Researchers have demonstrated that photonic time crystals, materials whose permittivity is periodically modulated in time creating momentum bandgaps for light, can operate at optical frequencies by exploiting quasi-bound states in the continuum within nanophotonic metasurfaces, according to a paper published August 12 in Science Advances.
Photonic time crystals have been theorized for their ability to amplify incident radiation and enable exotic phenomena like lasing without population inversion. But observing momentum bandgaps at optical frequencies has been considered impractical: the required temporal modulation amplitudes—near-unity relative permittivity changes at twice the frequency of the probing light—are far beyond what conventional materials can sustain without damage. The paper shows that metasurfaces composed of either multilayered aluminum-doped zinc oxide spheres or germanium cylinders supporting qBICs reduce the threshold modulation amplitude by orders of magnitude compared to homogeneous PTCs.
In the cylinder geometry, the team achieved a full momentum bandgap spanning all in-plane wave vectors at modulation amplitudes reachable through the optical Kerr effect in germanium without laser-induced damage. The work adds to a growing body of research where AI is revealing new physics in complex material systems, and sits alongside efforts to make AI models follow fundamental physical laws rather than brute-force data fitting.