The two main research questions on time crystals are: What is a time crystal? Is there a material that is spontaneously crystalline in time? This study synthesizes a photonic material of topological Floquet time crystals and experimentally observes its indicative period‐2T$2T$ beating. A single‐particle picture is explicitly reconstructed of discrete time‐crystalline phase and is revealed using an appropriately‐designed photonic Floquet simulator the rigid period‐doubling as a signature of the breakage of the discrete time‐translational symmetry. Unlike the requirement of the many‐body localization, the photonic Floquet time crystal is derived from a newly defined single‐particle topological phase that can be extensively accessed by many pertinent nonequilibrium and periodically‐driven platforms. The observation will drive theoretical and technological interests toward condensed matter physics and topological photonics.
Topological insulators (TIs) with robust boundary states against perturbations and disorders have boosted intense research in classical systems. In general, two-dimensional (2D) TIs are designed on a flat surface with special boundary to manipulate the wave propagation. In this work, we design a 2D curved acoustic TI by perforation on a curved rigid plate to localize the edge state by means of the geometric potential effect, which provide a unique approach for manipulating waves. We experimentally demonstrate that the topological edge state in the bulk gap is modulated by the curvature of space into a localized mode, and the corresponding pressure distributions are confined at the position with the maximal curvature. Moreover, we experimentally verify the localized edge state is still topologically protected by introducing defects near the localized position. To understand the underlying mechanism for the localization of the topological edge state, a tight-binding model considering the geometric potential effect is proposed. The interaction between the geometrical curvature and topology in the system provides a novel scheme for manipulating and trapping wave propagation along the boundary of curved TIs, thereby offering potential applications in flexible devices.
The ability to manipulate the interaction between light and optical emitters is essential for enhancing the capability of optical devices. Multifarious metallic and all-dielectric structures have been proposed frequently to enhance the emission of electromagnetic dipoles through the Purcell effect, in which its performances depend on two confinement mechanisms: temporal confinement (photon cavity period) and spatial confinement (localized light in an enclosed space), which can be described by the quality factor and mode volume, respectively. Here, we demonstrate that a hollow spoof plasmonic spiral structure in deep-subwavelength scale, which is constituted by periodically inserting spiral-shaped metallic arms into a hollow silicon cylinder, can drastically enhance emission of magnetic dipoles. Particularly, ultrahigh quality factor and ultrasmall mode volume of the magnetic resonance can be realized by further increasing the spiral degree of metallic arms. The results indicate that the quality factor of magnetic dipole mode in the structure can be enhanced to 2600 (silicon
∼
5.5
for same scale) for spiral degree
4
π
, and the Purcell factor can be enhanced to
5
×
10
6
(silicon
∼
5.1
) for a magnetic dipole emission. These results may provide a new avenue for designing optical cavities and enhancing magnetic dipole emission in low frequency.
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