Researchers from École Polytechnique, the Collège de France and the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) have experimentally produced the first all-optical photonic time crystal — a material whose optical behavior changes rapidly and repeatedly over time. The achievement, reported in Nature, was enabled by HZDR's TELBE superradiant terahertz source.
Conventional photonic crystals are nanostructured materials with repeating optical patterns in space, which let scientists block, guide or amplify selected wavelengths of light — much as semiconductors control electrons. The new device extends that idea from space to time: its reflectivity and resonance frequency can be altered dynamically on picosecond timescales, close to the oscillation period of light itself.
To achieve this, the team built a plasmonic metamaterial: micrometer-scale gold crenelated structures above an insulating layer and an indium–antimony semiconductor. The gold forms tiny cavities that confine light, while surface plasmons — collective electron waves in the semiconductor — capture and sustain the trapped photons. Exposing the device to intense terahertz pulses from TELBE changed its optical properties both strongly and rapidly, a combination that had previously been a major technical obstacle.
A theoretical model developed at the Collège de France reproduced the experiments and showed that modulating the material over time halved the photon dissipation — the fraction of photons lost through the metamaterial's surface.
The terahertz band sits at the frontier between electronics and photonics, roughly 1,000 times faster than the frequencies used in electronic components, yet remains technologically underdeveloped. The team says the approach could lead to ultrafast optical computers, advanced telecommunications, new terahertz lasers and medical imaging tools, as well as light whose "color" and intensity can be tuned almost instantly.
The researchers now aim to cut dissipation further and trap more photons, with the eventual goal of producing sufficiently strong amplification to serve as the basis for a new generation of highly adaptable lasers.
