Light can carry enormous amounts of information at extreme speeds, making photonic technologies promising for the development of faster communications, more powerful computing systems, and more sensitive sensors. But for light to be useful for these purposes, engineers need to be able to control where it goes and redirect it quickly. A new device built by Caltech researchers uses a beam of light to steer another to a different angle in just 74 femtoseconds (74 quadrillionths of a second). That's about the time it takes light to travel 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 (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 team describes the work in a paper published on June 22 in the journal Nature Nanotechnology. The paper's lead author, Claudio Hail, completed the work as a postdoctoral scholar in Atwater's lab at Caltech and is now an assistant professor of mechanical engineering at UC Berkeley.
Most technologies that steer or modulate light, such as the liquid-crystal panels in projectors or the optical chips used in telecommunications, rely on altering a material's electronic properties to change how light passes through. In this process, electrons get excited to higher energy states and then relax back down, releasing the excess energy. That relaxation process limits how fast light can be redirected, typically restricting modulation speeds to nanoseconds or picoseconds (trillionths of a second).
Atwater's group decided not to rely on an electrical signal. Instead, they used one intense beam of light, called the pump, which had a carefully selected pattern to modify the optical properties of a target material. Then a second weaker beam, called the probe, could pass through the material and get deflected according to the pump's projected pattern.
The approach is enabled by a phenomenon called the optical Kerr effect, in which an intense beam can briefly and ever so slightly change the refractive index of a material-a measure of how much light slows down, and thus bends, as it travels through a medium. The beam does this by changing the motion of electrons within their orbitals, regions around an atom's nucleus where electrons have a high probability of being located. None of these electrons are excited into separate longer-lived states, so the effect appears and disappears almost as quickly as the light pulse itself. There is no waiting for the electrons to relax to lower energy states. The hitch is that the Kerr effect on its own is not strong enough to redirect a beam of light enough to be meaningful for practical applications.
To amplify the effect, the researchers patterned a thin film of amorphous silicon into a meta-surface-specifically into a sheet covered with nanoscale pillars, each smaller than the wavelength of the pump's light. The scientists sized and spaced the tiny pillars in such a way that the light would linger a bit and recirculate within the meta-surface rather than passing straight through uninterrupted. That extra time effectively magnifies the impact of the small refractive index change in silicon, creating a signal strong enough to redirect a beam of light.
The scientists used the meta-surface and approach to steer beams at angles of up to 13 degrees in as little as 74 femtoseconds and showed that the speed of the light modulation is limited by the pulse of the pump beam (which was also 74 femtoseconds).
The researchers note that the current modulation speed is still set by the duration of the laser pulses that drive the system rather than by the meta-material's intrinsic properties. With additional work, the speed could be improved and pushed toward a regime that would put it in the company of emerging photonic concepts such as time crystals and synthetic time-varying optical materials.
The paper is titled "Ultrafast, reconfigurable all-optical beam steering and spatial light modulation." Along with Hail and Atwater, Lior Michaeli is also an author of the paper. He completed the work as a postdoctoral scholar at Caltech and is now an assistant professor of electrical and computer engineering at Tel Aviv University. The work was supported by funding from the Air Force Office of Scientific Research and its Meta-Imaging Multidisciplinary University Research Initiative, the Swiss National Science Foundation, the Fulbright Fellowship program, and the Breakthrough Foundation. The Kavli Nanoscience Institute at Caltech provided infrastructure and support for the work.
