Optical metasurfaces can manipulate electromagnetic waves in unprecedented ways at ultra-thin engineered interfaces. Specifically, in the mid-infrared (mid-IR) region, metasurfaces have enabled numerous biochemical sensing, spectroscopy, and vibrational strong coupling (VSC) applications via enhanced light-matter interactions in resonant cavities. However, mid-IR metasurfaces are usually fabricated on solid supporting substrates, which degrade resonance quality factors (Q) and hinder efficient sample access to the near-field electromagnetic hotspots. Besides, typical IR-transparent substrate materials with low refractive indices, such as CaF₂, NaCl, KBr, and ZnSe, are usually either water-soluble, expensive, or not compatible with low-cost mass manufacturing processes. Here, we present novel free-standing Si-membrane mid-IR metasurfaces with strong light-trapping capabilities in accessible air voids. We employ the Brillouin zone folding technique to excite tunable, high-Q quasi-bound states in the continuum (qBIC) resonances with our highest measured Q-factor of 722. Leveraging the strong field localizations in accessible air cavities, we demonstrate VSC with multiple quantities of PMMA molecules and the qBIC modes at various detuning frequencies. Our new approach of fabricating mid-IR metasurfaces into semiconductor membranes enables scalable manufacturing of mid-IR photonic devices and provides exciting opportunities for quantum-coherent light-matter interactions, biochemical sensing, and polaritonic chemistry.

光学超表面可以在超薄工程界面上以前所未有的方式操纵电磁波。具体来说，在中红外 （mid-IR） 区域，超表面通过增强谐振腔中的光-物质相互作用，实现了许多生化传感、光谱学和振动强耦合 （VSC） 应用。然而，中红外超表面通常是在固体支撑基板上制造的，这会降低共振品质因数 （Q） 并阻碍样品对近场电磁热点的有效访问。此外，典型的低折射率红外透明衬底材料，如 CaF₂、NaCl、KBr 和 ZnSe，通常要么是水溶性的，要么是昂贵的，要么与低成本的大规模制造工艺不兼容。在这里，我们提出了一种新型的独立式 Si 膜中红外超表面，在可接近的气孔中具有很强的捕光能力。我们采用布里渊区折叠技术来激发连续谱 （qBIC） 中可调谐的高 Q 准结合态共振，我们测得的最高 Q 因子为 722。利用可接近的气腔中的强场定位，我们展示了具有多个 PMMA 分子的 VSC 和各种失谐频率下的 qBIC 模式。我们将中红外超构表面制造成半导体膜的新方法实现了中红外光子器件的可扩展制造，并为量子相干光-物质相互作用、生化传感和极化激子化学提供了令人兴奋的机会。