As sighted creatures, humans have always used light for communication through body language, writing, smoke signals, and more. Harnessing invisible frequencies of light such as radio massively expanded our communication range, while hyper-focusing light into lasers tremendously increased communication fidelity. And sending lasers through fiber optic cables meant communication that was free from the tyranny of straight line-transmission.
Now, researchers at the California Institute of Technology (Caltech) have thwarted that tyranny even more profoundly, by devising an ultra-fast way to steer light, using light itself as the steering wheel. And not even Han Solo's reflexes during the Kessel Run could match Caltech’s speed: a mere 74 femtoseconds (quadrillionths of a second) to change the angle of a beam of light.
As lead author Claudio Hall writes in his team’s Nature Nanotechnology paper, the optical Kerr effect – the tendency of intense light to alter a medium’s refractive index for less than a femtosecond – is key to overcoming slow response times that have defied other attempts at light-steering. Previously a Caltech postdoctoral scholar, Hall is now an assistant professor of mechanical engineering at the University of California at Berkeley.
Until now, light-steering methods typically altered the electronic properties of optical chips or liquid crystal panels, exciting electrons and releasing their additional energy to shift how light passed through those media. Such as approach had its own speed limits – at the slowest, nanoseconds (billionths) and at the fastest, picoseconds (trillionths).
But to beat those limits, Hall, Harry Atwater (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), and their colleagues replaced the electron “steering wheel” with a “pump” made of light.
By calibrating an intense beam light to match the optical composition of a medium, the team could then shoot a second, less intense “probe” beam through the medium at a new deflection angle. That’s the Kerr effect at work: altering a medium’s refractive index entails altering the movement of electrons within their orbitals, which slows and bends light passing through that medium. "Steering light with light is very challenging,” says Atwater, “because light typically interacts very weakly with matter.”
To boost the Kerr results, Hall, Atwater, and their co-researchers needed to boost the refractive index even further, which they did by applying amorphous silicon onto a sheet of nanoscale pillars that were smaller than the pump’s wavelength, spacing them to slow and recirculate the light as it passed at a deflection angle up to 13 degrees.
“Using optical meta-surfaces,” says Atwater, meaning “ultrathin carefully nanoengineered sheets, we can up the interaction strength to make this possible with much higher efficiency.”
With improvements, the Caltech team claims it could drop the modulation speed below 74 femtoseconds, a realm associated with time crystals and other artificial optical materials that alter time. Such progress could mean a near future in which previous information-transmission records can easily be smashed, allowing image-processing well beyond a billion images per second, and vastly superior sensors mounted on moving air and sea vehicles that currently require gravity-bending to work.
Source: Caltech
