Physicists Built the First Photonic Time Crystal That Works at Terahertz Speed

A team led by researchers at École Polytechnique has built the first photonic time crystal that operates at terahertz frequencies, a device whose optical properties flip back and forth so quickly that the material itself changes faster than the light passing through it can complete a few dozen oscillations.

Ordinary photonic crystals control light through structure in space — layers, lattices and holes arranged to reflect some colors and pass others. A photonic time crystal does the same job in time, by driving the material's reflectivity and resonance frequency up and down periodically on ultrafast timescales. The idea has existed on paper for years; the obstacle has been that the modulation has to be both very strong and very fast, and until now nobody had achieved both at once in the terahertz range.

The device is a plasmonic metamaterial: micrometer-scale gold structures with a crenellated, castle-battlement profile, sitting on an insulating layer above a mixture of indium and antimony semiconductor. The gaps between the gold and the semiconductor act as cavities that trap photons, while surface plasmons — collective waves of electrons rippling across the metal surface — provide the coupling that makes the whole assembly respond to a driving field.

The drive came from TELBE, a high-field terahertz source at the Helmholtz-Zentrum Dresden-Rossendorf. The researchers describe the resulting modulation as simultaneously "very strong (like forcing an object to emit an entirely different color) and very rapid (on the picosecond scale)." Terahertz frequencies are roughly a thousand times faster than the clock rates used in conventional electronics, which is the range where this kind of temporal control has to happen if it is going to matter for optical signal processing.

The measurable payoff was a reduction in photon dissipation of about 50 percent. Loss is the standing problem with plasmonic devices — gold is a good place to confine light and a bad place to keep it, because electrons in a metal shed energy as heat almost immediately. Halving that loss by driving the system in time, rather than by finding a better material, points at a different design strategy than the field has been pursuing.

The work was published in Nature on July 30 by lead author Tingwen Guo, a doctoral student at École Polytechnique, with collaborators including Yannis Laplace, Marco Schiró and Jan-Christoph Deinert, and involving the Collège de France, HZDR and Thales' Laboratoire Albert Fert. Potential applications the group points toward include amplifying weak light signals, building ultrafast optical switches, and engineering states of light that are difficult to produce with static materials.