Trapping light in air with membrane metasurfaces for vibrational strong coupling

TL;DR

This paper introduces free-standing silicon membrane metasurfaces for mid-infrared light trapping, achieving high-quality resonances and enabling vibrational strong coupling with molecules, thus advancing scalable, efficient photonic sensing and quantum applications.

Contribution

The authors develop novel free-standing silicon metasurfaces with high-Q resonances in air, facilitating efficient light trapping and strong light-matter interactions in the mid-IR range.

Findings

Achieved a maximum Q-factor of 722 for q-BIC resonances.

Demonstrated vibrational strong coupling with PMMA molecules.

Enabled scalable manufacturing of mid-IR metasurfaces.

Abstract

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 (q-BIC) 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 q-BIC 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.