Beyond traditional optics: Caltech scientists build 'space-time metasurface' that could be used to develop new wireless communication channels

According to news on July 29, a team from the California Institute of Technology built a nanoscale device covered with micro-tunable antennas that can reflect a beam of incident light into multiple beams of light, each with a different frequency. Able to spread in different directions. According to reports, this kind of "Electrically tunable space-time metasurfaces at optical frequencies" can control and change the frequency of light at the same time, and can point the way for future wireless communication channels. Relevant results have been published in "Nature: Nanotechnology" (attached DOI: 10.1038/s41565-024-01728-9).

As shown in the figure, the incident laser beam (green) hits this new "space-time metasurface" and is modulated by the tunable nanostructure antenna, which can produce controllable beams (blue) of different frequencies, which can be used as An optical channel for transmitting data on Earth or in space.
"With these metasurfaces, we have been able to show that one beam of light comes in and multiple beams of light go out, each with a different optical frequency and traveling in a different direction," said Otis Booth, Department of Engineering and Applied Science leader "It's like a complete array of communications channels," said Harry Atwater, chairman, Howard Hughes Professor of Applied Physics and Materials Science and senior author of the paper. "We found a way to transmit signals in free space rather than on optical fibers. method.”
This work not only points to a feasible path for developing new wireless communication channels, but also may lead to the development of new ranging technologies and even a way to transmit more data to and from space. New approaches to data point the way.
To understand this work, first consider the term "metasurface". The word "meta" is derived from the Greek prefix meaning "beyond."
Aiming to surpass the capabilities of traditional optical components, such as camera or microscope lenses, "metasurfaces" are multilayer transistor-like devices designed with carefully chosen patterns of nanoantennas that can reflect, scatter or otherwise control light. These planar devices can focus light (similar to a lens) or reflect light (similar to a mirror) by strategically designing a series of nanoelements that influence how light responds.
According to reports, this thing called a "space-time metasurface" can reflect light in a specific direction and at a specific frequency (frequency is defined as the number of waves passing through a point per second). The core of this device is only 120 microns long and wide, and operates in reflective mode at optical frequencies commonly used for telecommunications (specifically 1530 nanometers), which are thousands of times higher than radio frequencies, which means that it will bring more Available bandwidth.
At radio frequencies, electronic devices can easily steer light beams in different directions, such as radar navigation equipment used on aircraft, but there are currently no electronic devices that can do this at higher optical frequencies. So the researchers had to try a different approach, changing the properties of the antenna itself.
Sisler and Turegia somehow developed this "metasurface" consisting of a gold antenna with an electrically tunable indium tin oxide semiconductor layer underneath, which can be made by applying known The voltage distribution locally modulates the electron density in the semiconductor layer beneath each antenna, changing its refractive index (the material's ability to refract light).
"By having spatial configurations of different voltages across the entire device, we can redirect reflected light at specified angles in real time without having to replace any bulky components," Turegia said.
"We direct an incident laser beam at a certain frequency Hitting our metasurface then modulates the antenna signal with a high-frequency voltage signal in time, creating multiple new frequencies, or sidebands, carried by the incoming laser light that can be used as high-data-rate channels to send information. There's still spatial control, which means we can choose where each channel is in space," Sisler explained. "We're generating frequencies and directing them through space. That's the space-time component of this metasurface. "If optical metasurfaces become an achievable and widely used technology, then in ten years you will be able to sit in a Starbucks with a group of other people," said Atwater, director of the Liquid Sunlight Alliance at Caltech. Internet access, and what everyone gets is not a radio frequency Wi-Fi signal, but their own high-fidelity optical signal." "A metasurface will be able to emit different frequencies to everyone."

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