A major barrier to creating photonic time crystals in the lab has been overcome by a team of researchers in Finland, Germany and the US. Sergei Tretyakov at Aalto University and colleagues have shown how the time varying properties of these exotic materials can be realized far more easily in 2D than in 3D.
First proposed by Nobel laureate Frank Wilczek in 2012, time crystals are a unique and diverse family of artificial materials. You can read more about them and their broader implications for physics in this Physics World article by Philip Ball – but in a nutshell, they possess properties that vary periodically in time. This is unlike conventional crystals, which have properties that vary periodically in space.
In photonic time crystals (PhTCs), the varying properties are related to how the materials interacts with incident electromagnetic waves. “The unique characteristic of these materials is their ability to amplify incoming waves due to the non-conservation of wave energy within the photonic time crystals,” Tretyakov explains.
Momentum bandgaps
This property is a result of “momentum bandgaps” in PhTCs, in which photons within specific ranges of momenta are forbidden from propagating. Owing to their unique properties of PhTCs, the amplitudes of electromagnetic waves within these bandgaps grow exponentially over time. In contrast, the analogous frequency bandgaps which form in regular, spatial photonic crystals PhTCs, cause waves to attenuate over time.
PhTCs are now a popular subject of theoretical study. So far, calculations suggest that these time crystals possess a unique set of properties. These include exotic topological structures, and an ability to amplify radiation from free electrons and atoms.
In real experiments, however, it has proven very difficult to modulate the photonic properties of 3D PhTCs throughout their volume. Among the challenges include the creation of overly complex pumping networks – which themselves create parasitic interferences with electromagnetic waves propagating through the material.
Reduced dimensionality
In their study, Tretyakov’s team discovered a simple fix to this problem. “We have reduced the dimensionality of photonic time crystals from 3D to 2D, because it is much easier to construct 2D structures compared to 3D structures,” he explains.
Key to the success of the team’s approach lies within the unique physics of metasurfaces, which are materials made from 2D arrays of sub-wavelength sized structures. These structures can be tailored in size, shape, and arrangement in order to manipulate properties of incoming electromagnetic waves in highly-specific and useful ways.
After fabricating their new microwave metasurface design, the team showed that its momentum bandgap amplified microwaves exponentially.
These experiments clearly demonstrated that time-varying metasurfaces can preserve the key physical properties of 3D PhTCs, with one key additional benefit. “Our 2D version of photonic time crystals can provide amplification for both free-space waves and surface waves, while their 3D counterparts cannot amplify surface waves,” Tretyakov explains.
Technological applications
With their host of advantages over 3D time crystals, the researchers envisage a wide ray of potential technological applications for their design.
“In the future, our 2D photonic time crystals could be integrated into reconfigurable intelligent surfaces at microwave and millimetre wave frequencies, such as those in the upcoming 6G band,” Tretyakov says. “This could enhance wireless communication efficiency.”
While their metamaterial is designed specifically for manipulating microwaves, the researchers hope that further adjustments to their metasurface could extend its use to visible light. This would pave the way for the development of new advanced optical materials.
Looking further into the future, Tretyakov and colleagues suggest that 2D PhTCs could provide a convenient platform for creating the even more esoteric “space–time crystals”. These are hypothetical materials that would exhibit repeating patterns in time and space simultaneously.
The research is described in Science Advances.
