Researchers at Caltech have developed an ultrafast optical device capable of using one beam of light to redirect another in just 74 femtoseconds.
The technology uses a nanoengineered silicon metasurface to amplify interactions between light and matter, potentially offering a new approach to controlling light in future communications, computing and sensing systems.
At 74 quadrillionths of a second, the switching time is roughly equivalent to the time it takes light to travel across the width of a human hair.
‘Steering light with light is very challenging because light typically interacts very weakly with matter,’ said Harry Atwater, Professor of Applied Physics and Materials Science at Caltech.
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‘Using optical meta-surfaces – ultrathin, carefully nanoengineered sheets – we can up the interaction strength to make this possible with much higher efficiency.’
Conventional technologies for steering and modulating light typically rely on changing the electronic properties of a material. Electrons are moved into higher-energy states before subsequently returning to lower-energy states.
The time required for this process creates a bottleneck, typically restricting modulation to nanosecond or picosecond timescales.
Caltech’s researchers instead eliminated the need for an electrical signal.
Their system uses an intense patterned beam of light, known as the pump, to temporarily change the optical properties of the material. A second, weaker ‘probe’ beam passing through the material is then deflected according to the pattern created by the pump.
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Central to the technology is the optical Kerr effect, where intense light produces a small and temporary change in a material’s refractive index.
Rather than pushing electrons into longer-lasting excited states, the effect alters their motion within their orbitals. The change can therefore appear and disappear almost as quickly as the light pulse producing it.
However, the Kerr effect is normally too weak to provide useful levels of beam steering.
To amplify it, the researchers fabricated a metasurface from a thin layer of amorphous silicon patterned with nanoscale pillars smaller than the wavelength of the incoming light.
The size and spacing of the pillars were engineered so that light circulates within the structure for longer instead of simply passing through it. This increases the interaction between the light and silicon, amplifying the change in refractive index sufficiently to redirect the probe beam.
Using the device, the team demonstrated steering angles of up to 13 degrees in 74 femtoseconds.
Significantly, the researchers say the switching speed was limited by the duration of the laser pulse used in the experiment rather than the metasurface itself, suggesting still faster operation could be possible.
The work, published in Nature Nanotechnology, could ultimately contribute to ultrafast optical systems in which light is manipulated directly by other light, reducing reliance on slower electronic control.
The research is published in Nature Nanotechnology.