To control and enhance light–matter interactions at the nanoscale, two parameters are central: the spectral overlap between an optical cavity mode and the material’s spectral features (for example, excitonic or molecular absorption lines), and the quality factor of the cavity. Controlling both parameters simultaneously would enable the investigation of systems with complex spectral features, such as multicomponent molecular mixtures or heterogeneous solid-state materials. So far, it has been possible only to sample a limited set of data points within this two-dimensional parameter space. Here we introduce a nanophotonic approach that can simultaneously and continuously encode the spectral and quality-factor parameter space within a compact spatial area. We use a dual-gradient metasurface design composed of a two-dimensional array of smoothly varying subwavelength nanoresonators, each supporting a unique mode based on symmetry-protected bound states in the continuum. This results in 27,500 distinct modes and a mode density approaching the theoretical upper limit for metasurfaces. By applying our platform to surface-enhanced molecular spectroscopy, we find that the optimal quality factor for maximum sensitivity depends on the amount of analyte, enabling effective molecular detection regardless of analyte concentration within a single dual-gradient metasurface. Our design provides a method to analyse the complete spectral and coupling-strength parameter space of complex material systems for applications such as photocatalysis, chemical sensing and entangled photon generation.

为了控制和增强纳米尺度的光-物质相互作用，两个参数是核心参数：光腔模式与材料的光谱特征（例如，激子或分子吸收线）之间的光谱重叠，以及腔的品质因数。同时控制这两个参数将能够研究具有复杂光谱特征的系统，例如多组分分子混合物或异质固态材料。到目前为止，只能在此二维参数空间内对一组有限的数据点进行采样。在这里，我们介绍了一种纳米光子方法，它可以在紧凑的空间区域内同时连续编码光谱和品质因子参数空间。我们使用双梯度超表面设计，该设计由平滑变化的亚波长纳米谐振器的二维阵列组成，每个谐振器都支持基于连续体中对称保护束态的独特模式。这导致 27,500 种不同的模式和接近超表面理论上限的模式密度。通过将我们的平台应用于表面增强分子光谱，我们发现获得最大灵敏度的最佳品质因数取决于分析物的量，无论单个双梯度超表面内的分析物浓度如何，都能实现有效的分子检测。我们的设计提供了一种方法来分析复杂材料系统的完整光谱和耦合强度参数空间，用于光催化、化学传感和纠缠光子生成等应用。