Nanoscale Device Points the Way to Future Wireless Communication Channels

California Institute of Technology, Pasadena, CA

An incident laser beam (green) hits Caltech’s new space-time metasurface and gets modulated by tunable, nanostructured antennas, creating steerable beams of light at different frequencies (blue) that could be used as optical channels for transmitting data on Earth or in space. (Image: Caltech)

Imagine you had a dedicated wireless channel for communication that was hundreds of times faster than the Wi-Fi we use today, with hundreds of times more bandwidth. That dream may not be far off thanks to the development of metasurfaces: tiny engineered sheets that can reflect and otherwise direct light in desired ways.

In a paper published in the journal Nature Nanotechnology, a team of Caltech engineers reports building such a metasurface patterned with miniscule tunable antennas capable of reflecting an incoming beam of optical light to create many sidebands, or channels, of different optical frequencies.

“With these metasurfaces, we’ve been able to show that one beam of light comes in, and multiple beams of light go out, each with different optical frequencies and going in different directions,” said Senior Author Harry Atwater, the Otis Booth Leadership Chair of the Division of Engineering and Applied Science, the Howard Hughes Professor of Applied Physics and Materials Science. “It’s acting like an entire array of communication channels. And we’ve found a way to do this for free-space signals rather than signals carried on an optical fiber.”

The work points to a promising route for the development of not only a new type of wireless communication channel but also potentially new range-finding technologies and even a novel way to relay larger amounts of data to and from space.

Co-Lead Author Prachi Thureja, a graduate student in Atwater’s group, said to understand their work, first consider the word “metasurface.” The root, “meta,” comes from a Greek prefix meaning “beyond.” Metasurfaces are designed to go beyond what we can do with conventional bulky optical elements, such as camera or microscope lenses. The multilayer transistor-like devices are engineered with a carefully selected pattern of nanoscale antennas that can reflect, scatter, or otherwise control light. These flat devices can focus light, in the style of a lens, or reflect it, like a mirror, by strategically designing an array of nanoscale elements that modify the way that light responds.

Much previous work with metasurfaces has focused on creating passive devices that have a single light-directing functionality that is fixed in time. In contrast, Atwater’s group focuses on what are known as active metasurfaces. “Now we can apply an external stimulus, such as an array of different voltages, to these devices and tune between different passive functionalities,” said Co-Lead Author Jared Sisler, also a graduate student in Atwater’s lab.

In the latest work, the team describes what they call a space-time metasurface that can reflect light in specific directions and also at particular frequencies (a function of time, since frequency is defined as the number of waves that pass a point per second). This metasurface device, the core of which is just 120 microns wide and 120 microns long, operates in reflection mode at optical frequencies typically used for telecommunications, specifically at 1,530 nanometers. This is thousands of times higher than radio frequencies, which means there is much more available bandwidth.

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