The speed of light in a vacuum is constant and cannot be exceeded, but in other media it can be slowed down, meaning more operations are possible. This is important in photonic chip design.
“As the light slows down, the energy density of the light increases,” said Dr. Li Guanyuan, a photonics researcher at the Shenzhen Institute who led the study.
“This means that for the same length of device, the effective distance for light to interact is longer, essentially improving the performance of the photonic device.”
Lee’s team designed a photonic chip in a way that slows down light by more than 10,000 times. According to the researchers, this was achieved with significantly reduced energy losses, which were only about 20 percent of those seen in previous attempts.
The current mainstream method of controlling the speed of light relies on metasurfaces, artificial materials made of nanostructures. When light hits these nanostructures, they resonate, changing their amplitude and phase.
However, the absorption and scattering of light by the materials produced by these artificial atoms can lead to light loss, thereby shortening the photon’s lifetime and limiting the extent to which the speed of light can be slowed down.
To avoid this, Lee’s team improved the materials used and the structural design.
“When it came to material selection, we chose materials that had low or no absorption loss at the wavelengths of interest,” Lee said.
“For example, metals have strong absorption in the infrared spectrum, so we used a lossless silicon material. However, silicon absorbs strongly in the visible light spectrum, so in those cases we used silicon nitride or silicon dioxide. We used transparent materials such as titanium.
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For structural design, the researchers refined a concept called surface lattice resonance, which involves periodic surface patterns to prevent energy loss.
“We designed a periodic surface structure of 100-nanometer silicon nanodisks,” Lee said.
“This design moves the path of light from inside the nanostructure to the surface, significantly reducing absorption losses in the material. Also, some photons lost due to scattering are recaptured by neighboring structures and It rejoins the propagation process and extends the lifetime of the photon.”
Li said the technology could reduce the manufacturing cost of photonic chips and expand their applications.
“Using metasurface technology, photonic chips can be made as thin as stickers or building blocks, enabling functional stacking,” he said.
Using that technology, researchers have recently been able to use photonic chips in devices such as sensors, lasers, and LEDs.
“The performance of these photonic devices is limited by the very limited interaction length of these ultra-thin photonic chips,” Lee said. “Using slow light effects can significantly improve performance.”