Dielectric microcavities with quality factors (Q-factors) in the thousands to billions markedly enhance light–matter interactions, with applications spanning high-efficiency on-chip lasing, frequency comb generation and modulation and sensitive molecular detection. However, as the dimensions of dielectric cavities are reduced to subwavelength scales, their resonant modes begin to scatter light into many spatial channels. Such enhanced scattering is a powerful tool for light manipulation, but also leads to high radiative loss rates and commensurately low Q-factors, generally of order ten. Here, we describe and experimentally demonstrate a strategy for the generation of high Q-factor resonances in subwavelength-thick phase gradient metasurfaces. By including subtle structural perturbations in individual metasurface elements, resonances are created that weakly couple free-space light into otherwise bound and spatially localized modes. Our metasurface can achieve Q-factors >2,500 while beam steering light to particular directions. High-Q beam splitters are also demonstrated. With high-Q metasurfaces, the optical transfer function, near-field intensity and resonant line shape can all be rationally designed, providing a foundation for efficient, free-space-reconfigurable and nonlinear nanophotonics.

具有数千到数十亿品质因数 ( Q因数) 的介电微腔显着增强了光与物质的相互作用，其应用范围涵盖高效片上激光、频率梳生成和调制以及灵敏的分子检测。然而，随着介电腔的尺寸减小到亚波长尺度，它们的谐振模式开始将光散射到许多空间通道中。这种增强的散射是用于光操纵的强大工具，但也会导致高辐射损耗率和相应的低Q因子，通常为 10 级。在这里，我们描述并实验证明了一种生成高Q 值的策略-亚波长厚相位梯度超表面中的因子共振。通过在单个超表面元素中包含细微的结构扰动，产生的共振将自由空间光弱耦合到其他束缚和空间局部模式中。我们的超表面可以实现Q因子 >2,500，同时将光束引导到特定方向。还演示了高Q分束器。使用高Q超表面，光学传递函数、近场强度和谐振线形状都可以合理设计，为高效、自由空间可重构和非线性纳米光子学提供基础。