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Researchers create new type of photonic time crystal that can amplify light

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A team of researchers from Aalto University, the Karlsruhe Institute of Technology (KIT), and Stanford University have created photonic time crystals that operate at microwave frequencies. The team has demonstrated that these crystals can amplify electromagnetic waves – such as light that shines upon them. This offers potential applications in technologies including lasers, integrated circuits, and wireless communications.

Reducing the dimensionality of photonic time crystals (PhTCs) from a 3D to a 2D structure enabled the team to actually create a PhTC and experimentally verify theoretical predictions about its behaviour. “We demonstrated that photonic time crystals can amplify incident light with high gain,” explains Xuchen Wang, the study’s lead author, a former doctoral student at Aalto currently working at KIT. “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 synchronized and coherent, which can lead to constructive interference and amplification of the light.” The demonstrated metasurface PhTCs could simplify laser designs by removing the need for bulk mirrors that are typically used in laser cavities.

Xuchen Wang adds that PhTCs have the potential to impact the transmission and reception of fiber-optic signals in several ways. A few considerations:

“Signal Amplification: PhTCs can amplify electromagnetic waves, including light. This property can be beneficial for boosting weak signals in fiber-optic communication systems. By integrating PhTCs into the network, it may be possible to enhance the signal strength, thereby extending the transmission distance or compensating for signal loss.”

“Signal Quality and Stability: The synchronization and coherence of photons within a PhTC can lead to constructive interference and improved signal quality. In fiber-optic networks, this can help maintain signal integrity over long distances and reduce signal degradation due to noise and other impairments.”

“Signal Processing: PhTCs could be employed for signal processing functions within fiber-optic networks. For instance, by manipulating the properties of the PhTC, such as the modulation frequency or the periodicity of the crystal, it might be possible to implement frequency filtering, wavelength conversion, or other signal processing operations directly within the fiber-optic system.”

“Overall, integrating PhTCs into fiber-optic networks has the potential to enhance signal amplification, improve signal quality and stability, and enable on-chip signal processing. However, it is important to note that PhTCs are still an emerging field of research, and practical implementations in fiber-optic networks are yet to be fully explored and developed. Further studies and technological advancements will be necessary to fully understand and harness the potential of PhTCs in fiber-optic communication systems.”

Another application emerges from the finding that 2D photonic time crystals don’t only amplify electromagnetic waves that hit them from surrounding space but may also amplify waves travelling along surfaces. By serving as an amplifier for surface-wave signals, which are known to suffer from severe losses, PhTCs could make an important contribution to future communications. Surface 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,” says Professor Viktar Asadchy, who generated the idea of 2D photonic time crystals. “With 2D photonic time crystals integrated into the system, the surface wave can be amplified, and communication efficiency enhanced.’

By amplifying electromagnetic waves, two-dimensional photonic time crystals could also make wireless transmitters and receivers more powerful or more efficient. Coating surfaces with 2D photonic time crystals could also help with signal decay - a significant problem in wireless transmission. The proposed concept is not limited to microwave frequencies - it can potentially be scaled up to subterahertz frequencies using tunable 2D materials and optical frequencies, using nonlinear effects.

Photonic time crystals (PhTCs), first conceived by Nobel laureate Frank Wilczek in 2012, are artificial materials whose electromagnetic properties (such as permittivity or permeability) are periodically and rapidly modulated in time while remaining uniform in space. The optical properties of PhTCs show strong periodic variations at single-cycle timescales. Conventional natural crystals have a structural pattern that repeats in space, whereas in a time crystal, the pattern repeats in time instead. Recent experiments have succeeded in creating PhTCs. In 2022 researchers at Aalto University created paired time crystals that could be useful for quantum devices.