Defect modes in two-dimensional periodic photonic structures have found use in diverse optical devices. For example, photonic crystal cavities confine optical modes to subwavelength volumes and can be used for enhancement of nonlinearity, lasing and cavity quantum electrodynamics. Defect-core photonic crystal fibres allow for supercontinuum generation and endlessly single-mode fibres with large cores. However, these modes are notoriously fragile: small structural change leads to significant detuning of resonance frequency and mode volume. Here, we show that photonic topological crystalline insulator structures can be used to topologically protect the mode frequency at mid-gap and minimize the volume of a photonic defect mode. We experimentally demonstrate this in a femtosecond-laser-written waveguide array by observing the presence of a topological zero mode confined to the corner of the array. The robustness of this mode is guaranteed by a topological invariant that protects zero-dimensional states embedded in a two-dimensional environment—a novel form of topological protection that has not been previously demonstrated.

已经发现二维周期性光子结构中的缺陷模式已经在各种光学设备中使用。例如，光子晶体腔将光学模式限制在亚波长范围内，可用于增强非线性，激射和腔量子电动力学。缺陷芯光子晶体光纤可以产生超连续谱，并可以无限循环地使用具有大芯的单模光纤。但是，这些模态非常脆弱：结构上的微小变化会导致共振频率和模态体积显着失谐。在这里，我们表明，光子拓扑晶体绝缘体结构可用于在拓扑学上保护中间能隙的模式频率，并最小化光子缺陷模式的体积。我们通过观察飞秒激光写入的波导阵列中局限在阵列角上的拓扑零模的存在，来实验证明这一点。这种模式的鲁棒性由保护在二维环境中嵌入的零维状态的拓扑不变量来保证，这是一种先前尚未证明的拓扑保护的新颖形式。