Light has long been recognized as a medium capable of transmitting staggering amounts of information at extraordinary speeds. Because of this inherent property, researchers across the globe are actively exploring photonic technologies to enable faster communications networks, significantly more powerful computing architectures, and highly sensitive sensing systems. However, to make these advanced systems fully functional and commercially viable, engineers and physicists need precise, reliable ways to control the direction of light and alter its properties on demand and at extreme speeds.
Addressing this critical bottleneck, a team of researchers at the California Institute of Technology (Caltech) has now developed an innovative device that uses one beam of light to redirect another in just 74 femtoseconds—equivalent to 74 quadrillionths of a second. To put this unimaginably brief duration into perspective, it is roughly the amount of time it takes for a beam of light to travel straight across the width of a human hair.
"Steering light with light is very challenging because light typically interacts very weakly with matter. Using optical meta-surfaces, which are ultrathin, carefully nanoengineered sheets, we can up the interaction strength to make this possible with much higher efficiency," explains 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 researchers detailed their groundbreaking findings in a paper recently published in the scientific journal Nature Nanotechnology. The study’s lead author, Claudio Hail, carried out the core research while serving as a postdoctoral scholar in Atwater’s laboratory at Caltech. Hail has since transitioned to a new role as an assistant professor of mechanical engineering at the University of California, Berkeley.
Why Conventional Light Steering Has a Speed Limit
Many of the existing technologies currently utilized for steering or modulating light depend heavily on changing the electronic properties of a material. Familiar examples of this approach include the liquid-crystal display panels commonly found in modern video projectors and the intricate optical chips integrated into contemporary telecommunications infrastructure.
In these conventional devices, electrons must be pushed into higher energy states before they can eventually return to lower energy states, a transition that results in the release of their excess energy. This relaxation process inherently takes time, creating a physical bottleneck that typically limits light modulation speeds to nanosecond or picosecond timescales—which represent trillionths of a second.
Atwater’s research team deliberately took a fundamentally different approach by eliminating the absolute need for an electrical signal to drive the modulation. Instead, the researchers deployed a powerful primary beam of light, referred to as the pump beam, which featured a carefully designed spatial pattern intended to temporarily alter the optical behavior of a targeted material.
Following this initial step, a second, noticeably weaker beam—known as the probe beam—was passed directly through that same modified material. As a result, the direction of the probe beam was changed in accordance with the specific pattern created by the pump beam, achieving optical control without relying on slower electronic switches.
Using the Optical Kerr Effect
The underlying mechanics of this novel system rely heavily on a well-documented physical phenomenon known as the optical Kerr effect. When an intensely concentrated beam of light passes through a specific material, it can briefly produce a very small, instantaneous change in the material’s refractive index. The refractive index is a crucial metric that describes precisely how much light slows down and bends as it travels through a given medium.
Crucially, this particular optical effect originates from subtle changes in the motion of electrons occurring strictly within their atomic orbitals—the specific regions around an atom’s nucleus where electrons maintain a high probability of being located. Most importantly, the electrons are not pushed into separate, longer-lasting excited states that would require time to decay.
Because the electrons do not undergo these prolonged state transitions, the resulting optical change can appear and vanish almost as quickly as the light pulse itself. There is no waiting period required for excited electrons to fall back down to lower energy levels, bypassing the traditional speed limit entirely.
On its own, however, the optical Kerr effect is naturally far too weak to redirect a beam of light by an angle or magnitude that would be practically useful in real-world electronic and photonic devices.
Nanoscale Silicon Pillars Amplify the Effect
To overcome this inherent physical limitation and significantly strengthen the optical response, the Caltech researchers engineered a specialized meta-surface crafted from a microscopic thin film of amorphous silicon. This highly engineered surface was densely covered with nanoscale pillars, with each individual pillar measuring smaller than the wavelength of the pump light illuminating it.
By exercising precise control over the exact size, geometry, and spatial spacing of these nanoscale pillars, the research team forced the light to remain inside the meta-surface for a slightly longer duration. Instead of simply passing straight through the material unhindered, the light circulated within the nano-structures.
This additional interaction time effectively amplified the otherwise minuscule refractive index change occurring within the silicon. Consequently, the resulting optical effect became robust enough to reliably redirect the secondary probe beam.
Using this sophisticated design, the researchers successfully steered light by wide angles of up to 13 degrees in as little as 74 femtoseconds. Furthermore, they demonstrated that the overall modulation speed of the system was primarily limited only by the actual duration of the pump laser pulse, which was also measured at 74 femtoseconds.
Even Faster Light Control May Be Possible
Looking ahead, the researchers note that the current operational speed limit of the device is dictated entirely by the external laser pulses used to drive the system, rather than by any fundamental physical constraints inherent to the meta-material itself.
This realization leaves open exciting possibilities for making the light-steering process even faster in future iterations. With continued research and technological development, the approach could eventually reach timescales relevant to emerging, cutting-edge photonic concepts, including time crystals and synthetic time-varying optical materials.
The published study is titled "Ultrafast, reconfigurable all-optical beam steering and spatial light modulation." In addition to lead author Claudio Hail and senior researcher Harry Atwater, Lior Michaeli also served as a co-author on the paper. Michaeli completed the collaborative research as a postdoctoral scholar at Caltech and is presently an assistant professor of electrical and computer engineering at Tel Aviv University.
The foundational work was made possible through financial support provided by the Air Force Office of Scientific Research alongside its Meta-Imaging Multidisciplinary University Research Initiative. Additional backing came from the Swiss National Science Foundation, the Fulbright Fellowship program, and the Breakthrough Foundation. Essential infrastructure and institutional support for the project were provided by the Kavli Nanoscience Institute at Caltech.
