A light-controlled meta-surface redirected another beam at femtosecond speeds without relying on slower electronic relaxation.
A pulse of light crosses the width of a human hair in roughly 74 femtoseconds (74 quadrillionths of a second). Caltech researchers have now redirected a beam in that same fleeting interval by using a second beam as the control signal.
The achievement points toward photonic technologies that move and process information with light rather than relying entirely on electrical currents. Such systems could support faster communications, more powerful computing, and more sensitive sensors. Reaching that potential, however, requires components capable of steering light with comparable speed and precision.
“Steering light with light is very challenging because light typically interacts very weakly with matter. Using optical meta-surfaces (ultrathin, carefully nanoengineered sheets), we can up the interaction strength to make this possible with much higher efficiency,” says Harry Atwater, the Howard Hughes Professor of Applied Physics and Materials Science and the Otis Booth Leadership Chair of the Division of Engineering and Applied Science at Caltech.
The work was reported in Nature Nanotechnology. Lead author Claudio Hail conducted the research while working as a postdoctoral scholar in Atwater’s Caltech laboratory. He is now an assistant professor of mechanical engineering at UC Berkeley.
Electronic switching creates a speed limit
Existing devices often steer or modulate light by changing the electronic behavior of a material. Liquid crystal projector panels and telecommunications chips, for example, alter how light travels by pushing electrons into higher energy states.
Those electrons must eventually return to lower energy states and release the energy they absorbed. This recovery period creates a built-in delay, typically limiting modulation to nanoseconds or picoseconds (trillionths of a second).
Atwater’s group looked for a way around that bottleneck. Instead of controlling the material with electricity, the researchers used an intense patterned beam called the pump. Its purpose was to briefly alter the optical behavior of the material.
A second, weaker beam called the probe then traveled through the same material. The pattern carried by the pump determined the direction in which the probe was deflected, allowing light itself to perform the steering.
A fleeting effect gains practical strength
The method relies on the optical Kerr effect. During this phenomenon, an intense beam produces a tiny and temporary change in a material’s refractive index, which describes how much light slows and bends while moving through that substance.
The pump creates this change by slightly shifting the motion of electrons within their orbitals, the regions surrounding an atomic nucleus where electrons are most likely to be found. The electrons do not move into separate, longer-lived energy states. As a result, the change begins and ends almost as quickly as the light pulse, removing the need to wait for electronic relaxation.
That speed solves one problem but introduces another. By itself, the optical Kerr effect is too weak to bend a beam far enough for many practical uses.
To strengthen it, the researchers fashioned a thin layer of amorphous silicon into a meta-surface covered with nanoscale pillars. Each pillar was smaller than the wavelength of the pump light.
The dimensions and spacing of the pillars were chosen so that light would briefly circulate within the structure instead of passing through immediately. Keeping the light inside for slightly longer amplified the small change in silicon’s refractive index until it became strong enough to redirect the probe beam.
Using this design, the researchers steered light by as much as 13 degrees in only 74 femtoseconds. They also found that the modulation speed matched the duration of the pump pulse, which lasted 74 femtoseconds.
Laser pulses now set the ceiling
The device itself does not yet appear to impose the main speed limit. Instead, performance is currently constrained by the duration of the laser pulses used to operate it.
Shorter pulses could therefore push the switching time even lower. Further development may move the system into the speed range needed to explore emerging photonic ideas, including time crystals and synthetic time-varying optical materials.
Reference: “Ultrafast, reconfigurable all-optical beam steering and spatial light modulation” by Claudio U. Hail, Lior Michaeli and Harry A. Atwater, 22 June 2026, Nature Nanotechnology.
DOI: 10.1038/s41565-026-02199-w
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