence’s polarization-independent metasurfaces have what’s known as a high quality factor, which means they trap light over a narrow band of resonant frequencies for a long time, generating a strong response to external stimuli. This sensitivity enables enhanced functionality that will open new applications for light shaping.
“We’re not just making metasurfaces smaller, we’re embedding them with new capabilities,” Lawrence added. “For example, by resonantly amplifying light inside our devices, we could make eyeglasses that translate and make sense of incoming information for the wearer or make programmable lenses that change focus or steer light exactly as the user wants.”
The team’s earlier iterations of highly resonant metasurfaces only achieve these advanced properties when illuminated with a specific polarization. But, with a new approach to metasurface fabrication, a polarizer is no longer required.
Lawrence and first-author Bo Zhao, a graduate student in Lawrence’s group, engineered metasurfaces with two cross-polarized modes that can be tuned and function independently. Careful alignment and tuning of the two modes allows the metasurface to manipulate light across multiple polarizations simultaneously without loss of efficiency or other desired qualities.
The implications of this work go beyond improving the versatility of metasurfaces. By enabling highly resonant polarization-independent wavefront shaping, the novel metasurfaces could help unlock new methods for nonlinear generation and mixing of light, potentially leading to breakthroughs in signal processing, the design of quantum devices, and other imaging and sensing applications.
