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Photonic 'time crystals' could improve lasers and wireless communication

Scientists have created photonic ‘time crystals’ capable of amplifying microwaves that could lead to more efficient and robust wireless communications, integrated circuits, and lasers.

While conventional crystals have a structural pattern that repeats in space, a ‘time crystal’ – first conceived in 2012 by Nobel laureate Frank Wilczek – has a pattern that repeats in time. 

Physicists were initially sceptical that such crystals could exist, however experiments have since succeeded in creating them. Last year, for example, researchers at Aalto University’s Low Temperature Laboratory created paired time crystals that could be useful for quantum devices.

However, so far, research on photonic time crystals has focused on bulk materials – three-dimensional structures. This has proven exceptionally challenging, and the experiments haven’t gotten past model systems with no practical applications. 

Now, a team including researchers from Aalto University, the Karlsruhe Institute of Technology (KIT), and Stanford University, have tried a new approach: building a two-dimensional photonic time crystal, known as a metasurface. Their work has been described in Science Advances.

“We found that reducing the dimensionality from a 3D to a 2D structure made the implementation significantly easier, which made it possible to realise photonic time crystals in reality,” said Xuchen Wang from KIT, the study’s lead author.

The new approach enabled the team to fabricate a photonic time crystal and experimentally verify the theoretical predictions about its behaviour. “We demonstrated for the first time that photonic time crystals can amplify incident light with high gain,” said Wang. “In a photonic time crystal, the photons are arranged in a pattern that repeats over time. This means that the photons in the crystal are synchronised and coherent, which can lead to constructive interference and amplification of the light.” This periodic arrangement of the photons means they can also interact in ways that boost the amplification.

Two-dimensional photonic time crystals have a range of potential applications, according to the researchers. By amplifying electromagnetic waves, they could make wireless transmitters and receivers more powerful or more efficient. Wang points out that coating surfaces with 2D photonic time crystals could also help with signal decay, which is a significant problem in wireless transmission. Photonic time crystals could also simplify laser designs by removing the need for bulk mirrors that are typically used in laser cavities.

Another application emerges from the finding that 2D photonic time crystals don’t just amplify electromagnetic waves that hit them in free space, but also waves travelling along the surface. Such waves are used for communication between electronic components in integrated circuits. 

“When a surface wave propagates, it suffers from material losses, and the signal strength is reduced,” said Wang. “With 2D photonic time crystals integrated into the system, the surface wave can be amplified, and communication efficiency enhanced.”


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