Caltech researchers created a chip that uses a patterned beam of light to modify the optical properties of a meta-material. A second beam can then pass through the material and get deflected according to the first beam's projected pattern.
Image Credit: Claudio Hail
Scientific Frontline: Extended "At a Glance" Summary: Ultrafast All-Optical Beam Steering
The Core Concept: Researchers have developed a novel photonic device utilizing an optical meta-surface that redirects a beam of light using a second light beam in merely 74 femtoseconds (74 quadrillionths of a second).
Key Distinction/Mechanism: Traditional optical chips modulate light by altering a material's electronic properties, a process fundamentally bottlenecked by the time required for electrons to relax to lower energy states. This new approach bypasses electronic relaxation by leveraging the optical Kerr effect, employing a patterned "pump" beam to momentarily alter the refractive index of a meta-surface, which instantly deflects a weaker "probe" beam.
Major Frameworks/Components:
- Optical Meta-surfaces: Ultrathin sheets of amorphous silicon patterned with nanoscale pillars smaller than the wavelength of the light, specifically designed to trap and recirculate photons to amplify interaction strength.
- Optical Kerr Effect: A phenomenon in which an intense beam of light alters the motion of electrons within their orbitals, briefly changing the material's refractive index without exciting the electrons into longer-lived energy states.
- Pump-Probe System: An intense, patterned light beam (the pump) modulates the optical properties of the material, while a secondary beam (the probe) passes through and is steered by the resulting modifications.
Branch of Science: Applied Physics, Nanotechnology, Materials Science, and Photonics.
Future Application: This technology paves the way for vastly faster telecommunications, ultra-high-speed optical computing systems, highly sensitive sensors, and emerging photonic concepts like time crystals and synthetic time-varying optical materials.
Why It Matters: By overcoming the physical speed limitations associated with electron relaxation, this all-optical modulation achieves redirection speeds measured in fractions of a trillionth of a second, laying the essential groundwork for the next generation of instantaneous data transfer and processing.
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. However, for light to be useful for these purposes, engineers must 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 is approximately 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 metasurfaces (ultrathin, carefully nanoengineered sheets), we can increase 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 the University of California, 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 it. In this process, electrons are excited to higher energy states and then relax, releasing the excess energy. That relaxation process limits how fast light can be redirected, typically restricting modulation speeds to nanoseconds or picoseconds (billionths or trillionths of a second).
Atwater's group decided not to rely on an electrical signal. Instead, they used an intense beam of light, called the pump, which featured a carefully selected pattern to modify the optical properties of a target material. Then, a second, weaker beam, called the probe, passed through the material and was 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 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, which are 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. Consequently, the system does not need to wait for the electrons to relax to lower energy states. The challenge is that the Kerr effect alone is insufficiently strong to meaningfully redirect a beam of light for practical applications.
To amplify the effect, the researchers patterned a thin film of amorphous silicon into a metasurface—specifically, a sheet covered with nanoscale pillars, each smaller than the wavelength of the pump's light. The scientists sized and spaced the tiny pillars so that the light lingered briefly and recirculated within the metasurface rather than passing straight through uninterrupted. This 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 metasurface and this approach to steer beams at angles of up to 13 degrees in as little as 74 femtoseconds, demonstrating 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 driving the system rather than by the metamaterial's intrinsic properties. With additional work, the speed could be improved and pushed toward a regime that would place it in the company of emerging photonic concepts such as time crystals and synthetic, time-varying optical materials.
Funding: 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 research.
Published in journal: Nature Nanotechnology
Title: Ultrafast, reconfigurable all-optical beam steering and spatial light modulation
Authors: Claudio U. Hail, Lior Michaeli, and Harry A. Atwater
Source/Credit: California Institute of Technology
Edited by: Scientific Frontline
Reference Number: phy070726_02