Department of Electronics and Nanoengineering, Aalto University, Espoo, Finland.
Institute of Nanotechnology, Karlsruhe Institute of Technology, Karlsruhe, Germany.
Sci Adv. 2023 Apr 5;9(14):eadg7541. doi: 10.1126/sciadv.adg7541.
Photonic time crystals are artificial materials whose electromagnetic properties are uniform in space but periodically vary in time. The synthesis of these materials and experimental observation of their physics remain very challenging because of the stringent requirement for uniform modulation of material properties in volumetric samples. In this work, we extend the concept of photonic time crystals to two-dimensional artificial structures-metasurfaces. We demonstrate that time-varying metasurfaces not only preserve key physical properties of volumetric photonic time crystals despite their simpler topology but also host common momentum bandgaps shared by both surface and free-space electromagnetic waves. On the basis of a microwave metasurface design, we experimentally confirmed the exponential wave amplification inside a momentum bandgap and the possibility to probe bandgap physics by external (free-space) excitations. The proposed metasurface serves as a straightforward material platform for realizing emerging photonic space-time crystals and as a realistic system for the amplification of surface-wave signals in future wireless communications.
光子时间晶体是一种人工材料，其电磁性质在空间上是均匀的，但在时间上是周期性变化的。由于对体积样品中材料性质的均匀调制的严格要求，这些材料的合成和物理实验观察仍然具有很大的挑战性。在这项工作中，我们将光子时间晶体的概念扩展到二维人工结构——超表面。我们证明，尽管时间变化的超表面拓扑结构更简单，但它们保留了体光子时间晶体的关键物理性质，并且还具有表面波和自由空间电磁波共有的常见动量带隙。基于微波超表面设计，我们通过实验证实了在动量带隙内指数波放大的现象，以及通过外部（自由空间）激发来探测带隙物理的可能性。所提出的超表面不仅为实现新兴的光子时空晶体提供了一种直接的材料平台，而且为未来无线通信中表面波信号的放大提供了一种现实的系统。
