Space-time crystal: Important piece of the puzzle on the way to new optical materials

(openPR) Photonic time crystals consist of materials that are the same everywhere in space, but whose properties change periodically over time. This periodic change over time allows the spectral composition of light to be specifically changed and amplified, both of which are crucial factors for optical information processing. “This opens up new degrees of freedom, but also presents many challenges,” says Professor Carsten Rockstuhl from the Institute for Theoretical Solid State Physics and the Institute for Nanotechnology at KIT. “The present study paves the way for using these materials for information processing systems in which all light frequencies are to be used and amplified.”

A little closer to four-dimensional photonic crystals

The central parameter of a photonic time crystal is its band gap in momentum space. To explain: The momentum is a measure of the direction in which the light propagates. A band gap describes the directions in which light has to propagate in order for it to be amplified: the wider the band gap, the greater the gain. “So far, we have had to intensify the periodic changes in material properties, such as the refractive index, in photonic time crystals for a large band gap.” “Only then is light actually amplified,” explains Puneet Garg, one of the two first authors of the study. “Since the options for this are limited for most materials, this is a major challenge.”

As a solution, the research team combined the photonic time crystals with an additional spatial structure and thus constructed “photonic space-time crystals”: ​​They built photonic time crystals made of silicon spheres that “capture” the light and last a little longer than previously possible. This means that the light reacts much better to the periodic changes in material properties over time. “We are talking here about resonances that strengthen the interaction between light and matter,” says Xuchen Wang, also the lead author. “In such optimally tuned systems, the band gap quickly extends across the entire momentum space, which means that the light is amplified regardless of its direction of propagation.” This could be the missing piece of the puzzle on the way to the practical use of new optical materials.”

“We are very pleased about this breakthrough in photonic materials and are excited about the long-term impact of our research,” says Rockstuhl. “In this way, the enormous potential of modern optical materials research can be exploited.” The idea is not limited to optics and photonics, but can be applied to many systems in physics and potentially stimulate new research in various areas.”

The research project was carried out in the special research area “Waves: Analysis and Numerics”, funded by the German Research Foundation (DFG) and is embedded in the Helmholtz research area of ​​information.

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Original publication:
X. Wang, P. Garg, MS Mirmoosa, AG Lamprianidis, C. Rockstuhl and VS Asadchy: Expansion of the impulse band gaps in photonic time crystals through resonances. Nature Photonics, 2024. DOI: 10.1038/s41566-024-01563-3
https://www.nature.com/articles/s41566-024-01563-3