Topological photonics offers the opportunity to control light propagation in a way that is robust from fabrication disorders and imperfections. However, experimental demonstrations have remained on the order of the vacuum wavelength. Theoretical proposals have shown topological edge states that can propagate robustly while embracing deep subwavelength confinement that defies diffraction limits. Here we show the experimental proof of these deep subwavelength topological edge states by implementing periodic modulation of hyperbolic phonon polaritons within a van der Waals heterostructure composed of isotopically pure hexagonal boron nitride flakes on patterned gold films. The topological edge state is confined in a subdiffraction volume of 0.021 µm³, which is four orders of magnitude smaller than the free-space excitation wavelength volume used to probe the system, while maintaining the resonance quality factor above 100. This finding can be directly extended to and hybridized with other van der Waals materials to broadened operational frequency ranges, streamline integration of diverse polaritonic materials, and compatibility with electronic and excitonic systems.

拓扑光子学提供了以一种不受制造无序和缺陷影响的方式控制光传播的机会。然而，实验演示仍然保持在真空波长的数量级。理论建议表明，拓扑边缘态可以稳健地传播，同时接受无视衍射极限的深亚波长限制。在这里，我们通过在图案化金膜上由同位素纯六方氮化硼片组成的范德华异质结构中实施双曲声子极化激元的周期性调制，展示了这些深亚波长拓扑边缘态的实验证明。拓扑边缘态被限制在 0.021 μm³ 的亚衍射体积中，这比用于探测系统的自由空间激发波长体积小四个数量级，同时保持谐振品质因数在 100 以上。这一发现可以直接扩展到其他范德华材料并与之混合，以扩大工作频率范围，简化各种极化子材料的集成，并与电子和激子系统兼容。