Caltech's Silicon Metasurface Steers Light in 74 Femtoseconds With No Moving Parts

A pulse of light crosses the width of a human hair in roughly 74 femtoseconds — 74 quadrillionths of a second. In that same interval, a device built at the California Institute of Technology can change the direction and shape of another light beam. The work, published in Nature Nanotechnology, was carried out in the laboratory of Harry Atwater and led by Claudio Hail, now an assistant professor at the University of California, Berkeley, with coauthor Lior Michaeli, now head of the Meta-Optomechanics Laboratory at Tel Aviv University.

The device is a metasurface — a thin optical layer patterned with silicon structures smaller than the wavelength of light — engineered to trap and concentrate light for a brief moment, intensifying its interaction with the material. A short control pulse then alters the silicon's optical properties through the optical Kerr effect, in which intense light momentarily changes a material's refractive index, the property that governs how light travels through it. The Kerr effect can occur in under a femtosecond but is normally too weak to be useful; the team's high-quality-factor design amplifies it enough for one pulse of light to control another.

In laboratory tests the researchers redirected a beam by up to 13 degrees to either side, choosing the direction through the spatial pattern of the control pulse, and used the same approach to reshape the outgoing beam into different patterns. "The key idea was to create a metasurface whose optical response is not fixed once it is fabricated," Hail said. Because the device has no moving parts and does not rely on long-lived electrical charges, it returns almost immediately to its original state after the control pulse ends.

Today's beam-steering hardware — rotating mirrors, rearranging liquid crystals, electronically tuned materials — is slow compared with the timescales of light itself, and communication networks repeatedly convert signals between light and electricity for processing. The measured switching time of the new surface was close to the duration of the laser pulse used in the experiment, and the authors note the metasurface itself may not yet have reached its fundamental speed limit.