An international team of researchers from École polytechnique, Collège de France, and the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) has achieved a world first: the experimental realization of an all-optical photonic time crystal (PTC), published today in Nature.
Unlike a conventional crystal, which has a periodic structure in space (a repeating lattice of atoms), a photonic time crystal has a periodic structure in time — its optical properties are modulated at ultrafast (picosecond) scales, comparable to the oscillation cycle of light itself.
"By extending photonic crystals from space to time, we open a new dimension for light control — and a novel path toward amplification and lasing," said Tingwen Guo, a PhD student at École polytechnique and lead author of the Nature paper.
The team built a "plasmonic metamaterial" — micrometer-scale gold crenellated structures atop a semiconductor layer of indium and antimony. These structures act as cavities that trap photons. By firing intense terahertz laser pulses from HZDR's TELBE superradiant source at the device, the researchers demonstrated that the material's reflectivity and resonance frequency could be modulated very strongly over picosecond timescales.
"TELBE's unique ability to generate high-field, phase-stable terahertz pulses was critical," confirmed Dr. Jan-Christoph Deinert, coordinator of the TELBE facility. "Without this infrastructure, achieving the coherent, ultrafast modulation needed for the PTC regime would have been impossible."
A theoretical model developed by Dr. Marco Schiró and his team at Collège de France confirmed the experimental results. The temporal modulation was shown to reduce dissipation of photons within the metamaterial by half.
The breakthrough could transform terahertz technology — a frequency range that sits between electronics and photonics and remains underdeveloped despite its enormous potential. Applications include:
- Ultrafast optical computing beyond the limits of electronic processors - New types of terahertz lasers with unparalleled modularity - On-demand tuning of light's color and intensity for medical imaging and communications - Smarter, more adaptable optical systems
"The THz range represents the frontier between electronic and photonic technologies," explained Prof. Yannis Laplace of École polytechnique. "It is a range full of opportunities for science and society, yet is still under-developed technologically."
The next challenge: further reducing photon dissipation and achieving amplification within the time crystal, which could lead to entirely new laser sources.
