Physicists Demonstrate the First All-Optical Photonic Time Crystal, Cutting Plasmonic Losses by More Than Half
An international team built a terahertz metamaterial whose optical properties flip on picosecond timescales, the first all-optical photonic time crystal, published in Nature.
Overview
An international team of researchers has built the first all-optical photonic time crystal, a metamaterial whose optical properties can be switched strongly and repeatedly on picosecond timescales, according to a joint press release from École Polytechnique and the Helmholtz-Zentrum Dresden-Rossendorf (HZDR). The work, led by Yannis Laplace’s team at École Polytechnique’s Irradiated Solids Laboratory, was published in the journal Nature under the title “Plasmonic metamaterial time crystal.”
What We Know
Photonic crystals, developed in the 1980s, are nanostructured materials built by alternating layers with different refractive indexes, giving them a fixed, spatially repeating pattern that determines how light moves through them, according to École Polytechnique. Researchers have long asked whether the same kind of periodic control could be applied not just across space, but across time, creating a “photonic time crystal” whose optical behavior changes on a repeating cycle. Until now, that idea had remained largely theoretical or limited to electrical-circuit-based devices, École Polytechnique reports.
The new device operates in the terahertz range, where light oscillates at frequencies on the order of 1,000 billion times per second, according to École Polytechnique. It consists of micrometer-scale gold crenellated structures sitting above an insulating layer and a semiconductor made from a mixture of indium and antimony; the gold structures form cavities that trap photons between the gold and semiconductor layers. By directing terahertz laser pulses at the device, the researchers showed that its optical properties, particularly its ability to reflect light, could be modulated very strongly over time, on the picosecond scale, one billionth of a billionth of a second, per École Polytechnique.
The mechanism relies on surface plasmons: when the semiconductor surface is hit by the laser pulses, its electrons form a collective wave that traps and sustains the light’s oscillations, École Polytechnique explains. In the published paper, the authors describe achieving “strong (near-unity) and coherent (sub-optical cycle) periodic driving of the plasmonic metamaterial,” with the carriers’ effective mass reaching “up to 80% of their rest mass,” according to the Nature paper. The paper further reports that spectroscopic measurements showed a transition into the photonic-time-crystal regime “mediated by an exceptional point, at which two Floquet-driven optical eigenmodes coalesce,” and that in this regime, “emergent gain is shown to reduce plasmonic losses by more than 50%,” per the Nature paper.
The experiment relied on HZDR’s TELBE superradiant terahertz source, part of the HZDR’s ELBE accelerator, to generate the intense, tunable terahertz pulses needed to drive the material into this regime, according to the joint press release. “TELBE’s unique ability to generate high-field, phase-stable terahertz pulses was critical,” said Jan-Christoph Deinert, the coordinator of the TELBE facility, according to HZDR.
Tingwen Guo, the PhD student and lead author on the publication, described the significance of the result: “By extending photonic crystals from space to time, we open a new dimension for light control,” Guo said, according to HZDR. Yannis Laplace framed the broader opportunity in the terahertz range: “The THz range represents the frontier between electronic and photonic technologies,” Laplace said, per HZDR. A theoretical model developed by Marco Schiró of Collège de France and his team supported the experimental findings; “the theory not only reproduces the experiment but also provides the basis for guiding future discoveries,” Schiró said, according to HZDR.
The research is a collaboration among École Polytechnique, Collège de France, HZDR, Thales’ Albert Fert Laboratory, and the Laboratory of Physics of Interfaces and Thin Films (LPICM), according to École Polytechnique.
What We Don’t Know
The researchers say they have already begun to observe signs of photon amplification within the structure and hope to observe a lasing effect soon, but École Polytechnique is clear that a working laser has not yet been demonstrated. The Nature paper similarly states only that the team “predict[s] plasmonic lasing to be within experimental reach,” not that it has been achieved. No timeline for a follow-up demonstration, a commercial device, or integration into telecommunications or computing hardware has been disclosed.
Why It Matters
Terahertz frequencies sit in a largely underused part of the electromagnetic spectrum, between conventional electronics and visible-light photonics, and are of growing research interest because they could offer new ways to probe and manipulate matter, ScienceDaily notes. Laplace’s team suggests that photonic time crystals could eventually be used to build new terahertz light sources or detectors, according to École Polytechnique, potentially feeding into ultrafast optical computing, telecommunications, and imaging applications, per ScienceDaily.